Bank Erosion in Mekong Delta

and along Red River in Vietnam

Report Mission 23 November - 6 December 2003

Delft, March 2004

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Bank Erosion in Mekong Delta and along Red River in Vietnam

Report Mission 23 November - 6 December 2003

Delft, March 2004

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Contents Main Report Preface Executive summary 1. Introduction 1.1 Background 1.2 Terms of reference 1.3 Composition of team 1.4 Approach 1.5 Set-up of report 2. Background information on the Mekong and the Red Rivers 2.1 Introduction 2.2 General information on Vietnam 2.2 Mekong River and Mekong Delta 2.4 Red River Delta 3 Main observations of the Mission to Mekong Delta and Red River 4. Additional technical observations 4.1 Introduction 4.2 River morphological and bank erosion aspects

4.2.1 Introduction 4.2.2 Morphology of Mekong and Red River, similarities and differences

4.2.3 Recent studies on Mekong bank erosion 4.2.4 Causes of bank erosion 4.2.5 Has bank erosion increased over the years and what are possible causes?

4.3 Prediction of bank erosion 4.3.1 Introduction

4.3.2 Present practice in Vietnam 4.3.3 Methods used elsewhere 4.3.4 Use of mathematical models 4.3.5 Need for additional studies

4.4 Geotechnical aspects 4.4.1 Introduction 4.4.2 Geological structure and effect on soil characteristics and erosion mechanism 4.4.3 Main geotechnical aspects 4.4.4 Modelling and stability prediction 4.4.5 Flow slides in sand 4.3.6 Observations and remarks from meeting and site visits 4.5 Bank protection works 4.5.1 Introduction 4.5.2 Measures to cope with or counter bank erosion and to reduce damages 4.5.3 Observations on bank protections 4.5.4 Bank protection materials and structures 4.5.5 Construction methods 4.5.6 Design manuals and guidelines

4.5.7 Recommendations for further implementation work

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4.6 Response of river to bank protection works and consequences for future 4.7 Maintenance 4.8 Need for setting up of data base 5 Additional non-technical observations and capacity building 5.1 Introduction 5.2 Institutional and legal aspects 5.2.1 Introduction 5.2.2 Institutional aspects 5.2.3 Existing legislation in Vietnam 5.2.4 Need for additional legislation 5.3 Socio-economic/environmental aspects of flooding, bank erosion and counter-measures 5.4 Master planning for bank erosion mitigation and river training

5.4.1 Need for a Master Plan and a long-term strategy for bank protection 5.4.2 Master plan as part of Integrated River Basin Planning and Management 5.4.3 Elements of a Master plan 5.4.4 Some details on some components of Master plan for River training and Bank Protection

5.5 Data and information management 5.6 Capacity building 5.6.1 Capacity building and cooperation 5.6.2 Staffing of Dike Department 5.6.3 Need for training of provincial staff 5.6.4 Use of models 5.6.5 Upgrading university curriculae 6 Proposed Action Plan 2004-2007 7 Conclusions and recommendations References Appendices I Mission participants II Contacts in Vietnam III Mission program and schedule IV Field visits Mekong River V Meetings Mekong River VI Field visits Red River VII Meetings Red River (including Final meeting) VIII Damage overview (collected data) IX Supplementary informations Contents Supplement: Short review on bank erosion and bank protection 1. Causes of erosion and failure a 2. Cliff erosion 3. Bank erosion in stable river systems

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4. Bank erosion and planform changes 5. Survey and data collection 6. Types of bank protection 7. Techniques of bank protection 8. References List of Tables 1 Comparison of river characteristics of the Mekong and the Red Rivers 2 Advantages of including the effect of sedimentary features in the prediction methods for the

Jamuna River (Sarker & Khayer, 2002) 3 Comparison of different protection types. 4 Proposed time schedule for identified actions in the fields of capacity building in bank

protection and river training in Vietnam List of Figures 1 Some information on flooding in Vietnam 2 Mekong Delta location, provinces and regions 3 Hydrology of the Mekong Delta 4 Flood map 1984 5 Final meeting at the Dike Department of MARD where the main conclusions from the

Mission were discussed with senior MARD/DDMFC staff 6 Faults and their possible impact on the planform of the Lower Mekong River 7 Water levels in the Lower Mekong during the year 1982 8 Flood levels Lower Mekong River 9 Some information on the composition of the Mekong Delta 10 Eroding banks along Mekong River 11 Eroding banks Red River 12 Different types of bank erosion along the Mekong River in relation to planform development 13 River planform and bank protection works along the Red River (taken from provincial map,

note that other province is not indicated) 14 Some information on flood peaks of the branches Lower Mekong River in the period 1978-

1998 15 Local bank erosion rates of the Mekong River at Sa Dec versus relative curvature (source Le

Manh Hung & Dinh Cong San (2002)) 16 Prediction method for bank erosion along outer bends in the Jamuna River as an example of

the methods developed by Sarker & Khayer (2002) 17 Alluvial stratification along the Tien river 18 Procedure for evaluating riverbank stability (US Army, 1981) 19 Example of physical components of bank erosion (US Army, 1981) 20 Example of slope stability calculation for a cross section of a dike with berm and slip circle 21 Schematization of flow slides for sand and clay 22 Plan view of a flow slide 23 Physical principle of liquefaction phenomenon 24 Sa Dec slope protection 25 Serious erosion near Vinh Long

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26 Cracks in dike left bank Red river between K82 and K84 in Hung Yen province 27 Bank cracking along the Red River at location (Ha Tay province) 28 Some examples of groynes in Mekong and Red Rivers 29 Floating factory for production of gabions 30 Upstream morphological development threatening Chandpur town protection, Bangladesh 31 Measuring vessel equipped with echo-sounding, GPS and data storage with pc 32 Decision diagram for corrective and preventive maintenance (Source: van Noortwijk et al,

1996) 33 Possible effect of planform correction measures

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Preface In 2002 the Department of Dike Management and Flood Control (DDMFC) of the Ministry of Agriculture and Rural Development (MARD) requested the Rijkswaterstaat (Dutch Public Works Dpt.) to review the problem of bank erosion in Vietnam and to assist in the preparation of a plan for tackling this problem. The mission took place in the period 23 November-6 December 2003. The goal of the mission was to get a more or less reprentative picture of these bank erosion problems and to give recommendations on how in the future to tackle problems, and it was certainly not intended to prepare a professional consultancy report on the bank erosion problems in Vietnam. The mission spent one week in the Mekong Delta and 4 days in the Red River area. Due to the limited time it was not possible to make a full inventory of problems on bank erosion in Vietnam. Therefore, to get a general (and hopely representative) picture of the problem it was decided to visit a number of representative sites in the upper part of Mekong Delta (non-tidal area) and in the lower part of the Red River area. Special attention and time was allocated for discussion with persons working for a long time in the visited areas (praciticians) or doing studies for these areas (researchers). The guides from Dike Deparment (DDMFC) provided useful information on legal, organizational and planning aspects. The direct findings of the mission members are reported in this report. Apart from common observations, observations are given using the disciplinary expertise of the different Mission members (hydrodynamic, morphology, erosion, geotechnics, bank protection). The mission likes to stress that such a short visit can not give the full overview of the erosional problems in Vietnam (and hence it cannot replace a more fundamental study on this problem). However, the work of the Mission and this report can be considered as a second opinion on this problem provided by an independent group from abroad. It has provided the opportunity to confront the actual Vietnamese approach to this problem with some foreign approaches/expertise, and has made it possible to draw some conclusions and recommendations in this respect. The mission hopes that their findings will be of use for further improvements of the Vietnamese approach. The results of this mission, conform to its terms of reference, are presented in this report. It should be stressed that it was a challenging task, which would have been impossible to realize without the close cooperation with Vietnamese experts and counterpart staff. A list of all contacts in Vietnam is given as Appendix 2. We wish to express our deep appreciation for the friendship and dedication of all mentioned persons who were involved in this complex effort.

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Executive summary This report present the findings of a combined Netherlands-Vietnamese mission to support MARD/DDMFC in formulating an approach to the bank erosion problems, especially concerning the improvement of the organizational and technical measures, and possibly, to obtain an international help for bank erosion problems in Mekong Delta and Red River Delta in Vietnam. In late 2003 the mission spent one week in the Mekong Delta and 4 days in the Red River area and had a number of meetings with the most important organizations active in the field of river training and bank protection in Vietnam. To obtain an idea of the scale and extend of bank erosion problems in Vietnam the Mission visited a number of representative sites in the upper part of Mekong Delta (non-tidal area) and in the lower part of the Red River area. Special attention and time was allocated for discussion with persons working for a long time in the visited areas (practitioners) or doing studies for these areas (researchers). The Mission recognizes the large scale of erosion problems in Vietnam, which are associated with many social and economic implications and consequences. The funds available for tackling these problems are extremely limited. It is surprising to see that even under such difficult conditions (under such strict financial and other constraints), the DDMFC, the provinces and the local agencies are able to generate acceptable results. However, the work done can be classified rather as emergency management than as a planned development of the river systems. The approach is not transparent enough (especially not how the urgency of problems is evaluated) and it is far from being optimal. Besides the financial and technical matters, the organizational matters and cooperation (and thus the optimal use op human potential and facilities) needs further improvement. The limited time allocated for this mission did not allow for making detailed studies of the challenges and problems experienced. This report provides the findings of the mission; these are more factual findings than an in-depth analyse. The information collected or the impressions of individual members with different backgrounds is preserved in this report for possible use in the follow-up studies. The main recommendations concern improvement of prediction and monitoring techniques, setting up of data base, strategic planning and capacity building. In Chapter 3 of the report the main findings of the mission are presented. More detailed observations and comments are given in the next two Chapters, where in Chapter 5 a lot of emphasis is placed on

the need for the development of Master plans for river training and a strategic plan for bank protection works both for the Mekong and the Red River capacity building for the Dike Department and the provincial offices active in the

design of bank protection works. In Chapter 6 an Action plan is formulated together with some suggestions for possible funding mechanisms and a tentative time schedule. Two major projects can be visualised, the first one a capacity building project for those at present active in the field of bank protection in Vietnam, and the second one a project in which the curriculum of the Hanoi Water Resources University is extended to include river training and river bank protection possibly in the wider context of integrated water resources planning and management.

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The capacity building project should be aimed at staff of the Dike Department, of the Water Resources Institutes and of the provincial engineering bureau’s, and should be applying a via learning by doing approach. The following activities are proposed under this project: Setting-up data bases (probably 2 types: one national, and another more technical per

river?) Master plan and long term strategy for river training Mekong River, Red River and

Central Vietnam Rivers, in cooperation with RBO’s Improved monitoring capability Improvement design manuals and standards Improved legislation Improvement cooperation Dike Department and the provincial design bureau’s Publication of yearbooks with progress and revolving yearly planning bank protection

works Training (two types: training courses in Vietnam for larger audience and MSc studies of

some students at UNESCO-IHE in NL) Training and exposure tours to the Netherlands NL (RWS/Delft Hydraulics/GeoDelft)

and to Bangladesh (CEGIS, for remote sensing and set-up data bases) For this capacity building project an international donor should be found. The second project is a proposal for curriculum development of HWRU (both in Hanoi and in HCMC), to be prepared together with TU Delft and UNESCO-IHE, and it would be an extension of an already existing cooperation between the two universities and UNESCO-IHE, for which possibly Netherlands funding would be available.

1 Introduction 1.1 Background The Mekong Delta in Vietnam is the most downstream part of the Lower Mekong River Basin and it is of great importance to the Vietnamese community and economy. It is potentially an area of great productive capacity and its development is of crucial importance to the nation’s economic prosperity and food balance. At the same time the Delta is a difficult area, with both considerable physical resources and environmental constraints: great annual variety in the Mekong’s hydrological regime, large tracts of lands with acid sulphate soils and vulnerable wetlands. It is also an area, which is heavily and frequently affected by flooding and bank erosion resulting in loss of live and high economical damage. The similar problems are noticed in the red River area. The Vietnamese Government has recognized this problem in recent years and it has been decided to undertake the necessary remedial actions. The Department of Dike Management and Flood Control (DDMFC) of the Ministry of Agriculture and Rural Development (MARD) requested the Rijkswaterstaat (Dutch Public Works Dpt.) to assist in the preparation of a plan for tackling the problem of bank erosion. This mission is the first step in this direction. The results of the mission, conform to its terms of reference, are presented in this report. 1.2 Terms of reference The mission aims at providing a framework for tackling the problem of bank erosion in the Mekong Delta and along the Red River, with reference to the following elements: To counteract present problems 1. Assessment through field visits of bank erosion problems in the Mekong Delta and along the

Red River, especially during or just after floods 2. General analysis of the bank erosion problems in an attempt to determine mechanisms,

causes and other factors which have a crucial impact 3. Assessment for the 6 Mekong problem locations at Thuong Phuoc, Tan Chau, Hong Ngu,

Sadec, Long Xuyen and Can Tho, and for some locations in the Red River, whether technical solutions are applicable and what are their limits and implications

To counteract future problems 4. Indication of which banks have a high risk on future bank erosion 5. Analysis of the used prediction method and suggestions for improvement 6. Proposal for a straightforward monitoring and data collection system to enhance insight in

bank erosion problems 7. Analysis of existing protection structures and techniques and suggestions for improvement 8. Recommendations for new protection structures and flood fighting strategies, which also

involve the local population 9. Definition of a suitable approach to bank erosion problems and assessment of the need for a

master plan in which the local population participates and which involves both bank protection and planform stabilization of river branches

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1.3 Composition of Mission The Mission (see Appendix 1 for details about its composition) was composed of both Netherlands and Vietnamese members. Three of the Netherlands members were from Publics Works Department (Hydraulic and Road Engineering Department), whereas two Netherlands Mission members (one from GeoDelft and the other from UNESCO-IHE) were selected because of their specialized knowledge and experience. The Vietnamese members were from the Dike Department of the Ministry of Flood Control and Drainage. 1.4 Approach To comply with the terms of reference the following approach was taken: 1. Literature study about the general physical aspects as well as bank erosion along the Mekong

and the Red River before departure of the Netherlands members of the Mission; 2. Study of the available research data about bank erosion along the Mekong River, which was

done during the visit of the Mission to Vietnam; 3. Field visits to a number of provinces to inspect existing revetments and locations with bank

erosion along the Mekong and the Red River; 4. Meetings with the authorities of the DDMFC (Department of Dike Management and Flood

Control), the WRRI (Water Resources Research Institute), the HWRU (Hanoi Water Resources University), the SARD (Service for Agriculture and Rural Development) and the Southern Planning Institute (PI);

5. Preparation of a draft-report of the Mission by the Netherlands members of the Mission on the basis of the above activities plus formulation of an Action Plan for the years 2004 – 2007 and subsequent discussion of the main findings and the proposed plan with the Vietnamese members;

6. Preparation of the Final Report

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1.5 Set-up of present report This report presents the findings of the Mission, and as such it is the response to the Terms of Reference of the Mission. The set-up of the report is as follows. It is divided in two parts. The first part is the Main Report, which consists in total of 7 Chapters and 9 appendices. The Main Report also contains an Executive Summary. The main findings of the Mission are listed in Chapter 3 of this Main Report, but this Chapter is preceded by a Chapter 2 which was prepared in The Netherlands before the Mission left for Vietnam. It is a summary of information available on the Mekong Delta in Vietnam and the Red River, mostly obtained from the Internet. The main findings in Chapter 3 are elaborated in more detail in the Chapters 4 and 5, which present Additional technical observations (in Chapter 4) and Additional non-technical observations and capacity building (in Chapter 5), respectively. Based on these findings Chapter 6 outlines a Proposed Action Plan for the period 2004-2007. Chapter 7 summarizes the main conclusions of the Mission and presents a number of recommendations. The nine appendices present a.o. the mission program and time and travelling schedule, and short reports of the different field visits and meetings in relation to the Mekong Delta and the Red River bank erosion and bank protection works. Moreover a damage overview is presented on the basis of data collected during the visits to the different provinces. In a Supplement a number of Technical Chapters are presented, which often are a summary of exisiting literature on the different topics of interest, notably causes of erosion and failure a, Cliff erosion, bank erosion in stable river systems, survey and data collection, Types of bank protection, and techniques of bank protection.

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2 Background information on the Mekong and the Red River 2.1 Introduction In preparation of the Mission, available information on Vietnam and on the Mekong and the Red River was collected. Mostly information from Internet sites was used. In this Chapter a summary of the collected information is presented, thus providing some background information for the Mission. 2.2 General information on Vietnam Vietnam is situated in the tropical monsoon area of South East Asia with an average rainfall of 1800 to 2500 mm/year and moreover it is a typhoon-prone country. A large number of people, who are mainly involved in the agricultural and fishery sectors, live on the low lying river floodplains, deltas and coastal margins. The most important ports are located along the coast. The potential for disaster in these areas is high, as protective river and sea dikes are frequently overtopped or breached, which results in flooding. During floods serious bank erosion occurs in the lower reaches of the Red River in the North of the country and the Mekong in the South. Flooding and bank erosion cause loss of life and damage to agricultural land and infrastructure. Some information on floods in Vietnam is presented in Figure 1.

Figure 1: Some information on flooding in Vietnam

(Source: Pilarczyk and Sy Nuoi, 2002) 2.3 Mekong River and Mekong Delta

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The Mekong River is one of the largest rivers of the world and it is approximately 4,220 kilometres long. From its source in the Xizang plateau, the river flows through the Xizang and Yunnan regions of China, and it forms the boundary between Laos and Burma and subsequently between Laos and Thailand. Below Phnom Penh, the river divides into two branches, the Song Han Giang and Song Tien Giang, and continues through Cambodia and the Mekong basin before draining into the South China Sea through nine mouths. In Vietnam these nine mouths are referred to as nine ‘dragons’. The river is carries substantial quantities of silt, and has in Vietnam a gentle slope and a large water depth. It is navigable by seagoing craft of shallow draft as far as Kompong Cham in Cambodia. In historical times a connection developed near Phnom Phen between the Mekong River and the Tonle Sap Lake, a shallow fresh- water lake that presently acts as a natural reservoir to stabilize the flow of water through the lower Mekong. When the river is in the flood stage, its silted delta outlets are unable to carry off the high volume of water. Floodwaters back up into the Tonle Sap, causing the lake to inundate as much as 10,000 square kilometres. When the flood subsides the flow of water reverses and proceeds from the lake to the sea. This effect significantly reduces the danger of devastating floods in the Mekong Delta, where the river floods the surrounding fields each year to a level of one to two metres. Habitation of the delta remained restricted by these stagnant waters until canals could be constructed, at the end of the 19th century (Mburu, 2001). The Vietnamese part of the Mekong river has a length of 230 km and is called Cuu Long. It has two main branches, the Tien (North branch) and the Hau (South branch). The Mekong Delta is an important region in Vietnam; with its area of 39,000km2, it covers about 12% of the country’s total area and it provides about 50% of the national agricultural production. About 15 million people live within the delta with 3.5 million in the urban centres.

Figure 2: Mekong Delta location, provinces and regions

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The Mekong Delta is a low-level plain not more than three metres above sea level at any point and criss-crossed by a maze of canals and rivers for transport, irrigation, drainage and flood control. So much sediment is carried by the Mekong's various branches and tributaries that the delta advances sixty to eighty metres into the sea every year. The estimated amount of sediment deposited annually is about 1 billion cubic metres. About 10,000 square kilometres of the delta are under rice cultivation, making the area one of the major rice-growing regions of the world. To prevent detrimental effects to any of the riparian countries all developments on the main stream as well as the tributaries are co-ordinated by an inter-riparian Mekong Committee. The Mekong River has created a variety of natural landscapes, ranging from tidal flats, sandy ridges and tidal backswamps in the coastal plain, estuaries at river mouths, to river floodplains, broad depressions, peat swamps, alluvial levees and terraces further inland. Wetlands created by seasonal or permanent inundation have an important function in the Delta. They form a buffer between sea and land, trap river borne sediment brought with floods, play an important role in soil conservation and coastal protection, provide a habitat for wildlife, and serve as spawning and nursing grounds for fish. They are extremely fragile and could easily and irreversibly be affected by improper management.

Figure 3: Hydrology of the Mekong Delta

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Water regime and tides The most determinant features of the natural water regime of the Delta are depicted in the figure 3. Both rainfall and river flow in the Delta have a pronounced seasonal pattern with very high and very low rainfall and discharge values in respectively the wet season and the dry season which each last for roughly half a year. Periods of water excess alternate with periods of water shortage and all the necessary water control measures essentially originate from this regime feature. Mean annual precipitation ranges from about 2,400mm in the western part of the Delta to 1,300mm in the central part and 1,600mm in the eastern part. The duration of the rainy season is from April to November in the western part and from May to November in the rest of the Delta. Another important feature of the water regimes of the Delta are the tides of the surrounding seas. The tide of the South China Sea is predominantly semi-diurnal with an amplitude of some 2.5-3.0m. The tide of the Gulf of Thailand, however, is mostly of the diurnal type, while its amplitude is only some 0.4-1.2m. The tides have a significant influence on the river and connected canals in the coastal zone and also in the area adjoining the main Mekong river branches all the way into Cambodia. On the long-term, also the impact of the sea level rise on the Delta will be considerable, given the extremely flat topography and the tidal influence throughout the Delta (0.3m is often mentioned as the most probable sea level rise for this area). Flooding and saline intrusion The average elevation of the Vietnamese part of the Delta equals about 0.8m+MSL. During the period of high discharge, the banks of the Mekong in the north of the Delta, below Kampong Cham and above Can Tho, are overtopped and the land is inundated, up to depths of 4.5m. The inundation usually starts in July/August and ends in November/December. A positive effect of the flooding is the deposition of sediments in the flood plains. As the capacity of the river system increases downstream, there is a considerable attenuation of the water levels and less flooding. At the coast, the combined action of river deposition and the sea has created a slightly higher coastal belt which further reduces flooding. The flooding problem in the North is aggravated by high rainfall. In the South, excess rain water also leads to large scale inundation of the land outside the river flooding zone. This occurs especially in the South Western part of the Delta. In the poorly drained depression areas, the inundation may last as long as 6 months. During the March-May period Mekong discharges are low and for an important part required to prevent deep saline intrusion. Higher rates of abstraction would increase salinity intrusion which is already affecting large areas. Flood protection The Mekong Delta is largely unprotected and therefore characterized by widespread, uncontrolled and prolonged floods. A system of drainage channels and pumping stations is used to make agriculture possible. Houses are situated on high places such as along the canal banks, roads or sandy ridges but during the high floods they are usually still flooded. Alternatively, the

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houses are built on stilts or raised foundations above the flood level. Boats are used for communication during floods. Flood control usually consists of low embankments along the primary and secondary canals, while the secondary-tertiary canal connections are provided with simple sluices or temporary earth dam closures. These means provide partial protection to agricultural production during the early part of the rainy season. In the deeply flooded areas, embankments are overtopped later in the flood season during a normal flood year but the embankments may give year round protection in a low flood year. In the coastal area some flood protection schemes also prevent salt water intrusion. Quite a few large sluices in primary-secondary connections were built in that area and many more are under construction or planned. The main canal systems are generally planned, designed, constructed and operated by the national and provincial water resources development organizations (primary and secondary units). The tertiary canals/water control system and on-farm developments, on the other hand, are undertaken at the district/village/farmers group level. Flood control is generally practised at the level of provincial authorities (secondary unit). Development and flood protection strategies The need for economic growth and diversification of the economy in an environmentally sound and sustainable manner will govern the scope and pace of the development of the Delta’s resources. The main thrust of water resources development would be on-farm development and canal improvement to bring more irrigation water to the already irrigated areas and to improve drainage conditions and promote flushing of acid water. The development would also include embankment improvement in the deeply flooded areas, for the time being to prevent flooding till the end of August only and full year round protection in the shallowly flooded, already more developed areas. On good soils forest cannot compete with crop production or aquaculture in terms of income or employment generation. Its development potential lies in areas with low graded, acid sulphate soils and along the coast. Inland swamps (Melaleuca) and mangrove forests are essential for biodiversity conservation and to save the few natural reserves that have been left. The sustainability of shrimp culture and fisheries depends on their existence. In addition, they provide coastal protection. Because of the specific situation of the Mekong Delta it is neither economically justified nor environmentally sound to provide complete flood protection. Controlled flooding would still allow for acidity flushing, would maintain the natural fertilizing effect of sediment, and would minimize the disruption of fish migration and spawning. The actual policy recommendations include: 1. Low embankments in the deeply flooded parts to protect against early floods; 2. Full embankments in shallow agricultural areas to protect against 10-year floods; 3. No embankments on land that has potentially serious acid-sulphate problems; 4. Adequate forecasting and warning systems; 5. Adequate evacuation plans and an adequate number of escape/rescue facilities; 6. Maintenance of natural flooding regimes in sanctuaries and swamps and mangrove forests.

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Figure 4: Flood map 1984 Channel migration The high sediment load of the Mekong River system, estimated at 160 million tons per year, results in an inherently dynamic channel system with rapid rates of change. Commonly, such changes are associated with channel migration, whereby deposition along a riverbank is countered by erosion of the opposite bank. Susceptibility to channel migration and the type of mechanism responsible vary according to the location within the deltaic system. The upper delta experiences very rapid rates of channel migration (with banks erosion rates commonly up to 20 m/year), caused by the lateral accretion of point-bars and mid-channel bars / islands, and the downstream migration of mid-channel bars. Mid- and lower delta channels are more stable (bank erosion 5-10 m/year), and channel change here is mainly caused by the slow accretion of elongated point-bars and mid-channel bars. The slower current velocities and cohesive bank material, as well as the protection afforded by mangroves and nypa palms (Nypa fruticans) in saline reaches, are the principal reasons for the

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relative channel stability here. Near the mouths of the main distributaries, channel changes are common and result from the formation and shifting of distributary-mouth bars. Another group of channel change involves the abandonment of channel segments, which generally leads to their progressive siltation. At a small scale, channels separating a mid- channel or channel or distributary-mouth bar from the river bank may infill with sediment to eventually result in the coalescence of the bar with the bank. At a larger scale, individual distributaries may also become abandoned. The progressive sediment accumulation within the Ba Lai sub-branch of the Mekong is a manifestation of this. Also, many of the smaller rach-type channels along the South China Sea coast (i.e. Ca Mau peninsula and the area about the mouths of the Saigon and Vaico rivers) are prone to change in position and abandonment, as the large tidal range along this coast results in the progressive inward transport of sediment from the sea and eventual channel infilling. Mangroves are likely to assist in sediment accumulation within these channels. Sedimentation and erosion processes in the Mekong Delta are highly seasonal given the large annual fluctuation in both the river discharge and sediment load. Suspended sediment load of the river inflow varies from less than 100 mg/l during the dry season to 600 mg/l during the peak flood season. During the flood season, most bedload, consisting predominantly of sandy material, is transported and deposited on the channel bed and in bars. The finer suspended load is either deposited on the delta plain through overbank flooding or flushed out into the ocean. During the low-flow period, suspended sediments also get deposited in-channel. In the seaward parts of the channels, this deposition is aided by saline intrusion, which causes sediment flushed to sea during the flood season to be re-imported into the delta. In the larger channels, much of the dry-season deposition is ephemeral, as the fine sediment is reworked during the following flood season. In the smaller channels, tidal creeks and canals, mud deposition is more likely to be cumulative over successive dry seasons. Bank erosion Bank erosion is considered a serious socio-economic problem in the upper delta provinces of An Giang and Dong Thap provinces. Problems are especially severe at Tan Chau on the Mekong branch in An Giang, where erosion rates attain 30 m/year, and approximately 400 households have had to be relocated recently due to destruction of their dwellings through bank collapse. Bank erosion has resulted in major disruptions to local livelihoods, and financial burden on the provincial government by necessitating the relocation of inhabitants and localised bank protection works (e.g. Truong Dang Quang). Losses due to bank erosion appear to have increased in the last decade, probably due to the growing urban population and the resultant concentration of activity and capital along the waterfront (e.g. Truong Dang Quang). The severity of erosion at Tan Chau is largely attributable to the sharp meander-bend morphology, which focuses the river flow energy onto the concave bank (where the town is situated). The gradual downstream rotation of the point-bar on the opposite bank has resulted in a progressive downstream shift in the zone of erosion; stretches of river bank upstream of Tan

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Chau, which formerly experienced severe erosion are now experiencing bank accretion (e.g. Truong Dang Quang). Other erosion hotspots further downstream within An Giang (e.g. at Long Xuyen) are mostly associated with the downstream migration of mid-channel bars, which creates a shifting zone of erosion downstream and to the sides of the bar, and a zone of accretion to its upstream. Sand mining Sedimentation on the opposite bank, which accompanies bank erosion, also represents an economic cost in places, through the shoaling of navigation channels, the stranding of wharves, docks and other water transport infrastructure, and the blocking of entrances to canals. However, sedimentation in the main distributary channels is regarded by many as an economic benefit, given the predominantly sandy nature of channel sediments, and the increasing demand for construction sand driven by urban expansion. Numerous sand dredging operations exist along most of the length of both the Mekong and the Bassac branches. An individual operation may extract volumes in the order of 10.000 m3/year from the bed of the channels (e.g. Ky Quang Vinh). Local over-exploitation of sand is also blamed for the frequent occurrence of bank erosion in the Mekong Delta. 2.4 Red River Delta The two major rivers systems in the Red River Delta in the north of Vietnam are the Red River system and the Thai Binh River system. The Red River system consists of the confluents Da River, Thao River and Lo River and five branches, these being the Duong River, Luoc River, Tra Ly River, Dao River and Ninh Co River. The Red River is so named because of the high amounts of red sediment it carries. The Red River carries about 200 million tonnes of sediment each year. In the Red River Delta in, people have built 3000km of river dikes and 1500km of sea and estuary dikes to protect against flooding. Many of these dikes are old and were built by using inadequate manual construction technology and poor materials. Dike foundation conditions and stability have not always been properly evaluated before construction or improvement. River dikes often suffer damage from under-seepage and piping, slides or local collapse during high flood stages. Moreover, the construction of dikes has gradually reduced the areas of the flood plains that are available to accommodate excess flood flows, with the result that river-flood levels have become increasingly higher. Bank erosion Due to instability of the river channel the Red River is affected by siltation of the river channel below Son Tay as well as a general increase in bank erosion, threatening dikes at numerous locations. The most likely causes for this instability are an increase in slash-and-burn land clearing practices in the highlands of Vietnam and China and the release of high energy, sediment-poor water from reservoirs such as Hoa Binh. Slash-and-burn land clearing increase both runoff and sediment load, resulting in sedimentation below Son Tay, where the bed gradients are lower and the river can no longer cope with the sediment. This sedimentation may cause widening and meandering of the river, resulting in local bank erosion.

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The release of high energy, sediment-poor water from reservoirs leads to scour and bank erosion for some distance below the dam, and sedimentation beyond this area, when the bed gradient becomes smaller. Furthermore flow regulation by reservoirs implies an increase in the mid-bank to full-bank flow duration and therefore in bank saturation and bank erosion. Bank erosion and other negative impacts of channel instability are counteracted by structures, which should stabilize the riverbank and channel and protect the dikes. Usually, the following structures are applied: 1. Rock filled wire baskets (gabion mattresses), underlain with geotextile filter cloth to prevent

erosion. A concern with these gabions is the disintegration of the wire casings by corrosion, which eventually will lead to flowing away of the relatively small stones and failure of the structure;

2. Rock and concrete blocks and mats; 3. 2 to 3 meter thick bamboo platings, spaced 5 to 10 meters apart at the toe of the dike; 4. Groins or hard points. The dikes are protected usually by hard revetments, e.g. of the following types: 1. Interlocking rectangular blocks with raised rectangular surface; 2. Six-sided interlocking blocks with a raised triangular surface. 3. Riprap The function of the raised surfaces is the dissipation of wave energy. The revetments are usually underlain with a granular filter, or, in more recent structures, with a geotextile filter cloth to prevent erosion.

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3 Main observations of the Mission to Mekong Delta and Red River In this Chapter the main conclusions from the Mission are given, whereas in the subsequent Chapters these will be elaborated in more detail. The scope and findings of this Mission can be considered as complementary to the findings of the study on ”Environmental issues and recent infrastructure development in the Mekong Delta” (ARMC, 2001), in the sense that for this Mission the emphasis was on bank ersion and bank protection, whereas the ARMC study was considering infrastructure development in general. Still however many of the findings of the latter study are quite relevant. For this reason the first part of this Chapter consists of some texts taken from the ARMC report, but adjusted to emphasize the aspect of bank protection and morphological development. As stated in ARMC (2001) for the Mekong Delta alone, the Mekong Delta and the Red River have experienced an unprecedented increase in infrastructure over the last decades. Although this development has contributed significantly to economic growth within both deltas and also nationally, it has resulted also in a number of adverse impacts on the society and on the natural environment. In retrospect many impacts have originated from the failure to recognise and maintain the rivera and the deltas as dynamic biophysical systems, and an emphasis on rapid economic development based often on export commodity production. Another relevant matter is that the flooding risks and the morphological dynamics of both rivers threaten the further development at locations near to the rivers, and a balance has to be found between the maintaining the rivers as dynamic biophysical systems and a further economic development. This chapter provides an overview of major issues related to flooding, bank erosion and bank protection in both deltas, and sets out various approaches for protection, including institutional, managerial, technical, economic and scientific approaches. The roles played by public awareness and national and international co-operation are also discussed. In addition, it offers advice for aid agencies for priority activities technical and financial assistance for a better integration of bank protection concerns in environmental protection and infrastructural development activities. This is elaborated in more detail in Chapter 5, where a checklist and other pertinent reference materials is attached for environmental impact assessments of projects affecting the river environment. The main objectives of this report are to provide advice and assistance: • to strengthen national and regional capabilities in bank erosion prediction, prevention and management; • to develop a national bank erosion monitoring and information management network; • to strengthen the ability of country to implement and enforce international environmental and technical codes; • to develop and initiate sustainable financing mechanisms which will support ongoing activities beyond the life of the cooperation project between RWS and MARD. First and foremost conclusion and recommendation is that future infrastructure development should incorporate a greater appreciation of the system characteristics of the Mekong Delta and the Red River. An activity in one part of one of the rivers might generate impacts in other areas. The analysis of environmental and socio-economic costs of proposed projects will need to be carried out, not only for the project areas, but also in the entirety of both rivers (AMRC, 2001). An improved coordination of activities between the discrete project areas, promoted in the first

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instance by an increased level of inter-provincial cooperation possibly through the new RBO’s, would minimise the generation of cross-project and cumulative impacts. The prediction and gauging of physical, environmental and social impacts arising from infrastructure development in the Mekong Delta and Red River are often hampered by the lack of adequate baseline and pre- and post-implementation monitoring data. Monitoring of the existing situation and past and ongoing projects will assume increasing importance in providing input for future projects, as the increase in the number of projects within both deltas will bring about a corresponding increase in cumulative impacts. It is not to say that data do not exist; various government agencies and institutions both within and outside the Mekong Delta region and the Red River have carried out a number of studies, but their temporal coverage is often too short to enable trends to be identified. Data collection may not be of benefit initially, but their utility grows with time. There is also an urgent need to improve the coverage of environmental data on the Mekong and Red River catchments. This is especially important in light of uncertainty over the effects of current and future dam construction in the Mekong on the delta. It needs to be a fundamental change in the planning and design of projects, namely a move away from the “defensive” approach that pervades many recent infrastructure projects. Instead of total control, prevention and elimination, emphasis should be placed on partial control, amelioration and, in general, adaptation to the natural environmental conditions. For example, there is more long-term benefit in replacing the current approach of flood-control, involving much investment in hard infrastructure for defending the delta plain from overbank flooding, with strategies which involve re-routing overbank flow and partial protection.

Figure 5 Final meeting at the Dike Department of MARD where the main conclusions from the

Mission were discussed with senior MARD/DDMFC staff

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Hereafter the main observations of the Mission are listed: General observations on bank erosion in Vietnam Bank erosion is a serious problem in Vietnam. In addition to the Mekong and the Red River

there are many other rivers and channels with different geological composition, different soil conditions, and different pattern of siltation and erosion process, where bank erosion is substantial. A problem is flood and bank erosion damage is not always easy to separate from each other, as bank erosion might cause the flood defences to fail. The following classes of erosion are often applied for classification of erosion problems:

I 1-5m/year II 5-10m/y III 10-20m/y IV > 20m/y

In Mekong Delta (i.e., Thuong Phuoc area above Tan Chau, but also in Sa Dec and Vinh Long area) there are a number river reaches with erosion rate of 40m/year or even more, with over the last decade thousands of hectares loss of land, tens of casualties, and/or a few thousands of households relocated due to bank erosion and protective measures. In the Mekong delta the local population do not consider floods as a disaster as long as it is

not an extremely big one; on the contrary even, an annual flood is considered as a disaster when it is too low. Advantages of flooding are: water and soil improvement for crops, better fishery conditions, and improved navigation. Disadvantages are linked to major floods (when nearly the whole area/province(s) is flooded), but this only holds for extremely big floods. The 2000 flood was one of the biggest historical flood for Mekong Delta (most areas in the delta were inundated about 1.5m or more for a number of weeks)

Institutional aspects Government Decree nr. 86 (2003) gives the mandate to MARD/DDMFC to manage the

erosion problems of riverbanks and seashores nationwide (all over the country). The most relevant legal and institutional aspects/questions to be addressed are:

- how to anticipate and timely address bank erosion problems (instead of the present emergency management) - how to properly manage the flood plain, especially in the areas without dikes (which holds for the Mekong Delta and partly for Central Vietnam). In a Government Act from 1998 the new Water Law was formulated including the

establishment of the National Water Resources Council and Regional River Basin Organizations. However, these River Basin Organizations are not yet fully operational. They are still in a primary stage of institutional development and at present they cannot fulfill the anticipated coordination role in water management. However, in due time a number of proposed actions and future planning issues mentioned in this report can be issues which are addressed at a preferably basin wide and interdisciplinary level as part of the activities of these new agencies. In particular at this moment, where these River Basin Organizations are still in an early stage

of institutional development, they need strong support from the involved Ministries and Provinces to become organizations that can play their anticipated coordinatory role. Hence it is important for the Dike Department to clearly indicate in this early stage what is required in terms of spatial planning and resources allocation to cope with bank erosion and flooding

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problems. The Mission noticed that at present bank erosion problems are not high on the priority list of the RBO for the Mekong River determined via public consultation. However, in the opinion of this Mission master planning and a strategic plan for river training should be an important issue to be addressed at short notice.

Capacity building There is an urgent need in Vietnam for capacity building in understanding of river bank

erosion processes and design, implemetation and maintenance of bank protection works in the Mekong Delta, and along the Red River and other rivers. This capacity building should include increase and training of the staff of the Dike

Department, training of provincial staff making the designs of the bank protection works and carrying out the supervision of their construction, the use of models in making designs and upgrading of university curriculae

Social and socio-economic aspects, cost-benefit ratio assessment Because of lack of funds the actual policy of the Dike Department of MARD is focused

mainly on bank protection in urban areas (and very limited for agricultural areas). However, within these limited funds, it is even not possible to solve the all urgent cases. Bank erosion causes loss of houses and properties, loss of land, loss of work, possible

casualties, and may moreover require the re-settlement of the affected population. Resettlement might also be required within the frame-work of the construction of bank protection works. The cost of lost of land, houses and properties and of necessary resettlements should be

taken into account in the cost-benefit analyse, including social and socio-economic repercussions for the society. However, the unit costs should be better/more uniform defined, eventually taking into account the potential value of certain area/land with respect to the planned/prognosed/expected developments.

• Because of the limited financial means bank protection works should be considered very carefully and based on a proper cost benefit analysis. In principal the mission agrees on the basic entries for selection and priorization developed by the MARD ministry as explained during the meetings. However the result of the selection procedure being the priority list for locations to be protected in short term is not completely clear.

• It became very clear to the mission members that MARD has to operate with an enormous lack of budget both with respect to the Mekong delta and the Red River delta. It comes forward in the procedure for priorization and selection of protection projects and in the absence of sufficient budget for maintenance of protection structures. That’s why the ministry is unable to allocate the majority of protection projects proposed by the involved provinces. An illustrative example is the yearly budget of in total 150.109 VN dongs (10 million US$) that is available for the whole flood defence system in the Red River delta. This budget, which comprises all new projects and maintenance works including dikes, bank protection and sluices, represents far less than one percent of the total invested value in the delta.

• Due to the lack of budget the MARD ministry is forced to act in a fairly ad hoc way with primary attention for short term solutions. In some respect the policy seems to be based on disaster management in stead of taking measures in anticipation to future development and based on proper cost-benefit analyses.

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No studies could be identified on the present conditions of resettled population. The Mission advocates that a number of such studies are carried out to better evaluate the socio-economic consequences of resettlement and to take that into account in future cost-benefit analyses.

Towards a Master plan for bank protection and river training There is an urgent need for the preparation of masterplans for bank protection and river

training of the Mekong River in Vietnam and the Red River in the North. These masterplan should fit in the integrated water resources planning and management of the RBO’s in development. From the Master Plan a long-term strategy for the implementation of bank protection works

should be developed. Often bank protection works have a direct interrelationship with safety aspects (stability of dikes, boulevards, roads, urban and/or industrial properties, and other infrastructural objects) and hence the strategy should be considered for the total system. The interrelationship with dike safety is very strong in case of Red River. To some extent the local population has to ”learn” to live with erosion; as for the time being

only local protection measures related to stability of river system and its economic values will be implemented based on cost-benefit/cost optimization. It is practically impossible to protect everybody/everything. The strategy should focus on: - preserving natural systems as far as possible, and - stepwise regulation of rivers and canals (applying, groins, revetments, vegetation, etc)

Zonation along the riverbanks (free belts) should be stimulated/improved in view of space for bank protection but also reserve some areas for flood conveyance. Proper methods to delineate these areas should be developed, e.g. using mathematical models.

Possible causes for accelerated bank erosion In Mekong Delta as well as in Red River area it was suggested by people that the erosion

problems have increased from early 90-ies. Increase of population and associated increase of occupation of riverbank areas in combination with destroying or removing of the natural vegetation have probably resulted in accelerated erosion in recent years. Other causes may be: - increase of flood magnitude - increase of flood frequency - monsoon winds (surface waves) - navigation (ship waves) - sand mining (at some location it can be a main reason of local erosion problems) - increase of fishery area - increase of population and use of riverbanks, also for economic activities

In some areas in the Mekong Delta the increase in number of small dikes and embankments for roads must have reduced the area available for storage and conveyance of flood waters resulting in an increase of flood levels and flow velocities. An in-depth study should be carried out to identify the real causes of any increased bank erosion and counter-act these whenever possible. In Mekong Delta it appears that recently there are more extreme floods: extreme floods have

occurred in nearly every year in the period of 1977 to 2002. Because extreme floods accelerate morphological processes, this has, of course, resulted in a rise of the erosion problems is a very short time-period.

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In HCMC area (Saigon River), erosion problems are evident especially at sites where natural banks have been adapted for local urban and industrial developments. Besides bank erosion of the main rivers in the Mekong Delta, accelerated erosion of (inter-)

connecting canals was mentioned, which might suggest that the present stage of development of the Mekong delta is a transitional one and more erosion can be expected. During the dry seasons the erosion of lower parts of banks continue endangering the stability

of the bank. On the upper part of slopes the drying process results in cracking and slope erosion due to land saturation/overflow after heavy rains. In Red River area the hydrograph and the induced flow regime and morphological changes is

partly influenced by operation of Hoa Binh reservoir. Some people refer increase of erosion in recent years to the operation of Hoa Binh reservoir. However, this effect is not studied in detail yet. In Red River more erosion is observed in the downstream areas influenced by tidal movement (in contradiction to Mekong Delta). The main erosion problems/damages are observed at the end of the flood season. Bank erosion problems in Central Vietnam are somewhat different from the North and the

South due to specific characteristics of rivers, the specific soil conditions and the almost absence of dikes. The rivers in Central Vietnam smallcatchments and have steep gradients, which causes rapid increase of water levels and discharges during the monsoon rains. There is a strong relationship between river hydrograph and probability of erosion. Some local studies are carried out with support of HWRU Hanoi.

On the understanding of bank erosion processes The physical mechanisms causing bank erosion are not yet studied in depth in Vietnam.

Especially, the morphological triggers and the geotechnical aspects of erosion and stability (steep foreshore, saturation, sliding, etc.) need more attention. Much statistical data is available at various institutions (local agencies, research institutes,

universities, ministries) but a systematic overview and e.g. a proper data bank is not available. The Mission strongly advocates that such a database is established for two reasons: for monitoring the river development and for study purposes. The most common bank erosion mechanism appears to be the scour of the underwater

section of the slope, which induces the sliding of (saturated) upper part of slope, usually after drop-down the high water. Removing of natural vegetation accelerates erosion process. Possibly there is a risk of flow slides, which might result in rapid bank recession and risk of

dike breaches. The possibility of the occurrence of flow slides should be studied in the field. Methodological issues Prediction of erosion is a difficult task. Systematic surveys of both the river morphology and

of bank line changes will help to understand the erosion processes and will help to make a better predictions, which should be based on: - measurements and experience/data bank - data collection and processing; monitoring by surveying, satellite image, GIS/GPS, radar

etc.; the use of new monitoring techniques should be stimulated - analytical methods/formulae, and - modeling (in view of the Mission physical modeling can be considered as support for

projects (design purposes) but not as a tool for prediction of bank erosion). Available analytical predictors for bank erosion and scour prediction can be

improved/adapted to local conditions by calibration using survey data; purely experimental

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predictors have limited (local) value and are only applicable as indicator or help for extrapolation of collected data. There is already much experience on the use of the modern monitoring techniques in

Vietnam. However, this knowledge is dispersed over a number of Institutes and Universities belonging to different Ministries. Lack of cooperation is the main weak point and the main reason of not fully using these techniques for practical applications. Dissemination and exchange of knowledge and experience via cooperation/courses/ publications/reports, etc. should be further stimulated. These organizations will need to be monitored and assisted to increase their effectiveness. More attention should be paid to monitoring and prediction of scour and geotechnical state

of bank/dike structures. Proper and timely monitoring/surveying of scour (especially in bends) can help early definition of critical sections. Surveying capacity is not adequate to the problems, and the actual expenditure on

monitoring is very low. Surveying on Red River (1 x per year, about 170 standard river cross-sections) is executed by a survey vessel from Ministry of Transport. Systematic data processing and storage in central data bank should be improved. The diagrams on erosional trends of monitored srcoss-sections are not always available (or difficult to find). Dedicated surveys should be carried out in reaches of special interest. The information on critical sections is not available. Usually, the critical sections are

recognized as such after collapsing of a certain section. The monitoring of such a sections takes usually places in the scope of preparation of rehabilitation/mitigation measures/projects. With good survey strategy, definition of critical sections, data collection and proper/on time

data processing one may earn money/investment back due to better selection capability. Good communication between various (national and local) agencies is crucial for optimal use of available resources and creating a data base. Definition of critical sections can be done by analysis of surveying data, possibly supplemented with results from mathematical models (i.e. MIKE 21-C or DELFT-2/3D).

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4 Additonal technical observations 4.1 Introduction In this Chapter the technical points already raised in Chapter 3 are discussed in more detail. This Chapter deals with technical considerations, whereas in Chapter 5 some institutional matters and capacity building in the field of flooding, bank erosion and river training are addressed The following issues are discussed in this Chapter: river morphological and bank erosion aspects, addressing similarities and differences

between the morphology of Mekong and Red River, recent studies on Mekong bank erosion, causes of bank erosion and whether bank erosion has increased over the years and what are possible causes? prediction of bank erosion, dealing with the present practice in Vietnam, methods used

elsewhere, the use of mathematical models, and the need for additional studies Geotechnical aspects, addressing the geological structure of the Mekong River and effect on

soil characteristics and erosion mechanism, the main geotechnical aspects, modelling and stability prediction, flow slides in sand, whereas a number of observations and remarks from meeting and site visits are made. Bank protection works, addressing an overview of measures to cope with or counter bank

erosion and to reduce damages, some technical observations on bank protections, bank protection materials and structures, construction methods, design manuals and guidelines and recommendations for further implementation of bank protection work. Response of river to bank protection works and consequences for future Maintenance Need for setting up of data bases

4.2 River morphological and bank erosion aspects 4.2.1 Introduction The Mission has spent only a limited time along the Mekong and the Red Rivers. This did not allow for a detailed study of their characteristics. Moreover, no studies were (made) available in which the morphological features of the two rivers are described extensively. The Mission holds the opinion however that on the long run river training measures can be successful only when a good understanding of the behaviour of the considered river is available. Therefore a summary of the characteristics of both rivers is given in this Section, based on the limited data available to the Mission. Some background information on both rivers is already provided in Chapter 2. In addition to Chapter 2 some observations made during the various visits are included in the subsequent Sub-sections. Also the question is addressed whether bank erosion along the two main rivers in Vietnam has increased in the last decade, and if so, what could be the most probable cause(s). 4.2.2 Morphology of Mekong and Red River, similarities and differences

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Some morphological features of the Mekong and Red River are discussed hereafter, based on what was easily available (either as map or report, or via the Internet). First per river and in the last part of this Section a comparison between the two rivers is made. Mekong River For information on the Mekong River use was made of in particular a report by Le Manh Hung & Dinh Cong San (2002) and a paper by Ngaonh & Akira (2003), although also some other sources of information were used. The catchment area of the Mekong is about 800,000 km2, of which about 65,000 km2 is in Vietnam. The total length of the Mekong River is about 4200 km, of which about 200 km is in Vietnam. The planform of the Mekong River is characterised by two separate branches plus a number of lateral connections, mostly man-made. The river having two parallel branches is quite unique, and is probably is due to the fact that in geological times, the Mekong was not yet connected to the Tongle Sap lake and the Bassac river. In this assumption the Tien River is the remnant of the ”original” Mekong River, whereas the Hau river is the original drainage channel of the Tongle Sap system. The Tien river is slightly meandering, whereas the Hau river is virtually straight. Probably there is a strong influence of neotectonics and active faults on the river planform. This is shown in Figure 6. The Hau River and the lower reach of the Tien Rivers coincide with active faults. Near Tan Chau an active fault crosses the Tien River, and this may be the cause of the almost square angle the river channel is making at that location (see also Section 2.3). In the other reaches of the Tien River the curvature of the river bends is much larger and appear to correspond to a “normal” alluvial river (see also Section 4.2.3). The average discharge of the Mekong River is about 15,000 m3/s. Flood discharges of the two branches combined vary over the years between 20,000 and 35,000 m3/s. Hence the ratio between the flood and average discharges is small. The difference in flood levels in Vietnam is not excessive. The maximum water levels near the Cambodian border are about 4 m above Mean Sea Level (MSL) (see the Figures 7 and 8), and the lowest water level is almost MSL. This implies a flood-low water range of 4 m decreasing in downstream direction. As the length of the Tien and Hau Rivers in Vietnam is about 200 km each, water level slopes are very gentle, and are even gentler during the low flow season.

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Figure 6 Faults and their possible impact on the planform of the Lower Mekong River

Figure 7 Water levels in the Lower Mekong during the year 1982

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Figure 8 Flood levels Lower Mekong River The bed material of the Mekong River is fine sand. Le Manh Hung & Dinh Cong San (2002) show (in their Figure III.a) that the D50 of the Tien channel reduces in Vietnam from 0.25 to 0.1 mm. The sediment load of the Mekong River is low. According to data provided in Jansen (1979), the sediment load of the river at its mouth is about 80 million ton per year. In view of the catchment area of about 0.8 million km2, this corresponds to an average denudation rate of (only) 0.07 mm/year. Some other rivers in South-East Asia have much higher denudation rates (Yangtse Kiang 0.2 mm/yr, Yellow River almost 2 mm/year), but the Chao Phrya has a comparable denudation rate (0.05 mm/yr). Average sediment concentrations in the Mekong are about 200 ppm. This all suggests a river, which is morphologically not extremely active. In the Tien River some large islands are present where the river bifurcates. These islands appear to be quite stable. They are characterized by dense and old vegetation and many settlements. These islands, the areas in between the two Mekong channels and the floodplain on both sides of the river are flooded yearly. Flood protection is virtually absent, although the Mission feels that the road building which is going on over the last decade, is creating obstacles to the flow. The population is apparently adjusted to the regular flooding pattern. Local bank erosion was observed by the Mission at a number of places, mostly along the Tien River. Also at the upstream part of some stable islands bank erosion was observed. Typical bank erosion rates are in the order of 10 m/yr and apparently do not exceed 40 m/yr (see Appendix VIII, which is based on information provided by the provinces visisted by the mission). The alluvial reach of the Mekong is composed of sand with mud and clay (see Figure 9). Erosion depth of up to 30 m have been observed in front of eroding banks.

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Figure 9 Some information on the composition of the Mekong Delta

During their stay the Mission visited some locations where bank protection works had been carried out (Tan Chau, Long Vinh) and also one location where bank protection works were under construction (Sa Dec). The bank protection works are constructed where important areas are in danger of being eroded. At other less important locations population is removed from the eroding areas and re-located more inland.

Figure 10 Eroding banks along Mekong River Red River Some data on the Red River were found in Experco (1994) and Nguyen Tuan Anh & Tran Xuan Thai (2000), and these are summarized hereafter. The total catchment area of the Red River (locally called the Hong River) is about 169,000 km2, of which about half is located within Vietnam. The total length of the Red River is about 1150 km, of which about 500 km is in Vietnam. The main tributary is the Da River with a watershed area of about 53,000 km2. Most of the river basin is mountaneous and of a high altitude (70% over 500 m). The forest cover continues to decrease. Some data for the Da River show that the forest cover has decreased from

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77% in 1943 to 9% in 1981. This will have had an enormous impact on peak flows and sediment load of the river. Only in the lower part of the Red River the river is flowing through an alluvial valley. The Red River is flowing through a seismically active area. The general direction of the river seems to coincide with a fault like is the case with the Hau River as part of the Mekong. The Red River zone is characterised by a substantial earthquake risk, high levels risks being incurred by the various structures built in the Red River area (see Experco (1994). In the alluvial reach is made up of quaternary sediments and comprise of (in the Hanoi area, see Experco (1994)) a 1 to 6 m deep layer of clayey silts overlying fine sands and at some places a layer of impervious clay. During floods the floodplains are continuously supplied with sediments rich in organic matter. Experco (1994) mention representative particle sizes of 0.2 mm for the silty snad and 0.5 mm for the sand. The average annual discharge of the Red River is about 4300 m3/s, whereas the maximum recorded flood in the last decades is about 34,000 m3/s and the minimum (after construction of the Hoa Binh dam on the Da River) about 1200 m3/s. The most striking aspect of the river is the great difference between the flood and the low-flow discharges (about 20:1). The water level variation is between 1 to 3 m above the floodplain level during flood to 5 to 6 m below floodplain levels during the low flow season. The flow width during low flow varies between 200 and 600 m, while during flood the width of the river reaches 2 to 3 km (Experco, 1994). During flood the Red River is characterised by a single channel, which is essentially meandering. During low flow a transition to braided is noticeable, and islands can be observed. The location of these islands and of the main flow channel is changing over the years, and this contributes to changes in bank erosion location along the Red River. The sandy islands are quite unstable and hence they allow only for some minor vegetation. The slope of the Red River is slightly affected by the discharge of the river. Only in the lowest reach of the Red River is the slope affected by the effect of the sea level. During low flow the slopes of the Red River vary between 3 and 6 cm/km. During flood the slope of the reach upstream of Hanoi increases to about 10 cm/km. Sediment concentrations of about 2,000 ppm have been observed, indicating that at present the Red River is morphologically much more active than the Mekong River. According to Nguyen Tuan Anh & Tran Xuan Thai (2000) the Red River is aggrading with a rate of 1.5 cm/year. Bank erosion along the Red River is in the order of several tens of meter per year at those locations where bank erosion is actually taking place (of course at other locations bank growth is taking place or islands are increasing in width). According to Experco (1994) the bank erosion is governed by two aspects notably high velocities along the bank and the orientation of the velocity vector towards the bank (and it is expressed in these two parameters because Experco was using 2D mathematical flow computations; see also hereafter and Chapter 4 for other parameters of importance).

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Figure 11 Eroding banks Red River Nowadays a continuous dike system is present along the Red River. In some cases a retired embankment is present as well. At many places bank protection works are present. The Mission did not visit all locations where bank erosion was or is active and moreover the river was approached from one side only. Also the maps provided were for one province (and hence for one bank of the river) only. It is amazing that no maps were available where the bank protection on both banks was marked. Comparison of the Mekong and the Red River In Table 1 a comparison between the Mekong River and the Red River is given. The differences between the two rivers should and will have an impact on the long term strategy for coping with the river and for the river training works. See Chapter 5. Aspect Mekong River Red River Catchment area (km2) and length of river (km)

Catchment area 800,000 km2 Total length of river 4200 km of which about 200 km in Vietnam

Catchment area 143,600 km2 Total length of river 1150 km, of which about 500 km in Vietnam

Number of channels Two separate branches plus a number of lateral connections

Single channel

Planform Tien river slightly meandering; Hau river straight

Meandering, transition to braided

Geological setting Alluvial plain with strong effect of fauls and neotectonics

Alluvial plain with possibly some impact of geology

Discharge (m3/s) Average 15,000 m3/s; flood between 20,000 and 35,000 m3/s

Average 4,300 m3/s; flood discharge about 35,000 m3/s

Difference between flood level and low water

Maximum about 3 m and reducing towards the sea; tidal motion appreciable even near Cambodian border during low flows

Difference much larger; and tidal motion not appreciable in more upstream reaches

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Sediment load Low (0.07 mm/year = 80 million tons/yr); average concentration 170 ppm

Higher (about 0.6 mm/yr = 114 million tons year); average (flood?) concentration 2,000 ppm

Islands and bars Almost stable islands with dense vegetation and many settlements

Unstable sand islands with some vegetation; nevertheless about 600,000 people living on islands (to be checked)

Average bed level Stable Aggrading (1.5 cm/yr?) Flood protection Virtually absent (roads) Continuous dike system Bank protection Some local bank protection works Along all (?) outer bends

Table 1 Comparison of river characteristics of the Mekong and the Red Rivers 4.2.3 Recent studies on Mekong bank erosion Recently the Water Resources Research Institute of the Ministry of Agriculture and Rural Development of Vietnam published the first systematic report dealing with bank erosion along the Mekong River, called “Study on forecasting and preventing bank erosion for the Cuu Long River”, dealing with past and future bank erosion in the Mekong Delta. This study was commissioned by the Vietnamese Ministry of Science and Technology in cooperation with MARD. This chapter gives an overview of the results of this study. Bank erosion damage between 1990 and 2000 In 2000 a flood occurred with an estimated return period of 70 years. Bank erosion due to this and other floods between 1990 and 2000 caused 32 casualties and led to the destruction of 5 roads and several villages, comprising some 2,200 houses. Erosion locations In the report there are a number of figures which show the displacement of the Mekong river bed since 1890. There are some 60 to 70 locations with an average bank erosion up to 10 m per year and more. The historic flood of 2000 at one place resulted in 70 m of erosion. Most erosion takes place in the 'upstream' half of the Vietnamese Mekong (comprising the first 130 km from the border with Cambodia, the Tien and Hau branches have not yet further divided in this stretch), and mostly along the Tien branch. The 6 most threatened locations are Thuong Phuoc (30 to 50 m of erosion per year), Tan Chau, Hong Ngu, Sa Dec (all on the upstream reach of the Tien), Long Xuyen en Can Tho (both on the upstream reach of the Hau). Erosion mechanisms The dominant erosion mechanism on the upstream reach of the river is toe scour with consequent collapsing of the bank. On the downstream reach the tide causes an alternate inflow and outflow of groundwater, which washes out minerals and fine soil particles from the banks, after which sheet erosion occurs. Predicted erosion until 2010 From equations proposed by Ibadzade and Popov a specific equation was derived to calculate future erosion along the Vietnamese Mekong (for more detail see Chapter 4.4). For the problem locations of Thuong Phuoc and Sa Dec this equation yielded a land loss of respectively 120 and 140 ha. by 2010. For the 4 remaining problem locations a land loss between 10 and 25 ha. were predicted. Attempts to simulate the erosion at Sadec with a 1-D numerical river morphology

31

model failed, as only 2-D or 3-D modelling is expected to yield appropriate results if based on a better understanding of the erosion mechanisms. Protection structures To stabilize the riverbanks and avoid erosion at Tan Chau, Hong Ngu, Sa Dec and Long Xuyen, revetments, gabions, groins and floating breakwaters screens are applied or proposed. Conclusions and recommendations 1. Continue to investigate and evaluate the human impacts on the riverbank erosion on the Cuu Long River system. 2.. At least 2D numerical river morphology modelling needs to develop and apply for prediction erosion on the Cuu Long River system in next phase of the study. The erosion mechanisms are not clear and needs to clarify before develop and apply any numerical modelling. 3. Continue to develop and perfect the technology for forecasting, warning, protecting the erosion on riverbank on the Cuu Long river system. Follow-up/ongoing study: Program KC. 08: Enviroment and Natural Disaster Prevention ( National Program); project kc.08.15Project title: Research on the causes and the solutions to prevent river bank erosion and deposition for the Lower Mekong Delta River System (LMDRS). Project Duration: October 2001 – September 2004. Cooperation Institutes (main): - Sub-Institute of Geography – Center for National Science and Technology - Sub-Institute of Water Resources Research Institute– Ho Chi Minh City - Ho Chi Minh City University of Technology - Vietnam National University - Ho Chi Minh City - Southern Hydro-Meteology Station - Offices of Science Technology & Environment, Agricultural & Rural Development in the Lower Mekong Delta. 4.2.4 Causes of bank erosion During their stay in Vietnam the Mission discussed with several institutes their understanding of bank erosion along the Mekong and the Red River and the ongoing studies into bank erosion along these rivers. Based on these discussions the Mission holds the view that it is urgently needed that studies are carried out to better understand the cause of the different types of river bank erosion in order to come up with sustainable solutions to cope with this bank erosion. The causes of bank erosion along rivers can be discussed in different ways. Experco (1994) attempts to explain bank erosion from high velocities (in excess of the critical velocities) and the direction of the velocity towards the bank. According to many handbooks the immediate cause of bank erosion along rivers is the peeling off of sediments (banks of loose material) or the collapse of a bank due to a bank slide (cohesive banks). Along the Red River bank slides appear to be the major cause of the bank erosion. Along the Mekong River this cause is less clear (see also Section 4.2.4). With a slightly different perspective, bank erosion of cohesive banks can also be explained by the occurrence of deep scour holes in front of the bank, which might cause deep-seated slides. This might be enhanced by over-pressure of the groundwater after the recession of the floods. The cause of deep scour holes (in the Mekong and in the Red River scour holes with depths in

32

the order of 30 m have been observed) can be either outer bend erosion, confluence scour or other (see e.g. the overview in Hoffmans and Verhey (1997). One level higher the cause of bank erosion can also be explained in terms of the planform development of the river. The development of a curved channel will induce outer bend scour, which in turn will cause deep scour holes in front of the bank which will result in a collapse of the bank by a bank slide. The ultimate cause of the bank erosion in this reasoning is thus the overall planform development of the river, including the occurrence of islands. These islands become apparent during the low flow period, but probably they are also remain to be present during the flood period as well. Hereafter the cause of bank erosion is discussed in terms of the overall planform development. For the Mekong River bank erosion occurs mainly along the Tien channel. The normal condition is bank erosion along the slightly meandering course. In Figure 12(a) this shown for the Tien channel near Sa Dec. The bank erosion is located in the beginning of the bend, and it continues unless checked by bank erosion works. Another example of bank erosion along the Tien channel of the Mekong is shown in Figure 12(b). Here bank erosion is mainly along the upstream part of the island, but there is also some bank erosion of the bank downstream of the island. This can be interpreted as a slow movement of the island in downstream direction. In this case two islands are present and the channel between them appears to scour. The final example of bank erosion along the Tien channel of the Mekong River is given in Figure 12(c). As is shown in Figure 6 an active fault is present which crosses the river. As hypothesis it is suggested here that the fault causes a horizontal movement, and this would imply that the sharp bends (90 degrees, well in excess of the other bends of the Tien channel) and the corresponding deep scour holes and bank erosion at Tang Chau and at Nong Ngir are in the end caused by neotectonic movements. In the Red River also different types of bank erosion can be identified. The most common are linked to bank erosion along a meandering river. In the Red River the amplitude of the meanders is more developed and the radius of curvature is smaller than in the Mekong River (see Figure 13). The location of the strongest bank erosion can be concluded from the location of the bank protection works, which are also shown in Figure 13. Near the confluence of the Red River with the Da River also confluence scour may play a role. Downstream of the Hoa Binh dam degradation is taking place, which induces collapse of the riverbanks. The Mission observed that within Vietnam limited understanding is available on the ultimate cause of river bank erosion. The Mission strongly advocates that detailed studies should be carried into these underlying riverine processes, and that bak erosion rates should be linked to these underlying processes.

33

(a) Bank erosion along the Mekong (Tien channel) near Sa Dec

(b) Bank erosion of upstream part of island in Tein channel near Cao Lanh

(c) Planform changes and bank erosion (in red) in the Mekong (Tien channel) near Tan Chau and Nong Ngir

Figure 12 Different types of bank erosion along the Mekong River in relation to planform development

34

Figure 13 River planform and bank protection works along the Red River (taken from provincial map, note that other province is not indicated)

4.2.5 Has bank erosion increased over the years and what are possible causes? During the visits of the Mission some provinces in the South were claiming that bank erosion is increasing (and more specifically since 1994). Some possible causes for an increase in bank erosion are listed below:

• increase of frequency of large floods, e.g. as an effect of climate change • increase flow in main channels due to reduction in overland flow • change in flow distribution over channels in Mekong delta (acknowledged for

upstream reaches in Cambodia) • (substantial) degradation of river bed downstream of dam in Da River resulting in bank

erosion • change in flow distribution at confluence of Da and Red rivers • psychological: more damage due to development (houses, roads etc.)

Some observations on this matter are given hereafter, partly on the basis of some data on yearly floods in the Mekong River in the period 1978-1998, provided by the Southern Institute for Water Resources:

35

• the flood discharges of the Mekong River do not seem to have increased in the period 1978-1998 (see Figure 14(a));

• the distribution of the flow over the two Mekong channels does not seem to have changed (See Figure 14(b)).

(a) Combined flow of Tien and Hau channels

(b) Relative flood discharge distribution

TC = Tan Chau, VN = Vam Nao, MT = My Thuan, CD = Chau Doc, CT = Can Tho

Figure 14 Some information on flood peaks of the branches Lower Mekong River in the period 1978-1998

The Mission proposes to do more detailed studies on these and other aspects of bank erosion along the Mekong and Red Rivers, once the data base as proposed in Section 2.6 is available and can be used. 4.3 Prediction of bank erosion 4.3.1 Introduction During its visit to Vietnam the Mission has assessed how in Vietnam predictions are made for future bank erosion. Such predictions are required for decisions regarding the implementation of bank protection works or evacuation of the threatened population. In the following sections some observations made in Vietnam are presented and some suggestions for the improvement of the prediction of bank erosion rates are given.

00.10.20.30.40.50.60.70.80.9

1 3 5 7 9 11 13 15 17 19 21

Year

Q s

tatio

n / Q

Tan

Cha

u

VN/TCMT/TCCD/TCCT/TC

0

5000

10000

15000

20000

25000

30000

35000

40000

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

Year (starting from 1978)

Floo

d di

scha

rge

(m3)

TC+CD

MT+CT

36

4.3.2 Present practice in Vietnam Not much information is available on how prediction of future bank erosion rates are made in Vietnam. As far as understood by the Mission, until now predictions of future bank erosion rates are in particular an extrapolation of the river behaviour in the past. Recently however, some interesting studies into bank erosion along the Mekong River were carried out by the Southern Institute for Water Resources in Vietnam. This study was funded by the Ministry for Science and Technology and the most important findings were published recently in Le Manh Hung & Dinh Cong San (2002). Although the report is in Vietnamese, still the Mission observed that promising results have been obtained which pave the way for improved and more scientifically based prediction methods for bank erosion rates. The Mission would like to point at two highlights from this study. Figure IV.a of Le Manh Hung & Dinh Cong San (2002) the yearly bank erosion rates at Sadec are plotted versus the parameter R/B (radius of curvature over width of the river). This figure is copied here as Figure 15. This result is in line with the work of Hickin & Nanson (1984), but the interesting aspect of this study is that the data included in this figure relate to the bank erosion at any point along the bend, whereas the Hickin & Nanson (1984) method formally only only holds for the one location where the bank erosion is maximum.

Figure 15 Local bank erosion rates of the Mekong River at Sa Dec versus relative curvature (source Le Manh Hung & Dinh Cong San (2002))

Results like included in Figure 15 suggest that this method can be elaborated to a prediction method for a whole bend.

37

Another interesting idea in the study of Le Manh Hung & Dinh Cong San (2002) is the use of a method initially proposed by Popov (no reference available) is used which reads:

max

0

ixi

H HFBLT H Hα −

= −

(3.1)

where Bxi = yearly bank erosion at point i along the bend (m/yr), F = total eroded area (m2), L = length of eroding area (m), T = time elapsed between the two subsequent (…), Hmax1 = bend scour depth at point i, H = average water depth, and H0 = maximum bend scour depth. Although for any application still F and Hmaxi have to be predicted, this method when found appropriate opens the possibility to predict the variation of the bank erosion along a bend. The Mission understands that the study of Le Manh Hung & Dinh Cong San (2002) will be continued with another study with funding from the same source, and they are convinced that these studies, in combination with results from studies abroad (as summarized in Section 4.3.3) and the introduction of numerical models (see Section 4.3.4), will result in improved prediction methods for bank erosion rates. 4.3.3 Methods used elsewhere There is not much theoretical or even empirical work related to quantitative prediction of bank erosion rates. Brice (1984) found that bank erosion rate increased linearly with drainage area for a number of USA rivers. Here three different approaches are discussed. One approach deals with estimates of yearly erosion and was developed by Hickin and Nanson (1984) for rivers in Canada. Klaassen and Masselink (1992) used the same approach for the Brahmaputra River in Bangladesh. The second approach was developed in Bangladesh by Klaassen et al (1993) and improved and updated by Sarker and Khayer (2002). The third approach, included in numerical models in which the bank erosion is simulated, links the momentary bank erosion to the local conditions (either flow or bank height). The first method was developed by Hickin and Nanson (1984), who did an extensive study of bank erosion rates along sand-bed rivers in Western Canada using aerial photographs. They found that erosion rate was a function of the radius of curvature to width ratio R/W, with a maximum at R/W = 2.5. This can be written as:

( ) 2.5M R/B = M . f(R/B) (3.2)

For f(R/W) an empirical relation was derived: for 1 <R/B < 2.5 f(R/B) = 2/3 (R/B-1) (3.3a) for R/B > 2.5 f(R/B) = 2.5 B/R (3.3b) The maximum erosion rate occurs for R/B = 2.5 and is defined as M2.5 (in m/year) and this maximum rate is proportional to total streampower Ω, which is defined as:

38

5 5 = Q = g Q ihτ ρΩ

(3.4)

where Ω = total stream power (in Watt/m'), Q5 = discharge exceeded once in 5 years (m3/s). M2.5 is inversely proportional to a bank-strength parameter YB (dimension N/m2) which is a function of bed material size (given as a figure in Hickin & Nanson (1984)):

2.5B

= M h.YΩ

(3.5)

The use of this method in other parts for other river systems elsewhere must be done with care but can give some idea of possible bank erosion rates. Application in Vietnam can be considered once the method has been re-calibrated on the basis of bank erosion data of both the Mekong and the Red River. This may not necessarily result to the same calibration coefficients, in view of the different characteristics of the two rivers (see Chapter 4.2.2). The second method was developed for the Jamuna (=Brahmaputra) River, which is one of the largest braided rivers in the world. The river is very dynamic, complex and chaotic in nature and its changing bank lines cause major problems for the people living along its course and on its islands. Comprehensive studies on the Jamuna River started only in the 1960-ies. Studies by Coleman (1969) and Bristow (1987) are considered as milestones in the growing understanding of the morphological processes of the complex river system. Later, in the late 1980-ies and early 1990-ies, studies intensified in the context of the construction of the Jamuna Bridge and the Flood Action Plan (FAP). These studies gave particular attention to the interaction between the river and the different types of interventions and a number of empirical prediction tools were developed on the basis of low flow satellite imagery (see e.g. Klaassen & Masselink (1992) and Klaassen et al (1993). Most of the empirical prediction tools developed were derived using dry-season satellite images from the mid 1970-ies to the late 1980-ies, having a coarse resolution of 80 x 80 m. Although the Jamuna River is very dynamic, these coarse resolution images were not sufficient to estimate annual changes with reasonable accuracy. Therefore, intervals of 2 to 3 years were used. Due to the very chaotic and complex nature of the river, however, it was recognized that the longer the time interval, the more uncertain the predictions. The study by Sarker and Khayer (2002) aimed to develop and update those prediction tools using a series of annual dry-season satellite images with a relatively fine resolution of 30 x 30 m, collected during the period 1992-2000. Based on these images, more accurate predictions could be made of such morphological processes as: channel abandonment, migration of bifurcation, and bank erosion along outflanking curved channels. The study introduced so-called “sedimentary features” as a new prediction tools, helping morphologists to better interpret the behaviour of the river. These sedimentary features are contraction bars, sharpened bars, sand wings, sand tongues and “bank side” bars. Identified from satellite images, they can be considered indicators of morphological behaviour. Data extracted from the satellite images are divided into two groups based on the presence and absence of sedimentary features, and correspondingly, two sets of prediction tools were developed for each of the above mentioned

39

morphological processes. A summary of prediction techniques for the different morphological processes with and without sedimentary features is presented in the Table 2.

Bifurcation migration (m/y)

Bank erosion along concave bend (m/y)

Bank erosion along straight reach (m/y)

Bank erosion along convex bend (m/y)

Channel migration (m/y)

Presence of sedmentary feature

Probability (p) of channel abandonment as a function of deviation angle (ϕ)

Avg. value

Range Avg. value

Range Avg. value

Range Avg. value

Range Avg. value

Range

Yes p=0 for ϕ ≤20° p=1 for ϕ ≥65°

1,200 200 to 2,500

230 60 to 400

215 90 to 330

175 50 to 410

950 0 to 3,200

No p=0 for ϕ ≤20° p=1 for ϕ ≥110°

670 -1,400 to 2,600

130 0 to 330

--- --- --- --- --- ---

Table 2 Advantages of including the effect of sedimentary features in the prediction methods

for the Jamuna River (Sarker & Khayer, 2002) Table 2, copied from Sarker & Khayer (2002),shows that the presence of sedimentary features facilitates predictions with higher accuracy. Moreover, in addition to the predictions for channel abandonment, bifurcation migration and bank erosion along concave bank, they make it possible to predict for bank erosion along the straight and convexly curved reaches and channel migration within the braid belt. An example of the prediction of bank erosion along a curved channel, which is the most relevant case of bank erosion for the conditions in Vietnam, is given in the below Figure 16. * ‘m’ stands for migration rate

Figure 16 Prediction method for bank erosion along outer bends in the Jamuna River as an example of the methods developed by Sarker & Khayer (2002)

Type of bifurcation

Migration rate*m = 670 m/y m (90%) = -200 to 2000 m/y

long (L>20km)

Is any sedimentary feature available within 3 km downstream of the bifurcation?

m (50%) = 670 m/ym (90%) = -1400 to 2600 m/y for L = 0 to 10 km m (90%) = -500 to 1800 m/y for L = 10 to 20 km

no yes

m (50%) = 1200 m/y m (90%) = 200 to 2500 m/y

short(L<20km)

40

The use of the empirical predictions based on low flow satellite imagery either without or with sedimentary features relating to morphology of rivers and developed for the Jamuna River can be effectively used to make predictions of bank erosion rates and the celerity of other morphological processes. In principle it can also be applied to the Mekong and Red River. It should be studied whether it is feasible and required to include sedimentary features into the prediction method. The third approach uses local conditions like the local shear stress, the local velocity the local sediment transport and/or the local bank height for estimating the bank erosion rates. Typically also here calibration coefficients have to be introduced which represent a.o. the bank properties. Althought the approach may be more generally applicable, these calibration coeffieicnets have to be determined for each river separately. Prediction methods which take into account the local conditions have been proposed by e.g. Ariathurai and Arulanandan (1978), Crosato (1990), Mosselman (1992), DHI (1996) and Shishikura (1996). A typical example of such a predictor is the predictor of Mosselman (1992):

B ca

c

-n = Etδ τ τδ τ

(3.6) where nB = bank line position and in the right side the local shear stress and the criritical shear stress are present. The coefficient Ea has to be calibrated to the local conditions. DHI & SWMC (1996) have proposed an alternative prediction method which reads:

b b

b

z s= · + ·t H

Bnt

α β∂∂∂ ∂

(3.7)

where zb = bed level near the bank, sb = bed load transport near the toe of the bank and Hb = height of the bank. The first term agrees with a proposition of Mosselman (1992), but the generally accepted term with excess shear strength (see Equation (3.6)) is missing and the second term with transport capacity is questionable because the sediment transport capacity (sb) influences only indirectly the bank erosion mechanism and this indirect influence is already accounted for by the sediment balance. The DHI bank erosion relation implies that higher banks (larger Hb) are less prone to erosion. This is in accordance with Hickin & Nanson (1984). 4.3.4 Use of mathematical models Recently mathematical models have been introduced which also predict bank erosion and the subsequent planform changes. These models are all 2D models, in which the following features are included: • orthogonal or curvi-linear grid (where in bank erosion studies the curvi-linear grid is

preferred) • 2D flow computation • helical flow computed later (and impact of helical flow on main flow not accounted for;

acceptable for not too sharp bends)

41

• bed load • suspended load via the advection-diffusion approach • bank erosion. Different models are available, the best known commercial ones being Delft-3D Rivers from WL/Delft Hydraulics and MIKE21 from DHI (Danish Hydraulic Institute). The Mission understood that MIKE21 will be implemented in both Institutes for Water Resources Research in Vietnam under a DANIDA funded project. In MIKE 21 C (the curvi-linear version of MIKE21), the bank erosion can be simulated in parallel with the sediment transport and hydrodynamic simulations. In each time step, the eroded bank material is included in the solution of the sediment continuity equation. The bank erosion model incorporated is the one already described in Section 4.3, notably:

b b

b

z s= · + ·t H

Bnt

α β∂∂∂ ∂

(4.8)

The extra sediment which is discharged into the river from this source is included in the sediment continuity equation. The accumulated bank erosion causes a retreat of the bank line position and thereby also in the extent of the modeling area. The MIKE 21 C incorporates these plan form changes by re-generating the curvilinear grid during the simulation when the bank line changes exceed a certain pre-defined threshold. In this manner, the morphological model becomes a plan form model. The Mission welcomes the introduction of this numerical model, but warns that such models are not a panacee. Such models are as good as the ingredients on which they are based. In particular the bank erosion modelling part needs to be thoroughly tested and calibrated versus observations. In this respect it would be useful to use MIKE21C in the ongoing research project on bank erosion. 4.3.5 Need for additional studies For the improvement of methods for prediction of bank erosion rates, a good understanding of the underlying processes is a must. The Mission suggests that in addition to the further continuation of the study of Le Manh Hung & Dinh Cong San (2002), a number of other studies should be carried out. These should include, but are not limited to only:

• study of scour phenomena in the Mekong and the Red River, linking scour depths to overall planform features, to come up with prediction methods for scour depths as the trigger for bank erosion

• more studies into the time dependency and the probabilistic nature of scour processes • studies on the basis of low flow satellite imagery to develop prediction methods,

without or with the inclusion of sedimentary features • comparative study of bank erosion features in the Mekong and the Red River • modelling and prediction of geotechnical stability for sites where bank protection

measures are considered.

42

4.4 Geotechnical aspects 4.4.1 Introduction The basic causes for bank erosion are the tractive forces due to currents and waves. Primarily these forces result in surface erosion (grain by grain erosion). However, because the impact of the hydraulic loading often is higher at deeper levels at the riverbed and because of the difference in erosion resistance of the different soillayers, in many cases pure surface erosion will be accompanied by steepening of the river bank. Geotechnical instabilities may therefore be evoked as a more or less interrupting part of the total (combined hydraulic-geotechnical) bank erosion process. Many aspects, generic but especially local, may influence the response of the riverbanks among which the features and size of bank sliding and slumping and the rate of bank retrogression. Specific local conditions are also important for the suitability and applicability of the mitigating bank protection measures. In the foregoing chapters an overview of the important aspects in relation to “causes” and “bank protection” is given. Because the geotechnical features were described in a more qualitative way up till here this section treats the (entries to) a quantitative assessment of bank stability in some more detail. In addition information on bank erosion processes is described in the Supplements 1, 2, 3 and 4. After a brief description of the geological structures of the Mekong delta and the soil types in Sub-section 4.4.2, the main geotechnical aspects and some possibilities for modelling and stability prediction are described in Sub-sections 4.4.3 and 4.4.4, respectively. In Sub-section 4.4.5 special attention is paid to the flow slide mechanism which may initiate bank failure involving large bank retrogression within a short period of time. Therefore, in locations where this mechanism may occur, the consequences of bank erosion could be very serious. In Sub-section 4.4.6 some observations and experiences acquired during the meetings and site visits in the Mekong and Red river deltas are described and illustrated. 4.4.2 Geological structure and effect on soil characteristics and erosion mechanism The present surface level of the flat Mekong Delta plain varies between 1 to 5 m above mean sea level. The depositional sequence in the delta consists of thick silty to sandy layers originated during the Eocene to Pleistocene period, overlain by Holocene sediments of variable character. The depth of the Pleistocene – Holocene boundary ranges from 15 m near the Cambodian border and the Southeast region to about 110 m in the coastal areas. This geological sequence together with the relatively dynamic character of the Mekong rivers and their tributaries during the last millennia mean that the erosion processes in and along the rivers are predominantly governed by the young Holocene sediments. The Holocene sediments consist of clay, silt and sand, ranging from very clayey soil to clean sand. As in most deltas and estuaries the stratification is often very complicated and heterogeneous. For instance along the Tien river fine to middle sand is present below the top clayey or silty layers almost everywhere between the Cambodian border to Vinh Long. However the top boundary of the sand deviates to a high extent.

43

The dynamic meandering character of the main and secondary rivers in the Mekong Delta suggests that the present upper 10 to 20 m are deposited very recently from the geological point of view. It means that the consistency or in-situ density of the different types of sediments generally will be relatively low. As a result the soils will be rather vulnerable to erosion attack and bank instability and in case of sand, under special conditions, susceptible for liquefaction. From the geotechnical point of view the distinction between clay and sand is very important with respect to bank erosion and bank stability. Grain to grain surface erosion is very likely in granular soil as sand and gravel because of the absence of cohesion whereas the phenomenon is unlikely in clay because of cohesive resistance. Therefore, because surface erosion progresses slowly in clayey banks, in many cases erosion will progress by means of mass failures due to undermining in the sand layer underneath the clay. These mass failures may have different dimensions from one large failure to successive smaller ones. Another difference between clay and sand of great relevancy concerns the permeability to groundwater flow. Within the time scales involved in the erosion and bank stability phenomena this difference in permeability coefficient (factor between 10-2 to 10-5) leads to a complete different response with respect to seepage and in relation to origin and dissipation of excess pore water pressures. The variety in alluvial stratification is illustrated Figure 9, which for easy reference is repeated here as Figure 17, and showing soil profiles in a number of locations along the Tien river.

Figure 17 Alluvial stratification along the Tien river

3.4.3 Main geotechnical aspects

44

The geotechnical features and mechanisms will be explained by means of Figure 18 showing the subsequent stages of the erosion process (left side) and the procedure for determination of riverbank stability (right side). During flood conditions in the river, the initial riverbed and bank geometry (upper left in Figure 18) will be subjected to large currents and erosion. Bank recession with time can be estimated by using a procedure as shown at the right hand side. According to the flow pattern in the river the current will be highest at deeper levels. Together with the sensitivity for surface erosion of the different soil layers the erosion rate can be assessed. In the upper diagram at the right side, soillayer 1 is supposed to be weakest which leads to the eroded cross section at a certain point of time as shown in the middle left figure. However in case the erosion resistance of soillayer 2 or 3 had been the lowest underneath a cohesive toplayer, the eroded section would have been more of an undercutting type. For the determination of the erosion characteristics of the different soil types erosion tests in the laboratory have to be carried out, especially for cohesive soils. For granular soils data can be taken from literature. However, because this method for cross section determination as function of time requires detailed information on river and sediment characteristics, soilprofile and erosion properties, frequent measuring of bed profiles and interpretation from the river engineering point of view often is a better alternative.

flow slide

Figure 18 Procedure for evaluating riverbank stability (US Army, 1981) The middle left figure shows a bank geometry with such steep slope inclination that bank instability and slumping is obvious as is indicated in the lower left figure. The analysis of riverbank changes caused by collapse and slumping is analogous or similar to conventional slope stability analysis of an excavated slope. Slope stability analysis for (circular or non-

45

circular) slip surfaces can be used to judge the stability of the bankprofile caused by erosion (as well as that of a possible eroded profile in the future) and the safety of the slope of the slope protection design. It requires data about the shear strength as function of depth in the various layers, as is indicated in the lower right figure by the dependency of shear stress as function of normal stress. Bank failure results when the induced shear stresses exceed the shear strength of the bank soils. Increase in shear stress result from increase in slope height or steepness, increase in external loads, and rapid drawdown of the river. Decreases in shear strength of the soil result from an increase in pore-water pressure, soil expansion, or shear movements. In case a loose sandlayer is present also a flow slide may take place which often results in much larger bank regression in a short period of time. This is indicated at the right side in the lower left figure. For a prediction of the final bank recession for say after one year, the total erosion is equal to the cumulative bank recession caused by surface erosion and slope failures. In case of banks of natural (non-regulated) rivers, the river is in a continuous process of morphological changes and a large area can be affected by seasonal changes. Erosion processes of banks and shorelines are very complicated. In general, there is no one universal explanation and solution; each case needs its own analysis and treatment. As an example, in Figure 19 the complexity of the bank erosion problem is illustrated by a variety of processes that act together.

Figure 19 Example of physical components of bank erosion (US Army, 1981) As illustrated in Figure 19, a typical bank consists of different soils deposited in distinct layers, such as clay, sand, silt, etc. These soils do not permanently stand at a vertical face, but form an angled slope that varies with the soil and groundwater conditions. In many cases this slope is formed after a series of failures whose nature depends on whether the soil is cohesive (clay) or granular (sand, silt, gravel, etc.). Cohesive soils generally slide along a straight or curved plane and rotate or move downward. Granular soils will erode gradually as a result of surface erosion or will drop or suddenly flow downwards. Height is a factor because high steep slopes impose greater stresses and are likely to suffer more severe stability problems than low ones.

46

The internal strength of soils can be decreased by groundwater and seepage flows within the bank. For instance, rainwater or river water at high stages (also high tides) is naturally absorbed and seeps down to lower levels. Especially after a drop in the river level, the weight of saturated soil is increasing causing potential instability. Soils, such as coarse sand, are permeable and allow rapid and free passage of water. Impermeable soils, such as clay, do not allow the free flow of water except through cracks or other openings, and the over-pressured conditions may last for a long time leading at certain moment to the movement of a soil block (especially, in a case of rapid and large drop in the outside water levels). In addition the stability and the recession rate of the bank depends on: - the added weight of buildings and other structures near the top edge of the bank (extra soil

stress contribute to slope failure); - wave action, both due to wind (in estuaries, along coastal shorelines and river stretches with

a large fetch) and navigation will erode or undercut the bank. The effect of wave action will be more extreme at steep slopes;

- roots of trees penetrate the clay layer and provide a path for seepage to the sand layer beneath. On the other hand tree roots will also strengthen the soil and increase the slope stability;

- as the clay starts to fail, cracks are formed at the surface, providing a path for seepage to penetrate the soil, further weaken the clay, and accelerate the failure process;

- water-level fluctuations, especially rapid drawdown, will reduce the slope stability in impermeable soils. In these soils the internal porepressure cannot follow the external conditions through which the passive resistance reduces. In addition seepage out of the slope arises by which surface instability is introduced and increases the risk of slope failure. In a layered stratum seepage water penetrates the clay, reaches the permeable sand layer and exits by flow along the lower clay surface;

- in addition, surface flow can erode the bank face, causing gullies and deposits of eroded material on the beach below. The seepage leaving the bank at the upper clay layer can also cause surface erosion;

- also holes made by animals (rabbits, rats, worms, termites, etc.) may intensify the erosion, for instance because of widening of these holes by drainage water;

- intensive sand mining near the toe of the bank will create scour holes and subsequent (additional) erosion.

4.4.4 Modelling and stability prediction Modelling and geotechnical stability prediction is of great importance for sites where bank protection measures are considered. In this respect the stability calculation forms an essential part in the slope protection design process. It requires quantitative information of the local conditions both with respect to river and erosion characteristics (cross-sectional geometry) as to the stratification of the subsoil and the soil properties. The degree to which predictions are relevant and applicable strongly depends on the reliability of the quantitative input data. On the contrary quantitative modelling of possible bank failures and stability prediction is less relevant within an overall analysis of the erosion process and bank erosion rates along the river (as explained before in 4.2.4). Although insight in the geotechnical processes and failure modes (in relation to soil conditions based on borings) is required, these issues can be conceived as a part of the total erosion process. In such a context, pure geotechnical modelling and stability

47

calculations are less relevant and an empirical and more qualitative approach is much more appropriate. Nevertheless, indications can be obtained on basis of quantitative analysis by specific bank stability and toe erosion model. As an example reference is made here to the bank toe erosion developed by Simon & Langendoen at the National Sedimentation Laboratory (USA), available on the Internet (http://msa.ars.usda.gov/ms/oxford/nsl/cwp_unit/bank.html). A number of calculation methods to determine the slope stability have been developed in geotechnical institutes throughout the world. These are powerful tools for the everyday engineering practice for solving two-dimensional stability problems. The methods are based on well-established design methods and have been widely (and still are) used for a number of decades. For a stability calculation usually an assumption is made with respect to the plane along which the soil mass may slide, supposing that at each point of that plane the maximum shear stress or shear strength is available. The reciprocal of the degree to which this available strength has to be mobilized (as an average along the potential slip plane) gives the stability factor SF of that plane. By considering many of these planes the most critical sliding plane with the lowest stability factor SF can be found. With a powerful computer stability model the critical slip plan can be found very rapidly and accurately. Because of the simple rotational equilibrium the circular plane is taken as the basic (standard) assumption for most slope and subsoil conditions. However for more complicated slopes and complex soillayer and/or loading conditions also non-circular slip planes have to be considered. In special cases with structures of high importance including combination of stiff parts (concrete foundations of rock soil) and soft soils, finite elements methods with a realistic model of soil behaviour give better and more realisctic results (for instance the computer programma PLAXIS, Figure 20). For slope stability the Bishop method is the most usual. In this method the potentially sliding mass is divided in vertical slices and for each slice vertical equilibrium is found. The safety factor is found by comparison of the driving moment by the sum of slice weights (and external loading if present) with the resisting moment given by the shear stresses alomng the potental sliding plane. Besides accurate stability analysis several special options are available such as: - user-defined zones that the plane will not cross (for example sheet piling); - input of geotextiles; - introduction of special loading as water pressures, surcharge (distributed or pointload),

earthquake loading (by additional horizontal and vertical acceleration); - internal pore pressures and degree of consolidation; - (in addition to standard circular slip planes) module for slope stability with user-defined

slip plane; - import a settled geometry calculated by a separate settlement model or a pore pressure

distribution as a result of a groundwaterflow calculation; - graphical output of safety contours and of stress components along slip plane.

48

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Xm : 22,50 [m]Ym : 26,00 [m]

Radius : 35,20 [m]Safety : 1,41

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Figure 20 Example of slope stability calculation for a cross section

of a dike with berm and slip circle 4.4.5 Flow slides in sand A flow slide can be described as an instability that occurs in a fairly gentle underwater slope consisting of loose to medium dense sand, causing liguefied sand to flow out into an even more gentle slope. Compared to a common shear failure it generally involves a much larger mass of sand. It is therefore often accompanied by a sudden and large decline of the coastline or riverbank. A flow slide may occur after a change in slope geometry, for example steepening and/or deepening of the channel as a result of scour erosion. A rather small but quick change may trigger the liquefaction of a vast amount of sand. Also vibration or seepage out of the slope or a riverbank due to tidal water level variation may introduce the phenomenon. As a consequence the local subsoil conditions required for liquefaction are often of greater importance than the exact reason for triggering the liquefaction. Flow slides occur in many parts of the world, for instance along the banks of the Mississippi, in several regions in Bangla Desh and in the south western part of the Netherlands. In this respect some remarking events are reported in literature from which it can be concluded that the chance of occurrence of flow slides is largest where turbulent hydraulic conditions and young geologic sub soil conditions are combined. It is therefore not surprising that a number of well-known flow slides areas are situated in river deltas. For detailled information, see Stoutjesdijk et al (1994) and Stoutjesdijk et al (1998).

49

The phenomenon also regularly occurs because of human activity during dredging works in loose sand and along the slopes of artificial sand islands (e.g in Beafort Sea near Canada). It means that in case critical subsoil conditions are present or expected, special preventive measures have to be taken. Dimensions of a flow slide The possible size and dimensions of a flow slide seem to be unlimited. In literature extreme amounts of displaced sand of hundreds of millions cubic meters are reported. However the dimensions depend on the degree of liquefaction susceptibility of the sand (related to the density of the sand) and on geometrical conditions like the channel depth, the steepness of the channel and thickness of the sensitive sandlayer. In case the sandlayer is covered by a cohesive layer, the size of the slide is influenced by the thickness ratio of toplayer and loose sandlayer. A relative thick cohesive toplayer will discourage the continuation of the flow slide process (by sinking in the liquefied sand) and reduce the distance of retrogression of the river bank. An important geometrical feature for the damage is the average slope that remains after the slide. Depending on the conditions described above the average endslope may vary between 1 to 5 and 1 to 20 (occasionally 1 to 30). The final cross section can in most cases be characterised by a very gentle slope which passes into a rather steep upper end (see following figure). As an example: suppose a depth of 30 m of the river close to the bank consisting of low density sand. Because of erosion the slope has been steepened to 1 to 3 and a flow slide is initiated at a depth of say 20 m. Because the loose sand is present up to 25 m, the liquefaction may occur to this depth. If the top of the loose sand is at the surface, the end slope will be between 1 to 7 and 1 to 20. Application of this endslope means retrogression of the upper bank boundary beween approximately 50 m and more than 200 m (see Figure 19). Because this may happen within several hours to maximum one day, a flow slide often represents a drastic, far-reaching and sometimes dangerous mechanism. The same example but now with a cohesive clay layer above the sand (lower part Figure 19). This clay layer reduces the thickness of the sand susceptible for liquefaction. The final slope in the toplayer after the instability will be between 1 to 1 and 1 to 2. Suppose the thickness of the claylayer is 10 m covering 15 m of loose sand. Based on the most extreme endslope in the sand of 1 to 20, the maximum possible retrogression of the bank is reduced to less than 120 m. It will be even more reduced because of the restraining effect due to the sinking of the clay in the liquefied sand. The chosen thickness ratio of 10/15 means that the maximum possible endslope in the sand decreases to 1 to 10 resulting in a maximum bank retrogression of about 40 m.

50

Figure 21 Schematization of flow slides for sand and clay

after flow slide in sand

Shear failure

initial erosion

Average slope

after flow slide with clay on top

initial erosion

Average slope

Maximum 200 m

maximum 40 m

51

In plan view the result of a flow slide can be characterized by a shell form which is illustrated in the Figure 22.

Figure 22 Plan view of a flow slide

Physical explanation of a liquefaction flowslide Liquefaction may occur in loose sand because this material has the tendency to decrease in volume under influence of a change in shear stress τ’. This is shown in Figure 21.

Figure 23 Physical principle of liquefaction phenomenon However, in case the shear stress change takes place quickly, the decrease in volume ∆e (or reduction in pore volume) is not possible when the pores are fully saturated with water. It means that the tendency to decrease in volume leads to an increase in the pore water pressure u. From the basic soil mechanic rule σ = σ, + u (total stress = effective stress + pore pressure) an increase in pore pressure involves a decrease in effective stress. And because the effective stress is directly related to the shearing resistance, according to the Coulomb law τ’ = σ, tan ϕ, the effect of the change shear stress in saturated loose sand is a decrease in shearing resistance. If this sand is present in or just below a slope it will reduce the stability of the (underwater) slope.

CONTRACTION

' '

- e' τ’

52

The increase in pore pressure may be so large that the initial effective stress is fully cancelled. In this case the sand is liquefied and it reacts like a thick liquid. The consequence is that an apparently stable slope may flow into the river. Because it is the change in shear stress that causes the excess pore pressure, the initiation of liquefaction can be triggered in a slope that is much more gentle than the angle of repose. Therefore flow slides may happen in a slope with inclination 1 to 3 or 1 to 4. Because liquefaction means that the sand reacts like a liquid, the final slope after the event can be very flat: between 1 to 7 and 1 to 25. Flow slide prevention For the prevention of the flow slide mechanism two basic entries are available: - reduction of liquefaction potential of the sand: in general this means densification of the

loosely packed sand. In general this method (for instance vibro flotation) is rather expensive and not applicable in slopes because the densification itself may trigger liquefaction and the initiation of a flow slide;

- prevention of the triggering mechanism: For scour induced liquefaction of slopes it can be realized by full protection of the slope. In the Netherlands slope covering protection has been very sussessful because the occurrence of flow slides has been reduced almost completely.

4.4.6 Observations and remarks from meetings and site visits During the meetings in Long Xuyen, Cao Lanh, Vinh Long, detailled information regarding procedures for selection of river bank locations to be protected, protection design and protection methods have been provided by the provincial authorities (SARD) and the representatives of the Ministry for Agriculture and Rural Development (MARD). This information and the observations during the subsequent site visits give rise to the following remarks from the geotechnical point of view: • During the meetings with provincial authorities of SARD it became quite clear that bank

erosion takes place in a large number of locations along the rivers in the Mekong delta as well as in the Red River delta. Bank erosion should be conceived as a feature of the natural behaviour in the Mekong rivers and to a high extent also for the Red River within the man- made flood protection system. Bank erosion cannot be fully prevented without complete training of the river, which may have a negative impact on the dynamic biophysical systems (see also Chapter 3 and Section 5.3).

• In the design stage the stability of the original (unprotected) and future (protected) slope is assessed. Some information concerning these geotechnical analyses was supplied to the mission members. This information contains remarkable low values for the shear strength of the present soillayers. Regarding the types of soil and the unit weights we should expect at least 30% higher values. Additional information on types of sheartests and design procedure will be appreciated by the Dutch mission;

• Based on the reported surprising low values for the shear strength, the stability factor for the Sa Dec gabion protection is only about 0.85, which suggest that the slope is not stable. Therefore it appears very important to re-analyse the calculation procedure and the basis of the low shear strength values ;

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Figure 24 Sa Dec slope protection • During the mission information on subsoil stratification and soil properties has been

provided for some locations where protection works are carried out or recently completed (e.g. Sa Dec: see above). For other locations including those with serious erosion, information on subsoil stratification and soil properties is scarce, not available or badly accessible. It means that prediction of future erosion or explanation of former erosion is rather difficult. The set-up of an generic geotechnical/geological data base (on provincail or national level) is recommended;

• In front of the Vinh Long boulevard strong erosion takes place (Vinh Long province). From comparison of results of recent soundings with respect to those of 2000 it is known that scour has increased and reached a depth of about 30 m with bank steepness approaching 1 to 1 (Figure 25). To the mission members the situation appears to be very serious. However up till now only the upper meters of the slope are protected by gabions and real effective measures are not planned;

Figure 25 Serious erosion near Vinh Long (average slope of riverbed approaches 1 to 1 but no protection is planned)

concrete

gabions on geotextile

slope 1 to 3

minus 12 m

minus 25 m

Critical slip circle SF = 0.85

sandbags

concrete

gabions on geotextile

slope 1 to 3

minus 12 m

minus 25 m

Critical slip circle SF = 0.85

sandbags

some gabions

minus 30 m

bed profile 2003

unprotected

bed profile 2000

some gabions

minus 30 m

bed profile 2003

unprotected

bed profile 2000

54

• In many locations sand mining is carried out. These activities are primarily meant for

economic reasons supplying construction materials. In a number of locations however these activities seem to be very uncontrolled from the river management point of view. Concentrated and extensive sandmining can threaten the stability of the riverbanks and intensify bank erosion. It is therefore recommended to promote the sandmining activities in locations where it may help to shift the deep channels away from the bank, for instance the removal of sandbars in the river;

• Navigation, especially fast sailing ships close to the bank (of river and canals) may intensify the bank erosion and should be regulated;

• Concrete piles to prevent instability of slopes have been applied in the bank protection works in Tan Chau and Sa Dec. The mission is still not convinced that this is the most optimum solution. Moreover, in case piles are used, they should be driven to such a depth that the benefit is applicable for the entire length of the critical slope. In this respect the depth of the piles in the Sa Dec slope protection is far insufficient (see Figure 22);

• During the mission there were some indications that the flow slide mechanism could play a role in the bank erosion process and in the rate of bank regression in Vietnam. It is very important to recognize this special bankfailure mechanism because it may cause very large bank retrogression in a very short period of time. During the meeting with SIWRR on November 5 is has been explained that a bank sliding may involve 50 m retrogression as a maximum. At the eroding site near Phong Van village along the Red River many cracks have been observed up till a distance of 35 m from the bank edge. It seems very unlikely that this is caused by (the beginning of) a conventional shear failure;

• Because the flow slide mechanism may involve large bank retrogression in a short period of time and because the phenomenon often occurs unexpectedly, it is important to know the potential locations with subsoil conditions (loosely packed fine sand) highly susceptible for liquefaction. Although no strong clues for such locations

• Between K82 and K84 at the left bank of the Red River (district Chay Giang in Hung Yen province) a number of cracks in the dike are reported by the provincial authorities. The cracks arose after dike repair and the recent construction of an extensive berm at the riverside (see Figure 24). The local soil profiel contains a 6 m thick soft clayey (mud) layer at a depth of about 8 m below the dike crest. It was suggested by the authorities that the crack pattern was related to the initiation of a large shear failure through the soft mudlayer. The geotechnical specialist of the mission however believes this in not very likely because of the very gentle average inclination (including the new constructed berm) and the fact that the mud layer has been completely consolidated due to the loading by the dike for so many centuries. A more credible explanation for the cracking should be found in the compression of the mudlayer owing to the extra loading of the large berm and, as a consequence in settlement at the surface. Because this settlement also occurs at the toe of the dike and only to a minor extent at the dike crest, it means an increase of the distance in between these two points and longitudinal cracking in of the surface of the dike slope. The cracks at the surface of the berm most probably are caused by drying and shrinking of the toplayer, perhaps partly due to unsufficient densification. Although more unlikely, due to the fairly steep slope of the dike the cracks in the upper part of the dike might also be an indication of a shallow shear failure in the dike itself. Because the origin of the phenomenon has to be found in deformation (above the mudlayer) and compression (of the mudlayer) the execution of finite element calculations (e.g by using PLAXIS) can be very helpful to understand this problem;

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Figure 26 Cracks in dike left bank Red river between K82 and K84 in Hung Yen province. Compression of mud layer during centuries – extra compression of the mud layer because of (recent) berm accompanied by surface settlement and cracks which may go on for many years

Figure 27 Bank cracking along the Red River at location (Ha Tay province) 4.5 Bank protection works 4.5.1 Introduction In this Section a number of observations and suggestions regarding bank protection works along the Mekong and the Red River are made. In Section 4.5.2 an overview is given of measures to cope with or to counteract bank erosion and/or to reduce damages. This includes both technical and non-technical measures. The observations made by the Mission during inspection of existing bank protections will be summarized in Section 4.5.3. Section 4.5.4 discusses different

mud layerCompression of the mud layer mud layerCompression of the mud layer

56

types of bank protection materials and structures. In Section 4.5.5 construction methods are discussed briefly, based on some observations made during the filed visits. Design manuals and guidelines are discussed in Section 4.5.6, whereas Section 4.5.7 gives some recommendations for further implementation of bank protection works. 4.5.2 Measures to cope with or counter bank erosion and to reduce damages Various bank protection methods as structural measures and a number of non-structural measures can be considered for solving a particular problem of bank erosion. River bank protection works have the positive impact of saving valuable land, which would of been otherwise lost to the river. However, there is also a general concern that river bank protection works in one reach may cause increased erosion at another unprotected reach, and even on the opposite river bank, by changing river flow patterns. It can be considered that such effects are usually localized. However, as the river is naturally meandering and bank erosion occurs naturally, it is difficult, if not impossible, to measure the effects of bank protection works on other reaches. However, this can be true for smaller rivers and canals. There is a large number of non-structural and structural measures for coping with floods and bank erosion. Among them one may distinguish the following measures: Non-structural measures

- floodplain/riverbank zoning - code of regulation - changing people’s attitude on floods and bank erosion - flood forecasting - (early) flood warning/erosion warning - evacuation - flood proofing - control of human activities - organizational (emergency) measures - insurance - etc.,

Structural measures - summer dikes/embankments (structural or non-structural?) - dikes (levees) - floodwalls - discharge sluices/pumping stations - detention (retention) basins/areas - storage dams/reservoirs - chanalisation - channel re-sectioning (deepening/widening) - channel re-alignment - diversion channels - floodplain platforms/mounds - (temporary) geo-membrane barriers - bank protection

57

- vegetation - etc.,

Individual measures or combination of measures can be considered in particular situation. Some of the measures listed above will be discussed in following sections by placing them in the broader context of problem mitigation. Mainly, the structural bank protection measures will be discussed here on the basis of observations made during the mission. However, some remarks on non-structural measures will also be given. 4.5.3 Observations on bank protections Old protections The old bank protections along the branches of the Mekong River were constructed by the local people to protect the upper part of the bank against erosion by river current, ship- and wind waves mainly. These constructions have not been evaluated on their economic effectiveness. Indigeous methods using vegetation to protect the bank against erosion have been applied successfully to some degree. Probably the experiences with these methods have been documented in the standard 14 TCN – 84 – 91. No other design manual seems to exist. Often the bank is still in its natural state without a dyke or embankment with a flood free road on top. Recent bank protections: the revetments near Tan Chau and Sa Dec Recently bank protections have been designed to stabilize the bank, for example a revetment near Tan Chau and a series of groins or a revetment near Sa Dec. Their construction will be completed soon. Irrigation projects do not seem to extend towards a river bank. The overland flow is not considered as a disaster; therefore only bank protections near towns are designed to stop overland flow and to prevent inundation during floods. Design tools used for the Tan Chau revetment are experience and a mathematical model, while a mathematical model, a physical model and experience are applied for the design of the Sa Dec series of groins. Mission has learnt that representatives of the People Committee and the local DDMFC offices usually decide the choice of design tools, and to which extend For both protections the same design report has been prepared. It is a technical report with a lot of information, without figures. Separate books with design figures indicate that the protection has a length of a few hundred meters. The slope of the upper 10 m of the protection has a steep slope 1 in 2, the deeper part has a slope 1 in 3. The design depth of the channel in front of the protection is about 25 meter. For Tan Chau only a combination of a revetment and a stone gabion layer with bamboo trees had been elaborated. This stone gabion layer had been designed to accelerate sedimentation upstream of the protection. For Sa Dec two alternatives, a series of groins and a revetment had been compared. The revetment costs half the price of the series of groins. Therefore a revetment had been selected. Other protections

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Nowadays the riparian population use sand bags made of a fabric to construct emergency protections. If these bags are exposed to sunlight, the strength of the fabric will deteriorate in a few years. And the protection will get damaged and finally it will be lost. Simple wooden protections have some effect to protect against wave attack. The connection to the rigid concrete structure founded on concrete piles above the cages of a protection by gabions allows the formation of a gap. As part of maintenance of the revetment this gap should be filled and repaired. The Mission saw an example of this damage in the waterfront in Vinh Long close to Cuu Long Hotel. At a ferry terminal along the Mekong River the Mission saw some older gabions badly damaged. These gabions were made of unprotected metal wire. Rock or rip rap has been applied at several locations at a small scale Concrete piles and concrete sheet walls can be found at places where ships are moored. At the locations visited by the Mission the surface of the concrete did not show damage or irregularities. Besides the traditional materials also to use vegetation, as a bank protection should be investigated more in detail. Examples of type of vegetation, which can be used to improve the strength of the bank to resist the eroding forces by the river, are for example vertiver grass, catkin grass, palm trees or acacia trees. A disadvantage of vertiver grass is that it will die after more than 14 days of continued inundation during a flood. 4.5.4 Bank protection materials and structures On conceptual design of bank protections In the past bank protections along the branches of the Mekong River were designed to protect against wind- and ship waves mainly. Indigeous methods using vegetation to protect the bank against erosion have been applied successfully. Probably the experiences with these methods have been documented in the standard 14 TCN – 84 – 91. No other design manual seems to exist. Often the bank is still in its natural state without a dyke or embankment with a flood free road on top. Recently bank protections have been designed to stabilize the bank, for example a revetment near Tan Chau and a series of groins near Sa Dec. Their construction will be completed soon. Irrigation projects do not seem to extend towards a riverbank. The overland flow is not considered as a disaster; therefore only bank protections near towns are designed to stop overland flow and to prevent inundation during floods. Several basic concepts have been developed for bank protections along riverbanks, such as revetments, permeable and impermeable groins, falling aprons and bottom protections. A revetment has standard an upstream and a downstream termination to guide the flow and to anticipate the formation of a local scour hole by the flow separation at the downstream termination. Guidelines are missing for the selection of a particular concept or a combination of different concepts for a bank protection in a certain location.

59

In general, the designs appear to be properly chosen, however, often based on a limited number of alternatives (if any alternatives are considered anyhow). It is recommended to consider also alternative solutions in an early phase, like a combination of existing floating fish-houses with concept of underwater vanes (with variable depth/submergence). This may create a flexible solution which may be shifted elsewhere relatively easily. More standarization and large-scale equipment can reduce the cost of construction. The groynes present along the Mekong and the Red River, usually applied on limited scale/on too short stretches or too small in size, have little effect in solving problems. In some cases it should be recommended to use larger groins working as flow guiding structures to be able to deflect the flow/currents.

(a) Permeable groyne Mekong River

(b) Short groyne in front of revetment Red River

Figure 28 Some examples of groynes in Mekong and Red Rivers The Mission recommends the comparison of more elaborated alternative designs for a bank protection to be sure that the optimum will be selected considering both initial and maintenance costs, and its effectiveness. The slope of a revetment is tested for its geo-technical static stability. However, its morphological effects of different slopes are sometimes neglected, for example on the scour in front of the revetment. A gentle slope will reduce the depth of the scour hole and might result in a cheaper solution. On the use of gabions In the Netherlands much experience has been obtained with slope protection by gabions. However, gabions are only seldom used in heavily attacked conditions and not to protect slopes

60

over a depth of more than ten meters. It has never been used to a depth of about 45 m as is presently in construction in Tan Chau along the Mekong. For conditions like those in Tan Chau (province An Giang) the gabion slope protection puts high demands on the quality of the gabion wire (avoidance of rust), degree of filling, exact placement and prevention of downward moving of the elements (e.g. due to wave action), etc. Frequent monitoring and inspection of the performance of the gabions protection and adequate maintenance and repair of damage is therefore of vital importance. In general gabions are sensitive for damage by settlement of the stones inside the cage. This results in a space in the cage and consequently rocking of the stones by wave attack. This rocking results in due time nearly always in damage of the gauze. Therefore the lifetime is rather short and the construction will probably require maintenance measures. Gabion cages are flexible and after some they will slide slightly along the slope of the revetment. 4.5.5 Design and construction methods Along the Mekong and to a smaller extent also along the Red River, the hydraulic conditions are often very severe, as the velocities during floods can be up to 5m/s. Also the hydraulic conditions during execution can be difficult with velocities up to say 3m/s. These conditions require rather heavy designs and proper placement equipment. Usually, the execution takes place when velocities are reduced to less than 2m/s. The actual design codes must be upgraded to be able to tackle properly these heavy conditions in designs and construction. In a number of cases revetments and gabion protection are stopped just beyond the toe of the slope (maximum about 20 meters). Sometimes even before the toe. It is recommended to continue these protections to the deepest point in the river or to make a proper toe protection structure. The projects are not executed/implemented to the whole planned extend and the flanking erosion can still be a potential danger for the already realized structures/protections. On the whole, in the design stage one should better anticipate on possible future changes of the planform of the river after construction of the bank protection works and the subsequent changes in riverbed, e.g. the development of a scour hole (see also Section 4.6). Some additional remarks: Standard for big projects are mostly revetments made of gabion mattresses, whereas

sometimes also (closed/open) groynes are used. Gabion stone mattresses are usually of low quality (without corrosion/abrasion protection) and thus, of short life-time. Local/traditional materials and systems: bamboo, sandbags, bamboo’s anchored in

gravel/stone baskets, and dragon stone/bamboo rolls (cylinders filled with stone) are interesting alternatives fro smaller works.

The Mission visited on site where bank protection were made made with several floating factories (see Figure 29), where it appeared that the gabions were accurately and properly placed with the help of advanced equipment. It demonstrates that it is possible in Vietnam to make bank protection works on an industrial level.

61

Figure 29 Floating factory for production of gabions

Quality control of submitted designs is not always optimal. Approval is often provided after start of the work, and the realization of recommended improvements is not always controlled. 4.5.6 Design manuals and guidelines Standard Guidelines National data-base: A systematic approach to the design of protection and it is needed to include monitoring, and surveying with proper instrumentation, prediction and mapping of problems, proper selection of priority areas including the development of selection criteria, the application of cost-benefit analysis to identify urgent cases. Improvement of the design standards and guidelines are needed. For the design of bank protections national guidelines are available, notably the Standards 14 TCN – 84 – 91 and 14 TCN 130 – 2002. However, these standards are old and not representing the actual state of developments. In due time an update should be prepared of these Standard guidelines for bank protections. Preservation of the existing/natural vegetation should be stimulated and wherever possible, new appropriate vegetation should be planted (for example, vetiver, bamboo, etc). Also this should be reflected in the design manulals. Better-coordinated support by Research Institutes for the improvement of the design standards and manuals is needed. This includes better contacts with and support from the planning agencies as well as of design offices.

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4.5.7 Recommendations for further implementation work A critical location can be defined as one where the dike was immediately adjacent to an eroding riverbank, the dike crest had partially collapsed, causing a traffic hazard, and the dike could not be realigned because of adjacent properties. It is again emphasized that the first priority deserves the selection/definition of the urgent/critical phase works, which may have improved the flood protection facilities protecting urban and valued economic areas and may have reduced the risk of flooding or high economic damage. Therefore the recommendations for further work include: • Investigations for increasing the knowledge on the critical sites/locations, especially when bank erosion endangeres the level of flood protection or valuable properties. Therefore, so that it is more accurately known what are the erosion rates and the gradient of underwater slopes, it is recommended that additional river level gauging stations should be installed and the amount and frequency of monitoring should be increased as soon as possible; • the construction of further lengths of river bank protection works in critical areas to secure the flood protection facilities and prevent the loss of urban and agricultural land; • assistance with O&M of the dikes, structures and bank protection works to maintain the level of protection. Investigations for the riverbank protection works should include the possible (future) large-scale morphological changes of the rivers and the future development/destination of the areas under consideration. Probably the main design consideration with riverbank protection works is the cost as they are expensive to construct on rivers such as the Mekong or Red River. The use of rock and gabions is far cheaper than methods using concrete. However the geotextile and good quality mattresses have to be imported (i.e., rock filled Reno mattresses, 6 × 2 × 0.3 m thick galvanized wire baskets). For local projects, wherever possible, the use of local rock and local materials and alternative structures should be stimulated. However, in case of local solutions and materials the proper design guidelines should be established. Also, the placing of rock for transitions at the upstream and downstream ends of the mattresses and also at the toe of the works should be considered on a larger scale. It is concluded that still considerable further inputs are required to maintain the present/ required level of flood protection, or increase the level of protection to that required by a local circumstances (i.e., safety of cities) and also to secure the protection by the construction of further lengths of river bank protection works. 4.6 Response of river to bank protection works and consequences for future As far as the Mission understood bank protection works in Vietnam are carried out on the basis of where the need arises. When an important reach is eroding bank protection works are implemented over a certain length. Once bank protection works have been implemented, economic development of the protected areas is accelerated. This creates a commitment to properly maintain the bank protection and to prevent that the protected area is eroded in subsequent years. Bank protection works however cause changes in the river system, which are noticeable both downstream and sometimes in later stages also upstream and which may necessitate additional bank protection works. The continued morphological development of the

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reach with bank protection works may thus induce the need for additional river engineering works both upstream and downstream. It is important to realize this in advance in view of decision making on protection, for proper siting of the bank protection works and to limit the funds required later for maintenance and extension of the bank protection works the Mission holds the opinion that upstream and downstream effects of the bank protection works and morphological developments upstream and downstream of planned works should be taken into account in the design of the bank protection works. This also stresses the need for the development of a long-term strategy for bank protection works along both the Mekong River in Vietnam and the Red River.

Figure 30 Upstream morphological development threatening Chandpur town protection, Bangladesh

An illustrative example is the case of Chandpur bank protection in Bangladesh. Figure 30 shows a processed satellite image of the Lower Meghna along which Chandpur town is located. The number of inhabitants of Chandpur is about 1 million people. The Lower Meghna is a very large river which carries the combined flow of Ganges, Brahmaputra and Upper Meghna Rivers. The flood discharge of the river exceeds every year 100,000 m3/s. Probably due to neotectonics the Lower Meghna River is moving Eastward. Due to this shift of the river, the town of Chandpur becomes more and more exposed. Chandpur Town is being defended since mid 20-ies, but the upstream reach continues to erode. Since the 20-ies the scour depths are increasing and have now reached 70 m below floodlevel. At some

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places the slopes of the present bank protection are in the order of 1:1 and very unstable. The yearly extensive maintenance works are much in excess of initial estimate of the required maintenance. By now it is realized that also upstream bank protection and river training measures are needed, but these will be very expensive and in a recent study ithe option to abandon the town was included and not immediately rejected! Mirroring the situation around the diagonal of Figure 30 shows a striking resemblance with the conditions at Tan Chau and may indicate the future conditions at that location. At the same time though it should be realized that the Mekong River is morphologically much slower than the Lower Meghna. Nevertheless the Mission holds the opinion that upstream and downstream effects of the bank protection works and morphological developments upstream and downstream of planned works should be taken into account in the design of the bank protection works. This also stresses the need for the development of a master plan and a long-term strategy for bank protection works along both the Mekong River in Vietnam and the Red River (see Section 5.4).. 4.7 Monitoring and maintenance Monitoring of bank erosion is required for taking timely action in terms of the implementation of bank protection measures or to evacuate people from endangered areas. Monitoring of bank erosion however is also needed to get a better understanding of the causes of bank erosion. Once these causes are better understood also better prediction methods can be developed. The mission is not fully aware of where and how frequent bank erosion along the Mekong and the Red River are measured. Still the Mission proposes, if not done already, to measure bank erosion on a yearly basis. This can be done by carrying out bank line surveys. Two modern techniques can be used in this respect, notably: • hand-held DGPS • satellite imagery Hand-held DGPS provides nowadays a very elegant and cheap method to determine bank lines. By walking along the bank and regularly determining the location, bank lines can be established easily and very accurate. Possibly even normal GPS might suffice, but this should be checked. A disadvantage of this method is that only bank lines are determined, whereas for an improved understanding of bank erosion processes also the overall planform, inclusive the location of islands and the main channel is required. In this respect satellite imagery is much more usefull. Satellite imagery has another advantage: also historical data (from 1973) are available which implies that already on the basis of these historical data a prediction method can be developed. However, the oldest satellite imagery are not very accurate (pixel size of 200 m compared to 10 m for the present generation of Landsat satellite (sensors) and the most recent (and accurate) images are expensive. By using an intelligent mix of these two methods, yearly bank lines and hence yearly bank erosion rates can be obtained in addition to more general morphological characteristics, which determine the bank erosion.

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Figure 31 Measuring vessel equipped with echo-sounding, GPS and data storage with pc

Monitoring is also crucial for the maintenance of river systems and the different (bank protection) structures. In general the maintenance of river systems consist of maintenance of the following elements in an engineered river system: • maintenance of the main channel • maintenance of the floodplain • maintenance of the different structures. As soon as a part of the river system does not fulfill its requirements, it should be maintained, and preferably against minimal costs. Methods are available to determine cost-optimal maintenance decisions for parts of the river system which are subject to deterioration (such as sedimentation of the flood plain or reservoirs, hydraulic structures, river banks). Inspection, repairs, renovations and replacement are possible maintenance actions (Noortwijk et al, 1995; Noortwijk, 1996). Often two types of maintenance are distinguished: corrective maintenance (after failure) and preventive maintenance (mainly before failure). Corrective maintenance can best be chosen if the cost arising from failure is low, and preventive maintenance is best when costs of failure are high. In Figure 32 the decision diagram for the choice between these two types of maintenance is given. Preventive maintenance can be further subdivided into time and use –based maintenance (regular intervals of use or time) and condition-based maintenance (carried out at times determined by inspecting or monitoring the condition).

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Figure 32 Decision diagram for corrective and preventive maintenance

(Source: van Noortwijk et al, 1996) Maintaining (parts of) river systems against minimal cost depends on finding an optimum balance between (initial) costs now and future (uncertain) maintenance costs. This balance can also be applied on existing systems. In the cost function all kind of criteria can be applied. The Mission holds the opinion that maintenance and the related monitoring should get more attention in Vietnam. When and where maintenance is needed often indicated the weak points in the designs and the construction of the different works and can help improving design standards and construction methods. This requires however a critical attitude and the willingness to be open on partial failures of structures. 4.8 Need for setting up of data base The Mission observed that data relevant for bank erosion and its cause are scattered and not stored in one data base, which would allow easy reference and could form the basis for good studies into the causes of bank erosion. Data for studying river processes can be divided in (1) hydrological and sediment transport data and (2) morphological data. In Vietnam hydrological data are collected and stored by the Hydro-Meteorogical Service. The collected and stored data include:

• stages • discharge • sediment concentrations

and these can be used to determine rating curves, slopes and sediment rating curves, and subsequently be utilized to determine statistical parameters of stages and discharges plus summarized information on sediment transport characteristics of the rivers, like sediment loads. In addition to these data the Mission proposes to set up a special data base in which morphological data of relevance for bank erosion, bank protection works and the response of the

Costs of failure Times to failure Deterioration can be inspected

Failure-basedcorrectivemaintenance

Time-basedpreventivemaintenance

Condition-basedpreventivemaintenance

Improvement or redesign of structure

low known yes

nounknownhigh

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rivers to bank protection works are collected. This data base should include the following information:

• historical and recent maps • satellite images • cross-sections and special sounding maps • borings and other information on bank composition • bank erosion rates • scour depths • information on bank protection works: when constructed, typical cross-section,

construction method, as-made drawings, subsequent soundings, etc. figures on maintenance over the years, etc.

• drawings with impact of river training works on upstream and downstream reaches All this information should be included for as many years as possible. The proposed data base can be set up at the Ministry, but probably it is advantageous to make the Water Resources Institutes in Hanoi (for the Red River) and in Ho Chi Minh City (for the Mekong River) responsible for the filling of the data base with data and with the maintenance of it. A special study should be carried out how this data base should be set up, though it seems logic to use a GIS like ArcInfo or Arc as basis for this type of information (which is mostly spatial).

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5 Non-technical observations and capacity building 5.1 Introduction In the previous Chapter mostly technical observations by the Mission were reported. In this Chapter non-technical observations are given and suggestions are made for capacity building in the field of bank protection works and river training. The following aspects are discussed: Institutional and legal aspects (institutional aspects and approaches, existing legislation in

Vietnam and need for additional legislation) Socio-economic/environmental aspects of flooding, bank erosion and counter-measures Master planning for bank erosion mitigation and river training (need for a Master Plan and a

long-term strategy for bank protection; a master plan as part of Integrated River Basin Planning and Management; elements of a Master plan and some details on some components of Master plan for River training and Bank Protection) Data and information management Capacity building (technical and scientific approaches, staffing of Dike Department, need for

training of provincial staff, use of models and upgrading university curriculae). 5.2 Institutional and legal aspects 5.2.1 Introduction The effectiveness of national institutions in charge of environmental protection and erosion mitigation in many countries remains limited. In particular, their resources are often small with marginal influence on preparation of national development plans and development-related decision-making. The failure to create effective national infrastructures equipped with interdisciplinary expertise and adequate resources is a major constraint in river basin and coastal environmental management. Sustainable management of the river and coastal areas requires a variety of expertise and, above all, a good understanding of the cross-sectoral nature of environmental issues. While narrow, sectoral technical expertise exists in most countries, greater efforts should be devoted to training experts in interdisciplinary skills. This requirement for more training of interdisciplinary experts is certainly highlighted whenever developing countries have to respond to significant flood and erosion disasters and water pollution incidents. The national environmental policies and practices of most countries are embodied in legislation. Since environmental concern is relatively recent, environmental provisions are often scattered in a number of sectoral laws. Only a few countries have enacted comprehensive environmental laws, with clear definitions of the functions and powers of the authority(ies) responsible for their implementation and enforcement. Where environmental legislation has been enacted, fragmentary application and weak enforcement have been major problems. Often rivers and coastal environmental problems are shared by neighbouring countries. Therefore, their solutions require international (bilateral, regional or global) co-operation. Legally binding international agreements (conventions, protocols, etc.) can play a decisive role in organising and maintaining such co-operation.

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The flood and bank protection works will not be sustainable if the dikes, bank protection/ revetments and toe protection/ and drainage/pumping structures are poorly operated and maintained through lack of operation and maintenance. Maintenance programmes are necessary to ensure that the flood defence facilities are fully serviceable before flood periods. Operation procedures are necessary for the closing and opening of the water control structure gates or stop logs. It should be recommended that an Operation and Maintenance Sections (O&M) be formed at provincial agencies. The O&M must have finance, staff and resources to undertake many activities including the continual inspection of all the dike/bank facilities, to carry out routine and periodic maintenance, be able to perform emergency activities if necessary during the flood season, undertake system operation, monitor rates of river bank erosion and prepare survey, design, drawing and cost estimates for special works which may require additional inputs from consultants. It is also recommended that donor assistance is required for technical and financial assistance with O&M activities. Technical assistance is necessary to assist the proposed Agency with on-the-job and formal training and the rehabilitation of a number of existing water control structures located downstream of the Urgent Phase area that are in an extremely poor condition and in an urgent need of repair. Donor finance is required to fully equip and operate the Agency for several years and for the rehabilitation of the structures. 5.2.3 Institutional aspects The actual institutional arrangements should be reviewed as far as they concern bank erosion and protections. At present the responsibility for priority listing, design and tools as mathematical modeling and physical modeling, research, management and maintenance of bank protections has been distributed over several institutes and offices, while other organizations have to be consulted regularly (on navigational, financial, sociological (compensation for damage), ecological and agricultural aspects. A small redistribution of responsibilities might be considered. The (intended) Master Plan should include criteria for the selection of proposals for bank protection projects. The Provinces send these proposals to the Ministry for approval and funding. The decision making process might be reviewed including the funding of river training projects. At present no budget is available for maintenance of existing bank protections. The structural lack of funding for maintenance activities needs urgently attention. 5.2.2 Existing legislation in Vietnam A national water resources policy should state the principles, procedures and direction which will be taken with respect to broad issues in the sector. In many countries, policy is developed through a process of investigation and consultation and is used as a basis for legislation. In Vietnam, policy development should be in harmony with, and guided by, the provisions of the Law on Water Resources (LWR) and other legislation and with national goals and objectives. The LWR is a framework document, which requires secondary legislation to bring it into effect.

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While the LWR gives a great deal of valuable guidance for management and development of water resources, it does not answer all of the important policy questions. Further work will be required to develop both policy and legislation onimportant topics coming under the LWR. Policy development priorities should be established to ensure that the most important topics are addressed first. Although many of the responsibilities for water resource management have been centralized (through the LWR) in MARD, it will be important to develop policy recommendations through an open process in which all ministries, agencies and provinces with an interest in the issues are able to participate. Government Decree No. 86/2003/ND-CP (18 July 2003) regulates the functions, tasks and organizational structure of the Ministry of Agriculture and Rural Development (MARD). One of the functions is the state management over the irrigation and water services nationwide. The tasks on irrigation and water services include such items as:

a) Manage construction, exploitation, usage and protection irrigation works, drainage works for rural area in a unified manner.

b) Manage river basin, exploitation, usage and river integrated development per approved plans in unified manner.

c) Manage construction, dike protection, prevention flood and storm works, and tasks related to prevent and combat against flood, storm, drought, and landslide along the river and coastline in a unified manner.

There are also some tasks formulated on science and technology related to the development of necessary programs and new technology application in the domain of irrigation/water services and others, including management of standards and information of science and technology in the domain of the ministry in accordance with legal documents. Additionally, the international cooperation in the domain of irrigation/water services is mentioned as an important task of the ministry. The Department of Dyke Management and Flood Control (DDMFC) is the responsible implementing ministerial agency for flood mitigation and bank erosion problems in Vietnam, in collaboration with provincial units. Large-scale problems and resulting mitigation activities/projects are usually organized and financed by the MARD/DDMFC on behalf of the Central Government, in collaboration with relevant provinces. Some local projects are often organized by provincial, district or local bodies, or even by individual persons. However, the ministerial funds for tackling of flood damage and large-scale bank erosion are rather limited in comparison with the scale of the problem. The selection of the sites for national financial support/projects is based on the following (socio-economic) criteria: Priority I: Populated urban areas and/or safety of dikes Priority II: Densely populated rural areas (larger villages) and/or high economic values Priority III: Rural areas Priority IV: others In general, bank erosion is strongly related to the flooding and it is often difficult to separate the bank erosion damage from flood damage. The figures on damage amount are not very consistent but one has to think in terms of some millions US dollars per year, in tens of human casualties , some tens or even (locally)hundred meters of land losses, and hundreds of demolish houses, often in combination with relocation to other areas.

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These indicative damage figures must be confronted with the annual budget of DDMFC of only/about 10 million US dollars per year. The above stresses the hard need for national Master Plan on Spatial Planning and River Basins Management, which should also include the national long-term strategy for bank protection and risk-based economic selection criteria. The Water Resources Planning Institutes in cooperation with the Water Resources Research Institutes should prepare the basic ingredients to these new policy documents. Other relevant legal documents The following important legal documents for water resources management have been issued: - Law on Water Resources No 08/1998/QH10 dated 20/05/1998. - Ordinance on Exploitation and Protection of Hydraulic Works No 36L/CTN-dated 10/09/1994. - Ordinance on Dykes and Dams No 26/2000/PL-UBTVQH10 dated 7/9/2000. - Ordinance on Flood and Storms Controlling (amended and supplemented 24/8/2000). Decrees of Guidelines on the implementation of the Law on Water Resources and the above-mentioned Ordinances have also been enacted. Based on the Law on Water Resources, the Government has issued the Decree No 179/1999/ND-CP stipulating the implementation of the Law on Water Resources. The National Council of Water Resources was also established in accordance with the Decision No 67/2000/QD-TTg dated 15/6/2000. In conformity with the current situation and to the Law on Water Resources, the Standing Committee of the National Assembly has compiled three Ordinances (revised): - Ordinance on Exploitation and Protection of Hydraulic Works (currently being amended and to be approved). - Ordinance on Dykes and Dams (already amended and issued No 26/2000/PL-UBTVQH10 dated 7/9/2000). - Ordinance on Flood and Storms Controlling (amended and supplemented 24/8/2000). - There is also a legal system of water fees for use of water for irrigation. Water fees and their calculation are based on a national decree that the government cabinet promulgated in August 1984 (112 HDBT, 1984). This decree specifies that all organizations and individuals benefiting from irrigation, drainage and other hydraulic public services, have to pay a water fee to hydraulic companies. 5.2.3 Suggestions for additional legislation Legislation: The Province arranges an area for resettlement of the people who have lost their land by bank erosion. In addition the victims of bank erosion receive only small compensation for their loss (a few hundred thousand Dong per family). In the past no legislation had been made to prevent construction of buildings too close to the bank. In a new law it is decided that in urban areas a margin of 50 m from the riverbank should be free from (permanent) buildings. In practice it is difficult to apply this law strictly. Administrative and financial problems

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• A permanent shortage of finance even on several very serious/danger occasions (i.e., evidence of very steep underwater profile in front of road and/or buildings) . This results in a slow start to project activities and later on often , on partly realization (completion) of projects;

• The initial request from the local agencies for the river bank protection works is often refused because of lack of budget and it is waiting for collapse/failure to may apply for the additional finance from the national disaster funds;

• A number of small stretches were protected (or are under construction) with the grants provided by international donors.

• The main technical problems encountered were those with the existing designs in the inception period and the problems with the meandering of the Mekong River and red river.

• This mission concerns bank erosion and protection and not flood management and mitigation. However riverbank erosion should be a major concern when planning flood protection works and is worthy of further comment. This is because it is usual practice to locate the dikes as near to the river bank as possible in order to maximize the areas protected. The top-soils on the river banks are generally very fertile and intensively farmed. However if the dikes are located close to an eroding riverbank then they will eventually be undermined and collapse. For example it may be considered appropriate to locate a dike 50 m from a river bank. However Mekong River bankerosion (and some locations of Red River) can be up to 20 m a year (or even more) and if this is the case, then the dike will have to be moved in a few years. The same is the case when the properties are buit close to the bank.

• As stated previously, bank protection works are expensive and cannot be automatically considered in a flood protection project, particularly in rural areas, where in economic terms the cost can be higher than the value of the assets, which are to be protected. The final choice/decision, however, should be taken based on a proper cost-benefit analyse/criteria; the further development of the use of this technique should be stimulated.

• Evacuation and resettlement are often applied as emergency actions in Vietnam. An important aspect of resettlement is whether the resettled population is satisfied with their new socio-economic and environmental conditions, and whether sufficient assistance has been provided by the government/provinces (?), both financially and in logistic terms for the re-settlers to be able to regain acceptable living conditions. The Mission is not aware of any studies on the conditions of the re-settled population and hence they advocate that such a study is carried out. Only when the results of such studies are available, it can be assessed whether relocation of population threatened by bank erosion is an acceptable alternative.

• To reduce the problems associated with evacuation and resettlement the Floodplain zonation should be applied on a larger scale.

The mission identified also some issues which are not so explicitly stated in the Law: Development of national policy and policy-making mechanisms including floods and bank

erosion; Development of a strategy or action plan on bank erosion problems and mitigation, including

even the necessity of drastic measures (as illustrated with the Chandpur case below); Future management and maintenance, including financing; Development of a strategy or action plan to guide capacity building in water resources

specifically related to floods, bank erosion, river training, ecology, and environmental and socio-economic impact.

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Some of these issues will be discussed below (in the following sections). Floodplain zonation Floodplain zonation can be an important instrument to regulate and direct the use of the floodplain. This is important for at least two reasons. Firstly, the floodplain is important for the conveyance and storage of floodwater. This property should be maintained and hence it is important to indicate what is acceptable in the floodplains of the Mekong River and the Red River. Secondly, it should be prevented that people settle in areas which might be eroded within the foreseeable future and for which there are no plans to protect the area by bank protection works. This calls for legislation to regulate settling and other activities and it calls for enforcement of this legislation. In The Netherlands already in 1908 the so-called River Law was introduced, with the specific purpose of floodplain zonation of the floodplains of the main rivers in the Netherlands (Rhine and Meuse). An essential part of the River Law are maps on which it is specifically indicated which areas of the floodplain are important for conveyance and which for storage. Building of houses, raising of the ground level and building of roads in areas reserved for conveyance is in general not allowed unless a permit of Public Works Department is obtained. Such permits usually come with requirements which should be fulfilled before the permit can be used. Such requirements usually consist of compensatory measures like the lowering of other parts of the floodplain. In areas reserved for storage more activities are allowed, but still a permit is needed. In the River Law no provision is made for areas threatened by bank erosion because th emain rivers in The Netherlands had been regulated already in the 2nd half of the 19th century, and bank erosion is prevented by long series of groynes and revetments. The Mission has understood that in Vietnam no legislation exists that can be used for floodplain zonation. Nevertheless legislation similar to the Dutch River Law can be made and implemented. Different from the Dutch “maps” in Vietnam it is important to identify also areas, which are reserved for future bank erosion. In these areas settlements should not be allowed. A crucial aspect of such type of legislation is enforcement. The Mission holds the opinion that mechanisms should be developed which allow for better enforcement. Experience from The Netherlands (and other countries) can be of use in this respect. In conclusion, coordination is needed with regard to policy and legislation, information management, water resource planning, operational programs, and emergency response. At the central level this will involve such things as development of a more coordinated strategy for the water resources sector, approval of budgets, approval of investment projects and river basin management plans, and improved communication and dispute resolution between sectors and major water users. Coordination and clear definition of roles is needed between water management agencies and organizations from the central down to the local level. Water management agencies at all levels need training, facilities, financial support and operational guidelines for effective water management. It will be necessary to build coordination in both a “top-down approach” and a “bottom-up approach”. The National Water Resources Council (NWRC) can play a roll in this process and help to establish policies and processes for coordination at the ministry and river basin level. The Council can also initiate developing of methodology/principles and guidelines for inter-

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ministry and basin level coordination and for the necessary policy and technical documents (Master Plans, Strategic Bank Erosion Plans, Resettlement Plans, Monitoring Plans, etc.). In respect to the financing of bank protection projects the reference can be made to the Decree 112 HDBT, 1984. This decree specifies that all organizations and individuals benefiting from irrigation, drainage and other hydraulic public services, have to pay a water fee to hydraulic companies/agencies. It is worthy to investigate the possibility of applying this regulation also to bank protection projects where local interest of people is directly present. 5.3 Socio-economic/environmental aspects of flooding, bank erosion and counter-measures Flooding and resulting bank erosion are a natural and recurrent phenomenon in the Mekong and Red River Delta. Especially in the Mekong Delta, it is a process, which drives the evolution of the delta plain and provides constraint to the human and economic development of the delta (AMRC, 2001). Due to the low elevation and relief of the delta plain, floods in the Mekong Delta are typically prolonged and aggravate the problem of poor drainage. Another socio-economic effect of flooding and poor drainage is an increased cost of infrastructure development and maintenance. For example, major roads need to be constructed on an embankment, and buildings on high foundations, mounds or stilts. Roads, which are submerged during the flood season, require frequent maintenance and the prolonged period during which they remain impassable hinders communication, trade and transportation. Damages due to flooding and erosion amount to tens of billions of Vietnamese dong (VND) per annum. However, not all socio-economic effects of flooding are adverse. Sediment deposition effected by floods plays an important role in rejuvenating soil over geological time scales. Although it is debatable whether the annual contribution of soil nutrients through flood-related sedimentation is sufficiently significant to improve crop growth, it is without doubt that overbank flooding and the associated sedimentation contribute to improved soil properties in the long-term, through the creation of higher, better-drained land (e.g. along levees), by flushing out accumulated toxins in the soil, and by counteracting unfavourable changes to the physical and chemical properties of the soil, e.g. in the absence of replenishment with new material, the soil may become compacted and partially reduced with age, hindering root growth and nutrient uptake and increasing the possibility of toxicity. Furthermore, the annual flooding brings increased opportunities for fisheries activities. Sedimentation and erosion also present a challenge to the human utilisation of the Mekong Delta waterways. The upper delta experiences very rapid rates of channel migration (banks erosion rates are commonly up to 20 m / year), caused by the lateral accretion of point-bars and mid-channel bars / islands, and the downstream migration of mid-channel bars. Mid- and lower delta channels are more stable (bank erosion rates are commonly 5-10 m / year), and channel change here is mainly caused by the slow accretion of elongated point-bars and mid-channel bars. Bank erosion is considered a serious socio-economic problem in the upper delta provinces of An Giang and Dong Thap provinces. Problems are especially severe at Tan Chau on the Mekong branch in An Giang, where erosion rates attain 30 m/year, and a large number of households have had to be relocated due to destruction of their dwellings through bank collapse. Bank

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erosion has resulted in major disruptions to local livelihoods, and financial burden on the provincial government (cost up to the present amounts to hundreds of billions of VND) by necessitating the relocation of inhabitants and localised bank protection works. Losses due to bank erosion appear to have increased in the last decade, probably due to the growing urban population and the resultant concentration of activity and capital along the waterfront. The severity of erosion at Tan Chau is largely attributable to the sharp meander-bend morphology, which focuses the river flow energy onto the concave bank (where the town is situated). The gradual downstream rotation of the point-bar on the opposite bank has resulted in a progressive downstream shift in the zone of erosion; stretches of river bank upstream of Tan Chau, which formerly experienced severe erosion are now experiencing bank accretion. Other erosion hotspots further downstream within An Giang (e.g. at Long Xuyen) are mostly associated with the downstream migration of mid-channel bars, which creates a shifting zone of erosion downstream and to the sides of the bar, and a zone of accretion to its upstream. High flow velocities in main rivers and canals contribute to bank erosion and enhanced transport of sediment in the main canals. Upon entering the smaller tertiary canals, much of this sediment is deposited due to an abrupt drop in flow velocities, adding to the cost of canal maintenance. Large-scale bank stabilization through hard engineering produces often undesirable side-effects such as rapid channel aggradation, and accelerated downstream erosion/sedimentation. Sedimentation, which accompanies bank erosion, also represents an economic cost in places, through the shoaling of navigation channels, the stranding of wharves, docks and other water transport infrastructure, and the blocking of entrances to canals. However, sedimentation in the main distributary channels is often regarded by many as an economic benefit, given the predominantly sandy nature of channel sediments, and the increasing demand for construction sand driven by urban expansion. Numerous sand dredging operations exist along most of the length of both the Mekong and the Bassac branches and Red River; an individual operation may extract volumes in the order of 104

m3 /year from the bed of the channels.

Many of the environmental problems resulting from large-scale infrastructure development interventions in the Mekong Delta may be viewed as a consequence of failure to recognise the delta as an environmental system. At a larger scale, the entire delta may be viewed as a component of the river catchment system. In this context, the delta is a sink and a transfer zone for matter derived from the more upstream parts of the catchment and transported downstream. However, this sediment formerly distributed over a large area of the delta plain is now accommodated in canals, which manifests itself in rapid siltation rates and high cost of maintenance of canals. Given the inherent role of deltas as sediment sinks, and the rapid rates of geomorphic processes driven by a large river discharge and sediment load, the deltas are the highly dynamic biophysical environmental systems (AMRC, 2001). As such, the deltas are in a constant state of evolution. Such environmental change is apparent at many different spatial scales, for example, a mid-channel bar undergoes accretion and downstream migration within a channel system, which evolves through channel shifts within the meander belt and occasional avulsions, and which itself is part of the expanding delta system. Trends in geomorphic evolution may be progressive, cyclical or episodic, and there is commonly a link between the spatial and temporal scales of evolution; namely, that small-scale geomorphic features, such as mid-channel bars, evolve over short time scales while the evolution of larger-scale features, such as the channel, and the encompassing delta system takes place over longer time scales. Analogous relationships

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may be observed in the biological environment; for instance, the time required for the establishment of a viable forest ecosystem far exceeds that required for the establishment of individual trees, which compose it. Environmental risk and impact assessments should be mandatory for all projects in these areas, which are likely to affect human health and well being, environmental quality or biodiversity (see also Annex). The “precautionary principle” should be one of the main guiding principles in preparing the assessments, and the assessments should be carried out before any such project is implemented. Economic incentives and cost-recovering fees should also be considered as environmental management tools. In situations where an EIA is deemed necessary, a checklist may be used as guidance for the EIA work and the form/content of the EIA Report. An example of such a checklist is given in Box 1, based on a format devised by FINNIDA in their draft “Guidelines for Environmental Impact Assessment in Development Assistance” (1989). 5.4 Master planning for bank erosion mitigation and river training 5.4.1 Need for a Master Plan and a long-term strategy for bank protection Many locations along the Mekong Delta and Red River in Vietnam experience increased bank erosion in recent years. The need for priority ranking of these locations and the potentially complicated morphological interactions require a Master Plan or an Integrated Strategic Plan to realize the most economic and efficient control of the bank erosion problem. The Mission noticed that no long-term strategy for bank erosion mitigation and river training has been developed for either the Mekong River in Vietnam or the alluvial part of the Red River. As a consequence bank protection works are implemented when and where the need is highest and at places where at that moment the river is eroding. The effect of this is clearest for the Red River. Although the Mission did only visit one side of the river (the other side is ”out of view” for the province the Mission was visiting), it is clear that the many implemented river bank protection works are creating a fixed river with a highly curved pattern. This planform may not necessarily be the best planform of the river in view of the sedimentation taking place and the as a consequence continuously rising flood levels. Possibly a straighter course might have advantages. The Mission holds the opinion that for both rivers a long-term strategy for bank protection and river fixation has to be elaborated at short notice. Bank protection works implemented in the coming years should fit into such a long-term strategy. The potential benefits of a Master Plan for bank protections might not be fully recognized yet, because along the Mekong River only a few bank protections are under development. It is expected that in due time the response of the river to bank protections will become noticeable (see also Csection 4.8), and at that time costly corrections might be the only option. Therefore the Mission recommends to prepare a Master Plan for the construction of bank protections along the rivers in the Mekong Delta and Red River Delta. For example the SARD expects that the protection in Tan Chau in future has to be extended in upstream direction.

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ENVIRONMENTAL IMPACT ASSESSMENT REPORTS FOR PROJECTS AFFECTING THE RIVER AND COASTAL ENVIRONMENT Checklist from “Guidelines for Environmental Impact Assessment in Development Assistance” (1989) I. Existing environment and site selection considerations • Assess whether there are possible alternative sites or locations which could be considered in project siting. • For each alternative site, whether there are, within or nearby, assess natural conditions and man-made activities. • Whether the project location might cause conflicts with the abovementioned land or resource uses, interests, values or communities. • Whether the project locations are affected by major natural hazards (e.g. floods, hurricane, volcanic activity). • The extent of existing development in the area and whether there are already significant environmental problems (e.g. water quality, bank/coastal erosion, habitat damage, over-fishing) in the project vicinity. • Any relevant human health/disease concerns in the project vicinity. • Whether the surrounding area can provide adequate supporting facilities. • Whether there are relevant environmental policy (including EIA) guidelines. • Whether there are any relevant planning/land use policy considerations (including coastal zone management plan, economic development zones). • Whether there is relevant international legislation (international waterways, etc.). II Site preparation and construction • Identify relevant site preparation and construction activities and components. • Identify and predict impacts on natural and socio-economic conditions. III. Project operation • Identify alternative design, manufacturing processes, raw materials, fuels, etc., which could be considered. • Identify for each alternative relevant activities/components. • Identify the extent to which discharges (particularly to marine/estuarine environments) may cross regional or national boundaries. • Identify and predict impacts on natural and socio-economic environment. IV. Mitigation and monitoring measures • Plan adequate mitigation of harmful impacts.

Box 1 Checklist for Environmental Impact Assessment from “Guidelines for Environmental Impact Assessment in Development Assistance” (1989)

In a final stage this can be large protection work guiding the river bend. The same holds for the revetment protection now under construction in Sa Dec. Already now a new budget is requested

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for extension works in 2004. A Master Plan considering also the response of the river can be a tool to prevent unpleasant financial surprises for the decision makers. The long-term strategy for the Mekong and the Red River will probably be different. Although the Mission was not in a position to do an extensive study of the best strategy for each of the rivers, it suggests that the best strategy is determined by the river characteristics on the one hand and socio-economic aspects on the other hand. For the Mekong River the long-term strategy could consist of: • stabilization of the flow distribution around the stable islands (in view of vested interest) • stabilization of islands • protection of important areas with bank protection works and upstream river training works For the Red River the preferred strategy could be: • a continuous bank protection along all outer bends • in due time narrowing of the river to induce bed degradation to counter-act the present

aggradational trend. A proper and appropriate long-term strategy for bank protection works and river training can only be developed when the river characteristics are properly understood and this calls for a in-depth study of the morphology of both rivers and other relevant aspects. Unless such a study is carried out and such a long-term strategy has been developed and is accepted by the population, the risk is present that investments in bank protection works in due time may turn out to be a less effective than initially anticipated. 5.4.2 Master plan as part of Integrated River Basin Planning and Management Planning and management of water resources needs to be carried out in an integrated manner, taking into account all water issues, needs and possible solutions in a balanced way. Effective planning can help to achieve coordination between sectoral water use, resolution of conflicts at an early stage, and coordination between water related aspects such as land use, wastewater discharge, etc. Planning is also an important part of the state management of water, as specified in the Law on Water Resources (LWR). Improved models and other decision support tools and planning procedures are needed, as are improved stakeholder consultation and involvement practices. An Integrated Water Management Plan should be developed and implemented in relation to all activities associated with the flood mitigation and associated problems of bank erosion. This plan should include: - details of specific regional items and problems, and general national philosophy/strategy - details of monitoring of impacts on physical resources, human and economic development and quality of life values; - details of measures to mitigate any anticipated negative impacts. This Integrated Water Management Plan should be supplemented by other documents, such as: Master Plan on Flood and Bank Erosion Mitigation, Environmental Management Plans, Resettlement Monitoring Programs and Action Plans, etc.. The development of appropriate Management Plans should ensure monitoring of problems and possible consequences, and the implementation of appropriate mitigation measures as required. It will enhance the country’s sustainable economic and social development and contribute to the growth of the regions.

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Finally it is mentioned here that the provincial authorities are also interested a Master plan and strategic plan as they require guarantees for the continuation of financing of (already planned and future) bank protection projects. 5.4.3 Elements of a Master plan A Master Plan for river training should deal with the following elements, varying from a high conceptual level to a detailed level of the bank protection structure: 1 Objectives 2 Priority for implementation 3 Floodplain zonation 4 Selection type of bank protection 5 Design method and design parameters 6 Pilot testing 7 Update Design Guidelines Standard An Integrated Plan for bank protections should take into account the long term, short term, socio-economic and environmental aspects, while not neglecting the importance of the rivers and canal system for inland navigation should be recommended. And an Integrated Plan should be a part of a Master Plan for the whole delta (i.e., for Tien River, Hau River in Mekong delta and for Red River). This Master Plan should include: water management, drinking water supply, floods, dikes, bank erosion, zoning along riverbanks (free space for natural developments), navigation, ecology, relocation of riparian people to other areas, legislation for living along riverbanks, etc. It was mentioned in discussions that some regulations on that aspect already exists but the Mission was not able to see these documents. Moreover, in the learning process of improving the actual approaches more attantion should be paid to such aspects as:

- design methods and design parameters, - pilot testing, - emergency actions and flood fighting, and - (update) Design Guidelines Standards

5.4.4 Some details on some components of Master plan for River training and Bank Protection Objectives In general the objective is to control the bank erosion along the branches of the Mekong River for the lowest costs. What is the more specific objective or what are the objectives? The answer on this question can be for example: - A river alignment with the shortest total length of bank protections or - A complete conservation of the present alignment or - Mainly a conservation of the present alignment, but with small corrections in case of islands;

one branch will be close gradually and the procentual discharge in remaining branch will increase to 100 %.

A consequence is that finally in due time almost all banks have a bank protection.

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Priority for implementation Which bank protection works can be implemented independent of each other? Distinguish a protection of a straight bank, an outer bend, an inner bend and a bifurcation point. A sequence will be selected for the implementation of the works to avoid unwanted morphological consequences. The presently applied criteria for priority of river bank protection works: I dike safety II urban areas/cities III (densely) populated villages IV agricultural land Often bank protection works are composed of combinations of different structural measures, such as:

- bank protection in case of big problems (revetments, series of groins, etc) - local measures (even by individual person) for small/local erosion problems - removing of sand bands which reduce the cross-section and shifting the current to one of

the banks; it should be considered to make an economic study on the quality of sand and and selling possibility as buiding material or for land reclamation purposes,

- light protections: vegetation, such as trees, planting along threatened banks, - temporary protections, for example by floating trees.

An example of a small correction in the river planform resulting in some land reclamation is shown in Figure 33. Maintenance is a general problem in Vietnam; no or little money for maintenance is available. Because of scarce of funds the maintenance is considered as spending money without direct profit (thus wasting money). A convincing strategy must be formulated in the Plan where the role of proper maintenance can be expressed in terms of saving money. Floodplain zonation Also Floodplain zonation can be an important instrument to regulate and direct the use of the floodplain and riverbanks. This measure should be included in the Master Plan of the River training and bank protection. Floodplain zonation includes both zonation of the floodplain proper and putting limits to land reclamation. At some places private owners extend their plot into the river by a small reclamation. Often this an illegal activity but it not corrected systematically by the authorities. To support the legal framework a smooth curve should be determined to indicate the maximum land reclamation. This curve should be communicated and discussed with all stakeholders. If land reclamation crosses this curve, a correction to remove this illegal part has a strong backing. This will became an important aspect as in future space in a river becomes more rare and the unit price of land will increase.

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Sketch 1 Present situation Sketch 2 Example of potential river training measures: 1 Stabilization of the bifurcation with bank protections 2 Create sedimentation in the small branch, for example by a series of groins Sketch 3 Final situation where 2 small islands are unified into 1 island

Figure 33 Possible effect of planform correction measures

2 1

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Selection type of bank protection Which the type of hydraulic structure should be applied and where? Examples of hydraulic structures are revetments, series of permeable or impermeable groins, bottom screens, combinations with vegetation and other measures. Other measures might include an investigation in the effect of the floating fish farms on the hydraulic load on the bank. The main advantages and disadvantages of types of bank protections are summarized in Table 3.

Type Advantages Disadvantages Revetment Narrow width of the

structure. Fixation of the bank line

Solid groins Flexible to future changes in bank line.

Strong current along the bank in the groin field. Wide zone of the bank required.

Impermeable groins Flexible to future changes in bank line. Good sedimentation downstream of groin.

Wide zone of the bank required.

Bed protection Flexible for future extension of the protection.

Only partial solution for bank erosion problem.

Vegetation Economic Effect varies in time. A spacious area is required

Table 3 Comparison of different protection types.

Design method and design parameters For a deterministic design of a bank protection the decisive flow velocity is an important design parameter. This parameter has been determined as the maximum measured flow velocity (in 1997). A statistical analysis and an extrapolation to the selected frequency of re-occurrence is a better method, which allows a differentiation of safety levels. However, this method can only be applied if sufficient field data are available. Therefore a regular monitoring should be set up of the erosion process and the hydrodynamic load on the bank at all important places. Above a revetment the flow velocity varies significantly. Therefore it is important to mention where the maximum flow velocity was measured and to determine a maximum velocity (including an estimate of extreme turbulence intensity) as a function of the location above the revetment or bank. The maximum hydraulic load on a bank is a combination of a maximum depth averaged flow velocity and turbulence intensities, maximum ship waves and flow velocities and maximum wind waves. The design method should provide a standard calculation method for the maximum hydraulic load. The slope of the revetment had been tested for its geo-technical static stability. However, its morphological effects on scour in front of the revetment could not be found in the design report. The shape of a bank protection has often many geometric parameters to be selected. Not for all those parameters exist design rules, therefore some are selected by expert judgement only. It is recommend to develop methods to find the optimal shape regarding the technical aspects and the total costs of the investment and maintenance. This can be a sensitivity analysis for a limited number of parameters (main parameters). Such methods do not exist for bank protections.

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The Mission recommends the comparison of more elaborated alternative designs for a bank protection to be sure that the optimum will be selected between costs and effectiveness. In general a revetment has an upstream termination and a downstream termination. The Mission recommends to add a downstream termination to the design of the revetment near Tan Chau and to strengthen the terminations of the revetment near Sa Dec. Pilot testing The present state of the selection of a certain type bank protection is guided by limited knowledge on the behaviour of the structure and limited experience as well. It is proposed to select of a number of pilot structures with several sections, which are different in top layers, slopes, falling aprons and materials. In a pilot revetment these structures should be tested in different sections. Their performance should be monitored during a few years. Finally the results should be evaluated in a cost analysis including initial costs and cost of maintenance. Emergency actions and flood fighting In addition to the structural measures the Master Plan should also contain non-structural measures:

- prediction and warning systems for extreme floods and of erosion including better transfer of information from studies carried out by Water Resources Institutes and others

- flood fighting and evacuation plans during emergengy situations. Flood fighting is concentrated in the North of Viet Nam. Its development in the Mekong delta had just started. Local people take emergency actions by themselves. A manual guiding these actions and the preparation of these actions will improve the effectiveness to save the banks. 5.5 Data and information management Long-term policies and management decisions need, as far as possible, to be based on facts collected, analysed and interpreted according to scientific criteria. Knowledge of the main causes and effects of floods, bank erosion, pollution and physical degradation is, in most cases, sufficient to provide reliable advice as to practical control measures. Nevertheless, further research is needed because existing databases and our understanding of the processes shaping the natural state of the water environment are generally inadequate for reliable predictions about changes and trends. The importance of monitoring “key” morphological and environmental indicators cannot be over-stressed. By appropriate monitoring, in combination with modeling techniques, it is possible to track progress in achieving technical,environmental and economic objectives and to learn from this experience the relative effectiveness of different approaches/measures. Water resources data and information management need to be improved in order to support the policy, planning and operational needs of improved integrated water resources management. This will include improved inventories and assessments of surface water, groundwater and water quality, river hydrograph and morphology, flood and bankerosion damage, improved accuracy and electronic management of data, and in particular, better sharing and dissemination of data and information. Coordinated data systems, planning and decision tools and public information procedures are needed. The need for and the components of data base for bank erosion are discussed in Section 4.8.

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The National Water Resources Council and its Office (ONWRC) can assist MARD to coordinate actions toward the development of a water sector information system at the national and/or a basin level. Implementation of such a system would likely be the responsibility of MARD and other key agencies. Local and National data base is needed The morphological studies of riverbanks are usually conducted by (Water Resources) Research Institutes (including Universities). However, statistical and economic data on flood and bank erosion damage is not yet incorporated in the activities of the planning institutes. Integrated Water Management Planning should include flood and riverbank damage, protection and maintenance, including cost-benefit considerations. These information’s can be used as input for selection and planning of mitigation measures, and the optimization of the use of available funds. Little (or no) attention is paid to the optimization of the cost of the particular project; more alternatives should be considered before final choice is made. More attention should be paid to preservation of natural vegetation and the use of vegetation as a protective measure against erosion. Monitoring, data base and evaluation The Mekong River and Red River are actively meandering and the main technical problem is from riverbank erosion undermining the flood protection dikes and other protection structures. Therefore estimates of past and future erosion rates should be an important part of the investigations for all flood protection and bank protection projects on the Mekong River and Red River Monitoring and Evaluation of the works completed are important for long term management of the flood defence system, for setting priorities for new works and adjusting Operation and Maintenance (O&M) requirements. The monitoring of important physical processes affecting the flood protection facilities must include: rates of river bank erosion along the dike and in particular immediately upstream and

downstream of river bank protection works; dike crest levels, width and slopes;

As stated before, river bank erosion is the process that most threatens the flood protection facilities. Without such erosion the dikes (also boulevards, roads) would only require routine maintenance, except when overtopped by rare flood events. If river bank erosion undercuts the dike during high river levels then flooding may occur through breaches in the protection. Erosion rates must be measured at locations, especially all along the dikes, so that the examination of the records will indicate priority areas which require action soonest and those which appear stable or at least require no action for several years. Beyond the simple use of the rate of erosion, it is recommended that specialists be periodically requested/hired for an interpretation and evaluation of the records. A specialist may perhaps separate out various influences such as unusual bank materials, perhaps changes to the natural river regime, for example those caused by local mining of sand and gravels from the river, unusually high drainage discharges through the river bank caused by irrigation facilities and most importantly, the large, powerful effects of river meanders. Inspection and monitoring of areas immediately upstream and downstream of river bank protection works are important as often the length of works is affected by available finance

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and it may be that erosion is continuing upstream and downstream and that the works may become isolated and fail from the ends. Dike crest levels should be surveyed at least every two years or more frequently in areas subjected to settlement and/or excessive vehicle use, for example when mined sands and gravels are transported. The crest levels should not be allowed to fall more than 0.2 m below design levels as this is the amount of freeboard allowed for dike wear and tear. Excessive wear and tear in particular locations should be investigated. In sheltered reaches or protected by natural vegetation, the freeboard allowance for waves can be reduced and if circumstances change, such as buildings demolished or trees cleared, and such areas become exposed to waves, then the dike should be raised accordingly. 5.6 Capacity building 5.6.1 Capacity building and cooperation Education and training is an area of identified priority for the Vietnam and for MARD. As part of the process of reform, modernization and integration with the international community, Vietnam and in particular MARD is keen to access formal training from developed countries such as Australia, Denmark, and Netherlands. In the scope of strengthening of higher education and training in Vietnam a number of specific programs already exists (i.e., Coastal Engineering at HWRU Hanoi). In this perception it is also desirable to upgrade the existing curriculums at Universities (i.e., HWRU) and to establish scholarship programs abroad for flood mitigation, river training, bank erosion and protection, environmental aspects, new technologies and other related items. Implementation of these items will contribute to the capacity building on the bank erosion mitigation and tackling of future problems, both at the national level and at the provincial level. 5.6.2 Staffing of Dike Department Where the future River Basin Organizations (RBO’s) are the institutions, which will deal with overall river basin development, the Dike Department (DDMFC) is the institution which should have and take the responsibility for the long-term strategy for river training and bank protection of both the Mekong Delta and the Red River. This should be done in close cooperation with the RBO’s. An important task for the Dike Department in this respect is the preparation of a yearbook on the development of the two rivers, on the basis of information provided by the respective provinces and local districts. In this yearbook the progress of the construction of the bank protection works along the two rivers should be reported upon together with the morphological response of the rivers. Water Resources Research Institutes should support the Dike Department in these activities. Another important task for the Dike Department is the maintenance of the data base as proposed in Section 4.8, which probably would be outsourced to e.g. the Water Resources Research Institutes. However, the Dike Department should give proper guidance what should be included in this data base nad ensure timely maintenance and upgrading. Finally the Dike Department could serve as central point for the collection of experiences with the implementation of the bank protection works by the provinces. Possibly a regular meeting

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(e.g. twice a year) could be arranged where the officials of the different design offices of the provinces along the Mekong and the Red River exchange their experiences with planning and implementation of bank protection works and the morphological response of the rivers. This could be the basis of the yearbooks. For these central activities some additional specialized staff may be needed. If so, the Mission very much favours such an extension of the staff of the Dike Department, as it feels that the Dike Department must be allowed to play this central role. 5.6.3 Need for training of provincial staff The Mission has visited a number of provinces and has had discussions with a number of officials and staff of provincial engineering bureaus. Although in general the Mission is impressed by the dedication of the staff involved, at the same time the Mission feels that the design work is done on the basis of a number of standard (somewhat outdated) guidelines. There is limited scope for original solutions based on special local conditions. Moreover there is limited feeling for the special aspects of implementing bank erosion works in large and dynamic morphological systems like the Mekong and the Red River. For this reason the Mission proposes to prepare and implement a special training course for the different engineers involved in bank protection works along the Mekong and the Red River, where the experience gained by the individual members of the Mission in The Netherlands and abroad is transferred to the Vietnamese staff of the provincial engineering firms. This course should be designed in such a way that the experience presently gained by the provincial engineering firms is mobilized as well and a proper exchange of experiences is achieved. 5.6.4 Use of models During their field visits and during the visits to the Water Resources Institutes and the University both in Ho Chi Minh City and Hanoi several times the use of models as support for bank protection studies was mentioned. In principle two types of models can be used in bank protection studies, notably: • physical models, both fixed bed and mobile-bed; • mathematical models allowing to study flow, sediment transport, morphological changes,

bank erosion. The Mission favours the use of models, but at the same time wants to make a number of critical remarks on the use of models (see also Sections 4.3.4 and 4.3.5): • Physical models subject to scale effects (e.g. scour depth not on scale in distorted mobile

bed models which simulates the overall morphological behaviour correct) • It is an illusion to think that bank erosion can be studied in a mobile bed model: modelling

of bank erosion is at present (and probably also in the far future) not possible in a quantitative way.

• Physical models can be used to study local scour near bank protection works. Until now this cannot be studied with mathematical models.

• Mathematical models require schematisations and proper (bank erosion) equations. Bank erosion predictors are not developed up to the level that with mathematical models bank erosion can be predicted accurately.

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• Nowadays mathematical models can be used advantageously to simulate morphological changes, but they need proper calibration and verification.

• In large projects often a combination of models is used (e.g. a mathematical model for a long river reach and a physical model for details like local scour). This is called a hybrid modelling approach.

• Models need prototype data for calibration and verification. Models are as good as the field data available. This implies that much more field data before and after the construction of bank protection works are needed.

The present situation in Vietnam regarding modelling is the following. Within Vietnam some knowledge on physical modelling is available, but in view of the Mission this knowledge should be updated on the basis e.g. of the extensive experience with physical modelling for bank protection works in Bangladesh and the improved insight into scale effects obtained over the last two decades (Struiksma, 1983; Struiksma & Klaassen, 1987; Haque, 1999). A special course on possibilities and limitations of physical models seems appropriate. Within a support project for the Water Sector in Vietnam, DANIDA is funding the implementation of MIKE 21C in both Water Resources Research Institutes. This implementation includes the following modules of MIKE 21C: flow, sediment transport, morphological changes and bank erosion. A critical attitude towards these models will be required. These models are as good as the understanding of the relevant processes is, and as far as bank erosion is concerned this understanding is quite limited. The maximum these models can provide is an indication of where bank erosion might occur. Predictions based on field data (see Chapter 4) might for the time being be the better alternative. The Mission assumes that as part of the implementation sufficient training in the use of MIKE 21C will be given. This training should be sufficiently critical. 5.6.5 Upgrading university curriculae Research capabilities at the two Water Research Institutes are limited. The same holds for the funding of research into the characteristics of the two main rivers and the morphological response of the rivers to bank protection and river training measures, although the funding of Ministry of Science of a study into bank erosion in Vietnam is a very promising sign. Moreover the curriculum of the Hanoi Water Resources University should be extended to include also riverine flooding and bank erosion problems, plus different approaches for flood management and bank defence and protection. Within this frame-work the Mission favours the involvement of students from the Hanoi Water Resources Universities into the different studies into the studies into the characteristics of the two main rivers and the morphological response of the rivers to bank protection and river training measures. BSc and MSc student can carry out research into the field of bank protection works and river training at low costs, and the WRI’s could provide some funding for the students as an incentive. These studies should also include field studies to study the response of the river system to bank protection works. Simple measuring methods should be developed to monitor morphological changes. For the analysis of the data use can be made of GIS.

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6 Proposed Action Plan 2004-2007 Based on the findings of the Mission as described in the previous Chapters 3 through 5, it is possible to identify actions for the coming years. Different actions are needed ranging from pure technical studies to capacity building and reinforcement of university curriculae. For these different actions funding is required. One possibility is either from Vietnam itself or from the Netherlands Public Works Department (RWS) within the framework of the twinning agreement between MARD/DDMFC and RWS. However, these funds are limited. Other possibilities are to interest international donors to provide either technical assistance or loans. Under such arrangements the funding might be more substantial. RWS and some members of the Mission will assist MARD in the preparation of proposals, which depending on the interest of the potential donor could cover a number of proposed actions listed hereafter. The following actions were identified: 1. Project proposal on Capacity Building for flood and riverbank erosion mitigation: Intended

as a support project for the Dike Department of MARD and the different provincial design offices; could include some of the activities listed below; proposal should be ready mid 2004 and subsequently donors should be approached.

2. Number of MSc studies at UNESCO-IHE Delft; education in the field of flood control, river bank erosion and protection and river training for some engineers from Water Resources Research Institutes and possibly provincial design offices; funding either NFP (Netherlands Fellowship Programme) or RWS (start in September 2004 and September 2005, total duration of these MSc studies about 1.5 year);

3. Organizing of a Workshop/Short course on Erosion Prediction and Bank Protection, in combination with presentation of the final results from the National Program: Program KC. 08: Enviroment and Natural Disaster Prevention; project kc.08.15; Project title: Research on the causes and the solutions to prevent riverbank erosion and deposition for the Lower Mekong Delta River System (LMDRS); funding by RWS (proposed for November 2004);

4. Follow-up for National Bank Erosion Study Program for Mekong and Red Rivers, with additional funding from Vietnam Ministry of Science and possibly MARD; to be carried out by the two Water Resources Institutes (after 2004).

5. Vietnamese Working Group (supplemented with Dutch experts) on Upgrading Design Standards for bank protection; This working group will collect recent findings on bank protection works and relevant literature and apply these to the Vietnamese conditions, and monitor the behaviour of some selected structures along the Mekong and Red River; if budget allows some test structures can be constructed and monitored as well; time frame of this activity could be a start in 2005 (provided funds are available) but it will probably continue for a number of years concerning the updating.

6. Vietnamese Working Group (supplemented with Dutch experts) on Strategy of Bank Erosion Mitigation: This working group will collect recent findings on river training master planning and relevant literature and apply these to the Vietnamese conditions; project could be part of a capacity building project for the Dike Department supplemented by staff from the Water Resources Institutes and staff from provincial design offices to bring in field experience with river response to bank protection works; the final product of these (2005-2006)

7. Setting up of data bases for bank erosion processes and bank protection works: these data bases are discussed extensively in the Chapters 4 and 5; setting up of data bases should be precluded by a phase in which (in close cooperation with the potential users: DDMFC, MARD, Water Resources Institutes, provincial design offices and HWRU and with suppliers

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of data like Hydro-Meteorological Service) a set-up for the data base(s) is made; subsequently the data bases should be filled

8. Capacity building project for Dike Department of MARD; the following components have been identified for such a capacity building project on the basis of learning by doing: Capacity building staff DD and provincal engineering bureau’s via learning by doing Improvement design manuals and standards Improved legislation Setting-up data bases (probably 2 types: one national, and another more technical per

river?) Improvement cooperation DDMFC – provincial design bureau’s Master plan and long term strategy for river training Mekong River, Red River and

Central Vietnam Rivers, in cooperation with RBO’s Improved monitoring capability Publication of yearbooks with progress and revolving yearly planning bank protection

works Training (two types: training courses in Vietnam for larger audience and MSc studies of

some students at UNESCO-IHE in NL) Exposure tours (to NL RWS/Delft Hydraulics/GeoDelft and to Bangladesh (CEGIS for

remote sensing and set-up data bases) Some of these activities are already mentioned in the above points 5 through 7, but it would be advantageously to combine them in one capacity building project; supposing that funding can be assured at short notice, such a capacity building project could start mid 2005 and last about 2 years (until mid 2007).

9. Proposals for extending of Curriculum at HWRU-CE; curriculum to be extended with riverbank erosion and mitigation and possibly more general with integrated water resources planning and management, from which bank erosion and mitigation and river training are an integral part; this activity is foreseen as the second phase of the HWRU-Delft University of Technology-UNESCO-IHE project, which start in 2005 and may last a number of years; probably funding available through The Netherlands Embassy;

10. Second short course on bank erosion mitigation: system approach/methodology, prediction techniques, planning, design and monitoring, based on achievements of the different projects mentioned above (2007);

11. Implementation and evaluation plan (after mid 2007), including proposals for upgrading legal documents and regulations.

Table 4 provides a tentative time table for the different activities. It should be realized however that this time table depends heavily upon acquiring funding for the different activities.

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Year

2004 2005 2006 2007 Action

Possible funding 3 4 1 2 3 4 1 2 3 4 1 2 3 4

Remarks

Proposal for capacity building DD-MARD X

MSc studies at IHE NFP/RWS X X X X X X X X X X X

RWS-MARD-SWRI-Workshop RWS X

Research study Vietnam X X X X X X X X

WG Design of bank protection works donor/RWS x x x x x x x x x x Continued?

WG Master plan donor/RWS x x x x

Setting up of data bases donor X X X X X X X X X X Continued

Capacity building DD-MARD donor X X X X X X X X

Upgrading HWRU Netherlands X X X X X X X X X X

2nd workshop donor/RWS X? Implementation of recommendations and adaptation legislation No funding X X Continued

Table 4 Proposed time schedule for identified actions in the fields of capacity building in bank

protection and river training in Vietnam

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7 Conclusions and recommendations Based on a visit of two weeks to the Mekong River and Red River in Vietnam and a number of meetings with a.o. Vietnamese officials both from MARD and the provinces plus visits to the two Water Resources Institutes and the Hanoi Water Resources Universities in Hanoi and Ho Chi Min City, a good view on bank erosion problems in Vietnam and bank protection projects (either implemented or planned) was obtained. The overall impression of the Mission is that there are bank erosion problems at many places in Vietnam, whereas only limited funds are available to tackle these problems. The actual management of the river system and in particular how bank erosion problems are handled resembles more emergency (disaster) management rather than a planned management with a clear view on how the rivers should be developed in future. Nevertheless the Mission holds the opinion that DDMFC and the provinces under these very difficult conditions and with limited possibilities are doing good and responsible work and are providing important products for the developing Vietnamese society, notably an environment that allows the further development of the country. However, as discussed in the preceding chapters, still improvements in approach and working methods are possible to optimize the use of the limited funds. More general it holds that it is important to develop the riverine resources in a planned way because the experience from many more developed river systems shows that mistakes in river management (in particular as far as spatial planning is concerned) are usually very difficult to rectify later on. Detailed observations and suggestions are given in the preceding Chapters 3 through 5. Hereafter only the most important conclusions and recommendations, varying from institutional to quite technical, are summarized: The legal framework for bank protection works, floodplain zonation and river training

works, and the funding and prioritizing criteria needs more clarity and transparency There is a clear need for a Master Plan for the main rivers (Mekong and Red River), which

should result in a long-term strategy for bank protection At the Ministry level and possibly also at the provincial level a number of improvements are

needed to overcome some of the problems experienced: - (often) too limited number of technical staff - (sometimes) there is a need for training - create an open atmosphere in the offices, whereby also the ideas from staff lower in the

organization (but sometimes nearer to the problems and/or better trained) are welcomed - arrange for (better) technical support by Water Resources institutes - some improvement of capabilities of Institutes and co-operation (a.o., sharing

tools/models) is still needed - better support needed for designs and for doing cost-benefit studies as a basis for

prioritization More international (UNESCO-IHE) education and international exposure, on the basis of

Vietnamese experiences, data bases and better monitoring Learn from international experience, which can be either positive and negative (i.e. possible

application of permeable groins, floating screens combined with e.g. fish farming, environmental friendly protection) Decide on best technical approaches on the basis of the comparison of alternatives

(functioning and costs) in stead of using always the same technical solution A study into the effect of partly completed projects and the subsequent response of the river

is needed to draw up better plans

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There is scope for the improvement of surveying techniques and data processing More integrated approach in the planning and the designs of bank protection works, and

attention to visual aspects of bank protection works, in particular in urban area Continue to investigate and evaluate the human impacts on riverbank erosion (e.g. influence

of sand mining) and try to reduce this impact Need for studies of the long term morphological behaviour of the rivers (in the past)

including predictions of future developments with or without major interventions in the river systems Continue to develop and improve capabilities for forecasting, early warning and mitigation

of the erosion on riverbanks along the Vietnamese river systems. Establish a co-operation platform on river bank erosion problems (periodic consultations of

all agencies working in this field) There is a need for better monitoring of eroding riverbanks and yearly reporting on bank

erosion and progress of bank protection works (including local and/or provincial Data-base) Better prediction of bank erosion rates and scour hole development, also as affected by bank

protection structures Better insight in erosion processes including geotechnical aspects Numerical modeling of river morphology needs further development and can become an

important tool for prediction of erosion More clarity in non-structural measures and procedures (early warning, evacuation plans,

compensation, etc.) and insight in possibilities and limitations Improvement of design rules for structural measures (traditional versus advanced design

methods, including the use of vegetation) There is a need for improvement of Design and Maintenance Manuals (including

alternatives) and Guides for Environmental Impact Assessment of bank protection projects Maintenance aspects should be included in the design

Most of these recommendations were considered, either explicitly or implicitly, in the formulation of the Action Plan for the coming years as presented in Chapter 6.

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References AMRC (2001), Environmental Issues and Recent Infrastructure Development in the Mekong Delta: review, analysis and recommendations with particular reference to largescale water control projects and the development of coastal areas; Takehiko ‘Riko’ Hashimoto, Working Paper No. 4, Australian Mekong Resource Centre, University of Sydney, June 20. Ariathurai, R. & K. Arulanandan (1978), Erosion rates of cohesive soils, Journ. Hydr. Div., ASCE, Vol.104, No.HY2, pp.279-283. Bristow, C.S. (1987), Brahmaputra River: channel migration and deposition, In: Recent developments in fluvial sedimentology, Eds. F.G. Ethridge, R.M. Flores & M.D. Harvey, Soc. Economic Paleontol. and Mineral., Spec. Publ. No.39, pp. 63-74. Coleman, J.M. (1969), Brahmaputra River: channel processes and sedimentation, Sedimentary Geol., Vol.3, Nos.2-3, pp.129-239. Crosato, A. (1990), Simulation of meandering river processes, Communications on Hydr. and Geotech. Engrg., No.90-3, Delft Univ. of Technology, ISSN 0169-6548. DHI & SWMC (1996), Mathematical morphological model of Jamuna River; Jamuna Bridge Site. First forecast report for Government of Bangladesh, World Bank & Jamuna Multipurpose Bridge Authority, Danish Hydraulics Institute & Surface Water Modelling Centre. Experco (1994), Preliminary study Hanoi dyke rehabilitation sub-project, Irrigation and Flood Protection Rehabilitation Project, Volume 1. Hickin, E.J. & Nanson, G.C. (1984), Lateral migration rates of river bends, Journ. Hyd. Eng. ASCE, Vol. 110, no. 11, pp. 1557-1567. Hoffmans, G.J.C.M. and Verheij, H.J. (1997), Scour manual, Balkema, Rotterdam, 205 pp. Jagers, H.R.A. (2003), Modelling planform changes of braided rivers, Twente University, Ph.D. Thesis, 318 p., fig., tab., ref. + cd-rom , ISBN 90-9016879-6. Jansen, P.Ph., van der Berg, J., de Vries, M. & Zanen, (…) (Ed.), Principles of river engineering. The non-tidal alluvial river, London, Pitman Publ. Cy. (also 1994 reprint, DUT). Klaassen, G.J. & Masselink, G. (1992), Planform changes of a braided river with fine sand as bed and bank material, Proc. 5th Intern. Symp. on River Sedimentation, Karlsruhe, Germany, pp. …- … (13 pages). Klaassen, G.J., E. Mosselman & H. Bruehl (1993), On the prediction of planform changes in braided sand-bed rivers, In: Wang, S.S.Y. (Ed.), Adv. in Hydro-Sci. and -Engrg., Ed.), Publ. Univ. Mississippi, pp.134-146. Le Manh Hung & Dinh Cong San (2002), Xoi lo bo song cuu long (Bank erosion Mekong River), Southern Institute for Water Resources.

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Mosselman, E. (1992), Mathematical modelling of morphological processes in rivers with erodible cohesive banks, Communications on Hydr. and Geotech. Engrg., No.92-3, Delft Univ. of Technology, ISSN 0169-6548. Ngaonh, M.T. and Akira, Y. (2003?), River bank erosion along in the Mekong Delta, Origin unknown (Internet). Nguyen Tuan Anh & Tran Xuan Thai (2000), Some problems on river bed and flood passage of the Red River, International European-Asian Workshop Ecosystem & Flood 2000, Hanoi, Vietnam, 7 pages. Noortwijk, J.M. van (1996), Optimal maintenance decisions for hydraulic structures under isotropic deterioration, Delft, Delft University of Technology, Ph.D. thesis. Noortwijk, J.M., Cooke, R.M. & Kok, M. (1995), A Bayesian failure model based on isotropic deterioration, European Journal of Operational Research, Vol. 82, No. 2, pp. 270-282. Olesen, K.W. & Tjerry, S. (2003), Morphological modelling of the Chaktomuk junction. Pilarczyk, K.W. (1998), Dikes and revetments, A.A. Balkema, Rotterdam. Pilarczyk, K.W. & Sy Nuoi (2002), Experience and practice on flood control in Vietnam, 2nd Intern. Symposium on Flood Defences, pp. 774-785. Przedwojski, B., Blazejewski, R and Pilarczyk, K.W. (1995), River training techniques: fundamentals, design and applications, A.A. Balkema, Rotterdam/Brookfield. Sarker, M.H. & Khayer, Y. (2002), Developing and updating empirical methods for predicting morphological changes in the Jamuna River, EGIS, Dhaka, EGIS Technical Note Series 29. Shishikura, T. (1996), Morphological changes due to river bank protection, IHE Delft/Delft Hydraulics, M.Sc. thesis no. 285. Stoutjesdijk, T.P., De Groot, M.B. and Lindenberg, J (1994), Engineering Approach to Coastal Flow Slides, Proceedings Int. Conf. Coastal Eng., 1994 (ICCE'94),Kobe, Japan. New York, ASCE, pp 3350-3359. Stoutjesdijk, T.P., De Groot, M.B. and Lindenberg, J.(1998), Flow slide prediction method: influence of slope geometry, Canadian Geotechnical Journal 35, pp. 43-54. US Army (1981), Streambank Erosion Control Evaluation and Demonstration (Main Report), US Army Corps of Engineers, Final Report to Congress, 1981.

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Appendices

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Appendix I: Mission participants Mission participants from the Netherlands 1. Krystian W. Pilarczyk: Bank and dike protection expert, DWW 2. Maarten van der Wal: River engineering expert, DWW 3. Ruud (Marinus Cornelis Johannes) Bosters: Hydraulic engineering and hydrology expert,

DWW 4. Jaap (Jacob) Lindenberg: Geotechnical expert, GeoDelft 5. Gerrit J. Klaassen: River engineering and river morphology expert, UNESCO-IHE Mission participants from Vietnam 6. Nguyen Huu Phuc: Chief of Master planning division of DDMFC 7. Nguyen Huy Dzung: Hydraulic engineering expert, DDMFC 8. Nguyen Si Nuoi: Vice Director of DDMFC (only Red River mission) 9. Do Ngoc Thien: Vice Director of DDMFC (only Mekong mission) 10. Dang Quang Thanh: Hydraulic engineer, DDMFC (only Mekong mission) Adresses 1. DWW: Ministry of Transport, Public Works and Water Management; Directorate-General of

Road Public Works and Water Management; Road and Hydraulic Engineering Institute: Van der Burghweg 1; P.O. Box 5044; NL-2600 GA Delft. Tel. ++.31.15.2518437, Fax ++.31.15.2518555

2. GeoDelft: Stieltjesweg 2; P.O. Box 69; NL-2600 AB Delft 3. UNESCO-IHE: Westvest 7; P.O. Box 3015; NL-2601 DA Delft. Tel. ++.31.15.2151715,

Fax ++.31.15.2122921 4. DDMFC: Ministry of Agriculture and Rural Development (MARD); Department of Dike

Management and Flood Control: A4 Building; 2 Ngoc Ha Street; Ba Dinh; Hanoi; Vietnam. Tel. (84-4)7335690, Fax (84-4)7335701

Abbreviations DWW: Road and Hydraulic Engineering Institute UNESCO-IHE: Institute of Water Education MARD: Ministry of Agriculture and Rural Development DDMFC: Department of Dike Management and Flood Control

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Appendix II: Contacts in Vietnam Hanoi Water Resources University, Second Base, Ho Chi Minh City 1. Dr. Duong Van Vien: Vice Director 2. Nguyen Van Dien: Expert 3. Nguyen Van Thiet: Expert 4. Miss Vu Hoang Anh: Teacher 5. Miss Mai: Teacher Sub Institute of Geography, Ho Chi Minh City 6. Prof. Nguyen Sinh Huy: Expert on hydraulics 7. Miss Nguyen Thi Hanh: Secretary of Prof. Nguyen Sinh Huy Sub-Institute for Water Resources Planning, Ho Chi Minh City 8. To Van Truong: Director 9. Nguyen Ngoc Anh: Deputy Director 10. Nguyen Xuan Hien: Deputy Director of SIWRP 11. Tran Minh Khoi: Vice Head of Center for Water Quality 12. John B. Cantor: Consultant for AusAID 13. Brian Cummings: Consultant for AusAID Southern Institute of Water Resources Research (SIWRR), Ho Chi Minh City 14. Dr. Le Manh Hung: Director of Center for River Training and natural Disaster Prevention 15. Ass.prof. Dr. Hoang Van Huan: Deputy Director of SIWRR 16. Dr. Tang Duc Thang: Deputy Director of SIWRR 17. Nguyen The Bien: Deputy Chief of Department 18. Dinh Cong San: Deputy Director of Center for River Training & Natural Disaster Prevention Service of Agriculture and Rural Development of An Giang Province (SARD), Long Xuyen 19. Do Van Hung: Vice Deputy 20. Pham Van Le: Director of AN Giang Provincial Department of Water Resources 21. Vuong Huu Tien Engineering Company for SARD of An Giang Province, Long Xuyen 22. Mai Anh Vu: Expert of Consultant Company for Rural Development 23. Tran Huu Ha: Expert of Consultant Company for Rural Development People's Committee of Tan Chau 24. Do Thanh Trung: Vice chairman of People Committee of Chau District Service of Agriculture and Rural Development of Dong Thap, Cao Lanh 25. Huynh The Phien: Vice Director 26. Nguyen Chap Kinh; Deputy Director of Dong Thap Provincial Department of Water

Resources

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Service of Agriculture and Rural Development of Vinh Long, Vinh Long Town 27. Phan Nhat Ai: Director of SARD 28. Ha Thanh Thang: Head of technical department Sub-Department of Dike Management and Flood Control of Vinh Long Province 29. Nguyen Van Thanh: Director Engineering Company for SARD of Vinh Long Province 30. Nguyen Trong Minh: Deputy Director of Design Consultant Company Of Vinh Long

Province 31. Tran Quoc Hoai: Designer Service of Agriculture and Rural Development of Ha Tay 32. Luu Van Hai: Vice Director Sub-Department of Dike Management and Flood Control of Ha Tay Province 33. Vu Xuan Phieu: Deputy Director of Sub-Department 34. Nguyen Tuan Khai Service of Agriculture and Rural Development of Hung Yen 35. Bui Xuan Bai: Vice Director Sub-Department of Dike Management and Flood Control of Hung Yen Province 36. Doan Quang Viet: Director 37. Ho Trong Khai: Deputy Director 38. Hoang Xuan Vang: Deputy Director of DDMFC and Head of Hydraulic Construction

Project Management 39. Tran Van Bui: Hydraulic Contruction Project Management 40. Vu Van Hanh: Director of Consultant Company Vietnam Institute for Water Resources Research 41. Ass.Prof.Dr. Tran Dinh Hoi: Deputy Director 42. Ass.Prof.Dr. Tran Xuan Thai: Director of River Engineering Research Centre 43. Nguyen Ngoc Quynh: Vice Director of River Engineering Research Centre 44. Miss Dao Thu Trang: International Cooperation Section 45. Ho Viet Cuong: River Engineering Research Centre 46. Nguyen Dang Giap Hanoi Water Resources University, Hanoi 47. Prof. Dr. Le Kim Truyen: Rector of HRWU 48. Ass.Prof. Dr. Vu Minh Cat: Head of Division of Science and International Co-operation 49. Tran Thanh Tung: Lecturer on river and coastal engineering 50. Ass.Prof. Dr. Le Dinh Thanh: Head of Coastal Engineering Faculty 51. Dr. Le Xuan Roanh: Secrectary of Coastal Engineering Faculty 52. Ass. Prof. Dr. Vu Thanh Te: Hydraulic Engineering Faculty 53. Ass. Prof. Dr. Do Tat Tuc: Head of Hydrology and Environmental Faculty

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Adresses 1. Hanoi Water Resources University (HWRU), Second Base; 2 Truong Sa; Binh Thanh

District; Ho Chi Minh City. Tel. (08)8404743, Fax (08)8400542 2. Sub Institute of Geography: 1 Mac Dinh Chi Street; District 1; Ho Chi Minh City. Tel.

(08)8234347 3. Sub-Institute for Water Resources Planning (SIWRP): 253A, An Duong Vuong; District 5;

Ho Chi Minh City. Tel. (84-8)8350850, Fax (84-8)8351721 4. Southern Institute of Water Resources Research (SIWRR): 2A Nguyen Bieu St.; District 5;

Ho Chi Minh City. Tel. (08)8380990 (Dinh Cong San), (08)8362821 (Nguyen The Bien), Fax (08)9235028

5. Hanoi Water Resources University (HWRU): 175 Tay Son Street; Dong Da District; Hanoi; Vietnam

6. Service of Agriculture and Rural Development (SARD) of An Giang Province: 4 Nguyen Du Street; My Binh Ward - Long Xuyen City. Tel. (076)853257, Fax (076)856705

7. Engineering Company for SARD of An Giang Province: 2-3 Le Hong Phong; Long Xuyen city; An Giang Province.

8. People's Committee of Tan Chau: Van Phong HDND - UBND Huyen Tan Chau, An Giang: Duong 1/5 Thi Tran Tan Chau. Tel. (076)822201

9. Service of Agriculture and Rural Development of Dong Thap Province: 154 Quoc Lo 30; Xa My Tan - TX. Cao Lanh. Tel. (067)852532, Fax (067)853514

10. Department of Water Resources Dong Thap Province: 154 Quoc Lo 30; Xa My Tan - TX. Cao Lanh. Tel. (067)851092, Fax (067)853028

11. SARD of Vinh Long: 107/2. Pham Hung; P. 9; TX. Vinh Long. Tel. (070)830981, Fax (070)827635

12. Engineering Company for SARD of Vinh Long Province: 107/2. Pham Hung; P. 9; TX. Vinh Long. Tel. (070)830981, Fax (070)827635

13. Sub-Department of Dike Management and Flood Control of Vinh Long Province: 107/2. Pham Hung; P. 9; TX. Vinh Long. Tel. (070)830981, Fax (070)827635

14. SARD of Ha Tay: 15. Sub-DDMFC of Ha Tay Province: 16. SARD of Hung Yen: 17. Sub-DDMFC of Hung Yen Province: 18. Vietnam Institute for Water Resources Research: 171 Tay Son; Dong Da; Hanoi. Tel. (84-

4)8523766, Fax (84-4)5634809 19. River Engineering Research Centre: 299 Tay Son; Dong Da; Hanoi. Tel. (84-4)8536524,

Fax (84-4)5634478 20. Hanoi Water Resources University: 175, Tay Son Street; Dong Da; Hanoi. Tel.

(84-4)8533083, Fax (84-4)8534198

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Appendix III: Mission program and schedule Schedule for mission visit to Mekong Delta Date Meeting/location Program 24 HWRU, 8:30-9:00 Introduction Nov. Second Base, 9:00-10:00 Presentation on Mekong River bank erosion 2003 Ho Chi Minh City 10:00-11:30 Questions and discussion 12:00-13:30 Lunch SIWRP, 13:30-14:00 Introduction Ho Chi Minh City 14:00-15:30 Presentation on Mekong River bank erosion 15:30-16:30 Questions and discussion Overnight in Ho Chi Minh City 25 SWRRI, 8:30-9:00 Introduction Nov. Ho Chi Minh City 9:00-10:00 Presentation on Mekong River bank erosion 2003 10:00-11:30 Questions and discussion 12:00-13:30 Lunch Thanh Da Island 13:30-16:30 Bank erosion field visit to Sai Gon River Overnight in Ho Chi Minh City 26 (Travelling) 7:00-11:30 By car to An Giang province (260 km) Nov. 11:30-12:00 Check-in at hotel in Long Xuyen 2003 12:00-13:30 Lunch SARD of An Giang, 13:30-14:30 Introduction and short briefing Long Xuyen 14:30-16:30 Questions and discussion Overnight in Long Xuyen 27 Tan Chau 8:30-11:30 Bank erosion field visits by boat Nov. 12:00-13:00 Lunch 2003 Long Xuyen 13:00-18:30 Visits to revetment and bank erosion sites Overnight in Long Xuyen 28 (Travelling) 8:00-11:00 By car to Cao Lanh in Dong Thap province (50 km) Nov. 11:00-11:30 Check-in at hotel 2003 12:00-13:30 Lunch SARD of Dong 14:00-15:30 Introduction and short briefing Thap, Cao Lanh 15:30-16:30 Questions and discussion Overnight in Cao Lanh 29 Sa Dec 8:30-10:30 Visit to Sa Dec revetment system (50 km) Nov. (Travelling) 10:30-11:30 By car to Vinh Long province (30 km) 2003 11:30-12:00 Check-in at Cuu Long Hotel in Vinh Long town 12:00-13:30 Lunch SARD of Vinh Long, 13:30-14:00 Introduction and short briefing Vinh Long town 14:00-15:30 Questions and discussion Vinh Long town 15:30-17:00 Visit to revetment system and bank erosion sites Overnight in Vinh Long town 30 (Travelling) 8:30-11:30 By car to Ho Chi Minh city (140 km) Nov. 11:30-12:00 Hotel registration 2003 12:00-13:30 Lunch Afternoon Free time Overnight in Ho Chi Minh City 1 SIWRP, 9:00-12:00 Evaluation of meetings and field visits Dec. Ho Chi Minh City 2003 12:00-13:30 Lunch (Travelling) 17:00-19:00 By plane to Hanoi Overnight in Hanoi

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Schedule for mission visit to Red River Delta Date Meeting/location Program 2 (Travelling) 7:30-8:30 By car from Hanoi to Ha Tay province Dec. SARD of Ha Tay 8:30-9:30: Meeting with SARD 2003 Ha Tay Province 9:30-14:00 Visits to revetments and bank erosion sites 14:00-15:00 Lunch Overnight in Hanoi 3 (Travelling) 7:30-9:30 By car to Hung Yen province (60 km) Dec. SARD of Hung Yen 9:30-10:30 Meeting with SARD 2003 Hung Yen town 10:30-11:30 Field visit to revetment 11:30-13:00 Lunch time Hung Yen Province 13:30-16:30 Visits to revetments and bank erosion sites Overnight in Hanoi 4 VIWRR, Hanoi 8:00-11:30 Visit Vietnam VIWRR Dec. 12:00-13:30 Lunch 2003 HWRU, Hanoi 13:30-16:30 Visit HWRU Overnight in Hanoi 5 DDMFC, Hanoi 8:30-11:30 Presentation of mission results for DDMFC Dec. 12.00:13:30 Lunch 2003 Hanoi Afternoon Free time 19.00 Farewell party (DDMFC, ICD, PI, WRRI and HWRU of MARD; representative of RNE) Overnight in Hanoi 6 Hanoi Day Free time Dec. 20:00 Departure to Amsterdam End of mission Abbreviations HWRU: Hanoi Water Resources University SIWRP: Sub-Institute for Water Resources Planning SIWRR: Southern Institute of Water Resources Research SARD: Service of Agriculture and Rural Development DDMFC: Department of Dike Management and Flood Control VIWRR: Vietnam Institute for Water Resources Research

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Appendix IV: Field visits Mekong River Field visit 1: Thanh Da Island, Sai Gon River, Ho Chi Minh City, 25 November, 14:00 Participant from Southern Institute of Water Resources Research 1. Nguyen The Bien Report The visit consists of a boat trip around Thanh Da Island. Thanh Da is situated in a meander of the Sai Gon river, northeast of the center of Ho Chi Minh City, and became an island after the digging of the Kenh Thanh Da channel (in the 1960s), which short-cuts the meander. Presently, the meander (depth 20 m) takes 84% of the discharge of the Saigon, while the channel takes 16% (depth 10 m). The tidal difference is about 2,5 m. The right bank and the channel banks are mostly occupied by slums. On the left bank quite a few expensive villas are under construction or have been recently constructed. Further upstream on the left bank of the meander, there are a number of shipping docks. Approximately 200 small ships per day come through the meander, about 100 small ships go through the much narrower channel. Cargo vessels have a speed limit of 20 km/h. Tourist vessels have a limit of 40 km/h. The natural vegetation seems to be a trunkless palm species (cay dua), with leaves up to 5 m. Where the palms grow there seems to be no bank erosion. Where the palms have been cleared, bank erosion occurs. All bank protection has been constructed by the inhabitants of the banks and depending on their means, may vary from sandbags to stone walls, vertical or inclined. Locally, also private land reclamation has been practised. Field visit 2: Tan Chau, An Giang Province, 27 November, 10:00 Participant from People's Committee of Tan Chau 1. Do Thanh Trung Report After a short meeting with the People's Committee of Tan Chau, a boat trip was made up the river, to the North until the Cambodian border. Bank erosion is observed at Tan Chau, where the ruins of a French colonial building are now being swallowed by the river. At Vinh Hoa the West bank shows erosion and circular slides over a length of several kilometres. On the way back from the border, a side channel west of the main channel is entered. It is commented that this channel is getting wider all the time. Driving back to Long Xuyen, a stop is made at the Song Vam Nao, a connecting river which takes more and more of the discharge from the Tien river to the Hau river. High flow speeds occur and are accompanied by erosion on the southeast bank. Field visit 3: Long Xuyen, An Giang Province, 27 November, 16:30 Report Returning from Tan Chau, the revetment system of Long Xuyen is visited. It consists of interlocked concrete blocks on the waterline and gabions below the waterline. It seems in a good condition.

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After this, a boat trip is made on the Hau river, where the island of Cu Lao Ong Ho is sailed around. Bank erosion is observed on both the western and northern side of this island. The smaller and very narrow island which lies in between Cu Lao Ong Ho island and Long Xuyen has suffered from erosion at the north side, where it has shortened a few 100 m. Field visit 4: Sa Dec, Dong Thap Province, 28 November, 9:30 Report A construction site in Sa Dec is visited. On the site a dam was built to close a side channel. Presently on the river bank a new protection structure is built, consisting of sand bags, gabions and a concrete revetment. The sand bags are used to grade the underwater slope of the river bank. After the desired slope has been established, gabion matresses are let down from a barge. The matresses have a dimension of 2x10x0,5 m and are put in place making use of a GPS. Free-divers check the correct placement and make underwater pictures. The divers can go as deep as 30 m. At Sa Dec the construction depth is about 25 m. The concrete revetment consists of interlocked slabs of approximately 0,15x0,8x0,8 m, which are made on site. Field visit 5: Vinh Long Town, 29 November, 15:30 Report With a speedboat the Tien river is explored upstream. Some bank erosion is visible and furthermore some concrete permeable groins are being observed. After a recreational island is visited. Also here there is some bank erosion.

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Appendix V: Meetings Mekong River Meeting 1: HWRU, Second Base, Ho Chi Minh City, 24 November 8:30 Participants from HWRU, Second Base 1. Dr. Duong Van Vien: Vice Director 2. Nguyen Van Dien: Expert 3. Nguyen Van Thiet: Expert 4. Miss Vu Hoang Anh: Teacher 5. Miss Mai: Teacher Participants from from Sub Institute of Geography 6. Prof. Nguyen Sinh Huy: Expert on hydraulics 7. Miss Nguyen Thi Hanh: Secretary of Prof. Nguyen Sinh Huy Meeting report Mr. Duong Van Vien gives a powerpoint presentation about the Mekong Delta, concerning flow regimes, bank erosion and geology. In the presentation, a number of specific technical data are given. Miss Nguyen Thi Hanh gives some additional geologic explanations. HWRU will provide a copy of the presentation. Meeting 2: Sub-Institute for Water Resources Planning, Ho Chi Minh City, 24 November 13:30 Participants from Sub-Institute for Water Resources Planning 1. To van Truong: Director 2. Nguyen Ngoc Anh: Deputy Director 3. Tran Minh Khoi: Vice Head of Center for Water Quality 4. Nguyen Xuan Hien: Deputy Director of SIWRP 5. John B. Cantor: Consultant for Vietnam - Australia Water Resources Management

Assistance Project 6. Nguyen Dinh Thanh 7. Pham Anh Tuan 8. Dang Thanh Lam 9. Ho Trong Tien 10. Do Duc Dung 11. Luu Van Thuan Meeting report Mr. To van Truong opens the meeting with an introduction explaining about the planning institute. Mr. Nguyen Xuan Hien holds a presentation about flood control planning in the Mekong Delta. A print of the presentation is handed out. Mr. Tran Minh Khoi holds a presentation about the water quality monitoring project. Mr. Pilarczyk mentions that the Dutch embassy is interested in the Mekong water managament programs. The mission will concentrate on bank erosion and is wondering to what extent this is an issue for the Planning Institute. Mr. To van Truong indicates that it is hard to gather sufficient data and that planning is concentrated on sites which are economically important or where ther live a lot of people.

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Mr. Cantor indicates that bank erosion is an important issue also for the Mekong River Comission. A meeting is proposed on Monday morning 1 December at 9:00 at SIWRP. Meeting 3: Southern Institute of Water Resources Research, Ho Chi Minh City, 25 November, 8:30 Participants from Southern Institute of Water Resources Research 1. Dr. Le Manh Hung: Director 2. Dr. Hoang Van Huan: Deputy Director 3. Dr. Tang Duc Thang: Deputy Director 4. Nguyen The Bien: Deputy Chief of Department 5. Dinh Cong San: Deputy Director of Center for River Training & Natural Disaster Prevention Meeting report Mr. Hoang Van Huan opens the meeting and introduces SIWRR. Of the people present, Mr. Dinh Cong San can speak English. Mr. Nguyen The Bien will come on the afternoon excursion. Mr. Le Manh Hung holds a presentation about the research about bank erosion in the Mekong Delta. Mr. Dinh Cong San gives some additional explanation on the whiteboard. The following questions are formulated to the mission: 1. Prediction of bank erosion? 2. Critical velocity? 3. Dominant discharge/tidal movement? 4. Survey sediment transport? 5. Construction measures for bank protection? After the break, Mr. Pilarczyk proposes that one or two people from SIWRR join the final meeting on December 1 at the Sub-Institute for Water Resources Planning. SIWRR thinks this is a good idea. Mr. Hoang Van Huan holds a presentation on structures to counteract bank erosion. Mr. Lindenberg asks whether there is generally a deeper, non-cohesive, fine sand layer which is sensible to liquefaction (and which would induce collapse of the upper layer). What is the process of erosion? Is the erosion fast or slow? Mr. Le Manh Hung says that the erosion in the upper reaches, where there is no tidal influence is much more gradual and less sudden than in the lower reaches. In a few occasions, the bank has suddenly collapsed over a width of 50 m. Mr. van der Wal asks whether there are some design standards or guidelines. Furthermore, to what extent is there undermining of the existent protection? Mr. Hoang Van Huan answers that generally people drop sandbags right in front of the bank. There are some standards but not really guidelines. The standards take undermining into account extending the construction 5 to 10 m beyond the toe. Mr. Pilarczyk asks whether most erosion takes place where the river is deepest? What is the influence of sand-mining? Mr. Hoang Van Huan answers that this is correct. The problem is aggravated because sand mining takes place where the river is deepest because the sand is coarser. Mr. Pilarczyk asks why Vietnam resists to the application of Vetiver grass. Mr. Le Manh Hung answers that Vietnam also applies natural solution, e.g. coconut trees are apllied in the less flooded zones where the groundwater level doesn't vary that much. However, the Vetiver doesn't survive in the places which are flooded for continued periods, nor in the tidal area with saline

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groundwater. Still, he would like to receive more information, for maybe the possibilities are underestimated. Mr. Klaassen asks whether there is any legislation as to how far away from the river you should live. The answer is that in recent years there is some regulation, but it varies a lot and is very flexible. As to accreted areas, they may be used temporarily, but should not be used finally. When was the Vam Nao connection canal dug? Has the Tonle Sap recently developed and may its coming about be at the origin of bank erosion? Nobody knows when the channel was dug. The water always flows from Tien to Hau. Meeting 4: SARD of An Giang, Long Xuyen, 26 November 14:00 Participants from SARD of An Giang 2. Do Van Hung: Vice deputy 3. Pham Van Le: Vice Head of Department of Water Resources 4. Vuong Huu Tien Participants from Engineering Company for SARD of An Giang Province 5. Tran Huu Ha: Expert of Consultant Company for Rural Development 6. Mai Anh Vu: Expert of Consultant Company for Rural Development Meeting report Mr. Do Van Hung presides the meeting and does a presentation about An Giang province, flooding and bank erosion. A paper in Vietnamese is handed out, which contains the data about flooding and bank erosion. It mentions that since 1994 bank erosion has increased, 3.000 houses were relocated, etc. Dzung will later on make a written translation of the highlights of the data in the paper. Mr. Pilarczyk asks whether some event occured around 1994 which might be linked to the increased bank erosion. Mr. Do Van Hung says that since 1997 big floods seem to be an annual event, where before they only occured every 2 or 3 years. Due to (low) dike construction the floods are higher than before. Besides, the population increased a lot, which makes that the problem attracts more attention,esp. where people started living on the banks. The water level difference between dry and wet season is about 5 m. In 2002 some 600 m of revetment were constructed in the Tan Chau area. Cost: 102·109 VND. Heigth of revetment: 15 m, width 50 m, therefore the slope is about 1:3. In Long Xuyen City some 700 m of revetment were constructed. For the construction gabions and bamboo is used. Mr. Mai Anh Vu tells that they use a model (MIKE) for the calculation of flow velocities. This is used for the design of bank potections, according to standard 14TCN-84-91. Meeting 5: SARD of Dong Thap, Cao Lanh, 28 November 14:00 Participant from SARD of Dong Thap 1. Huynh The Phien: Vice Director Participant of Department of Water Resources Dong Thap Province (= sub-Department of Dike Management and Flood Control) 2. Nguyen Chap Kinh: Deputy Chief Meeting report

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Mr. Huynh The Phien opens the meeting. Kinh comments about bank erosion in Dong Thap Province, as explained in a Vietnamese paper, which is handed out at the beginning of the meeting. A brief bank erosion history is given about the problem locations Hong Ngu, Can Tho and Sa Dec. Sa Dec is the most affected place. Presently 900 m of bank protection exists and there is the desire to extend this. According to Mr. Huynh The Phien sand mining is the main cause for bank erosion. Mr. Pilarczyk observes that sand mining doesn't need to be a problem as long as it is done at the right place. Mr. Huynh The Phien says that sand mining is regulated, but hardly controlled. Meeting 6: SARD of Vinh Long, Vinh Long Town, 29 November 13:30 Participants from SARD of Dong Thap 1. Phan Nhat Ai: Director of SARD 2. Ha Thanh Thang: Head of technical department Participant of sub-Department of Dike Management and Flood Control of Vinh Long Province 3. Nguyen Van Thanh: Director Participants from Engineering Company for SARD of Vinh Long Province 4. Minh: Consultant 5. Hoai: Designer Meeting report Mr. Phan Nhat Ai opens the meeting. Mr. Nguyen Van Thanh gives a presentation about Vinh Long Province and the occuring bank erosion. A Vietnamese paper with bank erosion is handed out during the meeting. Meeting 7: Sub-Institute for Water Resources Planning, Ho Chi Minh City, 1 December 2003, 9:00 Participants from SIWRP: 1. John Cantor: Vietnam - Australia Water Resources Management Assistance Project 2. Brian Cummings: Vietnam - Australia Water Resources Management Assistance Project 3. Dr. Le Manh Hung Participant fromSIWRR: 4. Dinh Cong San Meeting report Mr. Cantor tells about the work of AUSaid for the River Basins Organization, dealing with water management in the Mekong Delta. A list of important water resources issues has been drawn up by the steering committee. Mr. Cantor mentions that there are a number of institutional reasons for which the River Basins Organization has not yet so much influence on the water resources management, meaning the policies descending from Hanoi.

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Appendix VI: Field visits Red River Field visit 1: Ha Tay Province, 2 December, 9:30 Report Driving East, the dividing dikes of the Day River were passed. The Day river plain serves as an outlet for floods on the Red River, which are then diverted through the Ha Binh reservoir, which is located at the entrance of the Day River. A first stop at the Red River is made near Chu Minh. Here the bank is high and near vertical, being completely dried out, despite the recent end of the wet season. Bank erosion has occured 'as far as the eye can see'. In front is the Minh Chau sand bar, which forces the flow to the eroded (South) bank. After, the bank protection at Phu Cuong is visited. It consists of loose stones in a concrete framework at the top of the bank, and of gabions with smaller stones on the toe of the bank. At one point the toe is being eroded and needs to be refixed. Next, a stop is made at Phong Van, where there is a severe bank erosion and collapsing, and where numerous cracks testify a large amount of slides. Near Tong Lenh some short groins from loose stone are observed, which have been constructed by the locals a few years ago. Presently, this bank is no longer threatened and the toe of the groins is not even submerged (the scour hole around it is clearly visible), the main river channel being shifted to the other side of the river. It seems that the river channel shifts to the other side every ten years or so. Field visit 2: Hung Yen Province, 3 December, 10:30 Report In the morning, a revetment is visited at the Red River near Hung Yen town. The revetment is located just North of a large bridge under construction. The revetment consists of pitched riprap in a concrete framework. The toe is protected with gabions which form a staircase. As a transition, riprap has been deposited on the 'steps'. The gabions are in bad shape, being corroded and damaged, although the structure is only a few years old. South of the bridge, the bank is eroding, showing cracks and slides. In the afternoon, the site of Thanh Cong is visited. Here, in 1982 groin construction was started to counteract bank erosion at the toe of the dike. The groins are approx. 100 m length and are covered in pitched riprap. The last 20 m are covered in gabions, which are in bad state. When after a few years the groins didn't seem to be effective, a retired ambankment was constructed in 1985. Eventually, the dike didn't collapse and groin construction was continued. Presently there are 6 groins, the last being constructed in 2002. Another 5 groins are visited at Tu Dan. They are of the same construction. Construction was started because even at a 1:3,5-slope the bank was not stable and continued to slide. After groin construction the sliding stopped. Finally, a visit is paid to the site of Phi Liet, where a revetment will be constructed. On the opposite site of the river, illegal sand mining is taking place.

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Appendix VII: Meetings Red River Meeting 1: SARD of Ha Tay, 2 December, 8:00 Participants from SARD of Ha Tay 8. Luu Van Hai: Vice Director Participants from Sub-Department of Dike Management and Flood Control of Ha Tay Province 9. Vu Xuan Phieu 10. Nguyen Tuan Khai Meeting report Mr. Nguyen Si Nuoi opens the meeting, introducing the participants. Mr. Luu Van Hai explains about Ha Tay Province and its geographic features. Along the Da River (a tributary of the Red River) bank erosion occurs on several locations over lenghts of several hundreds of m. The land loss was 40 ha since 1994. A number of other locations and rivers with bank erosion are summed up. Among the causes of bank erosion is the operating of Ha Binh reservoir, due to regulating the flow in the flood season and trappinf of sediment in the reservoir. Important aspects for bank erosion are the formation of sand bars in the middle of the channel and the fact that the banks are sandy. In opposition to the Mekong, most bank erosion in the Red River occurs in the dry season. 2.000 People were moved, another 2.000 still have to be moved. Groin construction is expensive, dry season water depth ranging between 10 and 20 m. Floods may occur in the Da, the Lo and the Red River on different moments. Monitoring of discharge, water level and flow velocity occurs hourly in one station. Over the whole Red River, 168 cross-sections have been surveyed since 1992. There are a lot of data, but it is not clear whether they are systematically organised nor whether anything is being done with them. Bank protection is not carried out with anticipation but only after damage has occured. Anually, approx. 55 109 VND is spent on bank protection in the whole Red River Delta (19 provinces), comprising both maintenance and new structures. Whether the money goes to maintenance or new structures varies greatly and depends on the severity of the floods. Meeting 2: SARD of Hung Yen, 3 December, 9:30 Participants from SARD of Hung Yen 1. Mr. Bui Xuan Bai: Vice Director Participants from Sub-Department of Dike Management and Flood Control of Hung Yen Province 2. Mr. Doan Quang Viet: Director 3. Mr. Ho Trong Khai: Deputy Director 4. Mr. Hoang Xuan Vang: Deputy Director of DDMFC and Head of Hydraulic Construction

Project Management 5. Mr. Tran Van Bui: Hydraulic Contruction Project Management 6. Mr. Vu Van Hanh: Director of Consultant Company Meeting report

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Mr. Bui Xuan Bai explains about Hung Yen Province and its geographic features. In the flood season, there are a lot of places where piping occurs. Furthermore, in many places bank erosion occurs and is eroding the toe of the dikes. The Province is surrounded by a ring dike. Breaches occured frequently in the Northwest of the province in the 19th century. The last dike breach was in 1945. Meeting 3: Vietnam Institute for Water Resources Research, 4 December, 8:00 Participants from VIWRR 1. Mr. Tran Dinh Hoi: Deputy Director 2. Mr. Tran Xuan Thai: Director of River Engineering Research Centre 3. Mr. Nguyen Ngoc Quynh: Vice Director of River Engineering Research Centre 4. Miss Dao Thu Trang: International Cooperation Section 5. Mr. Ho Viet Cuong: River Engineering Research Centre 6. Mr. Nguyen Dang Giap Meeting report Mr. Tran Dinh Hoi formally opens the meeting and tells about the history and activities of the institute. Mr. Tran Xuan Thai holds a presentation about bank erosion in the Red River. Within VIWRR, the River Engineering Research Centre is in charge of bank erosion research. A video shows the consequences of the flood season in 2002. Modelling attempts are being made, for example with MIKE-21, but a lot of problems were encountered. In the Red River delta, bank erosion does not only affect those who live on the banks, but also the dike system, and therefore much more people. For this reason, bank erosion in the rest of Vietnam is not so important. Apart from the natural causes (high flow velocity), an important cause of bank erosion is sand mining. There is more bank erosion in areas with tidal influence. Bank erosion has a tendency to continue in downstream direction rather than in upstream direction. In some cases bank erosion is continuous, in others it only occurs during some years. Bank erosion is strongest at the end of the flood season (end of september), when the water level starts going down. If nothing is done, the river bed will shift and bank erosion will continue downstream. The most serious bank erosion sites are enumerated and show how quickly the Red River may change its course. For bank erosion prediction the Russian formulas of Ibadzade and Popov were used. Bank erosion is combatted with revetments or groins. The revetments have improved a lot. The groins were constructed esp. in the 1960s. Mostly they are impermeable, recently also permeable groins are built to divert the river flow in service of navigation. The groin consists of a soil core covered by stone. At the top a drainage system is provided for. The concrete revetment blocks have a thickness of 0,12 m. At the toe of the groins stones with a diameter of approx. 0,8 m ('dragon stones') are deposited to prevent scourholes. In Vietnam presently only short-term solutions are applied, but there is a great necessity for long-term solutions. A digital copy of the presentation will be provided. Discussion Mr. van der Wal asks how VIWRR managed to improve the revetments.

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Answer: Before everything was done by the local people in a purely empirical way. Besides there is more money now. Before, 3 109 VND per km was invested, now 25 109 VND. A groin of 50 m may cost 2 109 VND. The 'dragon stone' is essentially a large, cylindric gabion. Design is purely empirical. The dragon stone has a length of 10 m. Mr. van der Wal wonders whether this is not too short, as it can cover only a scour hole of about 4 m depth. It is stated that there is little money for satellite images to improve prediction. Mr. Klaassen indicates that also protection structures are very expensive, so it is not a matter of money but a matter of approach. Mike 21C is used for flow velocity, morphology and bank erosion. The data for the model and the calibration come from a measuring station. Presently the government is investing in more accurate data collection. Meeting 4: Hanoi Water Resources University, 4 December, 13:30 Participants from HWRU 1. Prof. Dr. Le Kim Truyen: Rector of HRWU 2. Dr. Vu Minh Cat: Head of Division of Science and International Co-operation 3. Tran Thanh Tung: Lecturer on river and coastal engineering 4. Prof. Dr. Le Dinh Thanh: Head of Coastal Engineering Faculty 5. Prof. Dr. Le Xuan Roanh: Secrectary of Coastal Engineering Faculty 6. Ass. Prof. Dr. Vu Thanh Te: Hydraulic Engineering Faculty 7. Ass. Prof. Dr. Do Tat Tuc: Head of Hydrology and Environmental Faculty Meeting report Dr. Le Kim Truyen opens the meeting and tells about the structure and activities of the University. Dr. Do Tat Tuc holds a presentation about bank erosion studies in central Vietnam, concerning the Thu Bon river. 13 Sections with severe erosion were defined. The Thu Bon river is a distributary of the Vu Gia river. Originally, ik took about 20% of the discharge of the Thu Bon river, but due to the shortcut of a meander, almost all the water started flowing into the Thu Bon river, changing the dynamics and causing bank erosion. It is the desire to stabilize the river mouth. In the flood season the flow velocity may reach 2 to 3 m/s, leading to surface erosion. Erosion prediction, or rather meander shifting is predicted in an empirical way and is not based on theory or models. The University (doing research in central Vietnam) and the two research institutes (research in respectively Mekong and Red River) apply different ways of prediction. The HWRU mainly uses remote sensing and survey data. Western formulae and theories badly apply to the Vietnamese rivers, due to differences in morphology etc. Mr. Pilarczyk pleads for setting up a good databank with hydraulic and morphologic data. There is a strong correlation between the hydrograph and bank erosion. Mr. Klaassen argues that this would be a good base to predict bank erosion. The University doesn't use erosion formulae (e.g. the Russian ones), because the erosion is also strongly influenced by factors like sand mining, which the formulae do not take into account. The available data consist of hydrographs and cross-sections. The maximum flow velocity in the central rivers is about 3 m/s, the maximum discharge about 20.000 m3/s. In the old city of Hoi An, vertical stone walls in combination with rock matresses (for bed protection) are used to stabilize the banks. Mr. Pilarczyk recommends to do some kind of systematic data research, e.g. by means of M.Sc. students. Furthermore he stresses that good cooperation with the Research Institutes (esp. the

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Second Base in Ho Chi Minh City) is of crucial importance. Mr. Cat says the funds for this are lacking. Final meeting: Presentation of mission results, DDMFC, 5 December 2003, 8:30 Participants 1. Mr. Nguyen Sy Nuoi: Deputy Director of DDMFC 2. Mr. Do Ngoc Thien: Deputy Director of DDMFC 3. Mr. Nguyen Huu Phuc: Head of Master Planning and Bank, Coastal Erosion Section 4. Ms. Cao Thi Lua: Director of Dyke Engineerin Consultant Center 5. Ms. Nguyen Thi Hien: Vice Director of Dyke Engineerin Consultant Center 6. Mr. Nguyen Tien Toan: Vice-chieft of Dyke Management Section 7. Some staffs of DDMFC 8. Mr. Tran Thanh Tung (HWRU) 9. Mr. Roanh (HWRU) 10. Nico Bakker (Dutch Embassy) Meeting report Mr. Sy Nuoi opens the meeting, introducing the participants and indicating that bank erosion is becoming a serious problem. DDMFC hopes that after the mssion a program may be started to upgrade the capacity to cope with bank erosion in Vietnam. DDMFC hopes that the Embassy can take an active part in this. Mr. Bakker indicates that the Embassy aims to support Vietnam in water management, esp. in flood control. For the Mekong river, the embassy is cooperating with the Mekong River Comission. Priority is given on the development of standards and guidelines, as well as capacity-building. The Embassy prefers to give financial aid to projects for which there is already basic funding by the ADB. Future financial aid is partly depending upon this principle. Mr. Phuc holds a presentation about bank erosion, treating backgrounds, contexts, causes, reasons and need for measures to be taken on various administrative levels. His presentation concentrates on planning and management. Mr. Bakker comments that it is important to link DDMFC to the planning institute in the south. Furthermore, China plans to build another 7 to 9 dams in the Mekong, so the hydrograph of the river will change. Data collection should be cooperated with the other Mekong countries. Mr. Phuc agrees that an umbrella structure should be set up and adds that this is also the case for the Red River, which also originates in China. The dilemma of Mr. Pilarczyk is that the master planning is very important but in practice even the money for the most urgent protection is lacking. Mr. Phuc says there is no fixed budget, the money which is available is disaster orientated. Mr. Sy Nuoi indicates that on average 2% to 3% of the GDP s spent on disaster mitigation. Mr. Pilarczyk holds a presentation on the findings of the mission. After lunch, Mr. Lindenberg gives a presentation on the geotechnical aspects of bank erosion. He is followed by Mr. Klaassen, who gives a presentation on river morphology. Mr. van der Wal gives a presentation on bank protection structures. Mr. Kerssens gives a presentation on the Second Red River Basin Sector Project.

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Mr. Sy Nuoi indicates that the work of the mission has been very useful to the DDMFC and thanks the mission for their effort. Mr. Pilarczyk says thanks on behalf of the mission for the warm reception of the mission and stresses that the most important thing is that DDMFC uses their own experience. He will do his best to to something iwth the suggestion to start capacity building on erosion prediction and monitoring, but of course it depends on money from the embassy as well. Furthermore that a nationwide approach should be followed comprising Mekong and Red River and other areas which suffer from this problem.

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Appendix VIII: Damage overview (collected data/not fully representative) An Giang Province, Mekong Delta Consequences of bank erosion between 1996 and 2003 1. Land loss: 123 ha 2. Flooded houses: 170 3. Damaged houses: 41 4. Number of families already relocated: 2.932 5. Number of families still to be relocated: 5.896 6. Approximate cost of damage: 18,4·109 VND Damage per location between 1997 and 2003 Location Land loss Nr. of families moved Tan Chau 68 ha 835 Cho Moi 23 ha 454 Long Xuyen 21 ha 1.312 Phu Tan 5 ha 180 Other locations 6 ha 151 Total 123 ha 2.932 Damage per year (in VND) 1996 1997 1998 1999 2000 2001 2002 2003 Total 2,4·109 2,5·109 0 0,3·109 7,3·109 1,4·109 4,0·109 0,5·109 18,4·109 Cost of bank protection 1. Tan Chau: 69·109 VND for a bank protection structure with a length of 0,61 km (113·109

VND per km) and a depth of approx. 45 m (slope ≈ 1:3); 2. Long Xuyen: 45·109 VND for a total protected length of 1,27 km (35,4·109 VND per km) on

3 locations with a depth of approx. 25 m (slope ≈ 1:3). Dong Thap Province, Mekong Delta Big floods occured in 1961, 1966, 1978, 1991, 1996, 2000, 2001 and 2002. During this up to 90% of the area of the province got inundated. Bank erosion occurs in 8 districts, over a total length of 106 km. Consequences of bank erosion 1. Total land loss: 50 ha 2. 1992: Big erosion in Hong Ngu with 28 persons dead or missing and 20·109 VND of damage 3. Sa Dec: Continuing erosion over a length of 10 km, destroying 2 bridges, 10 km of road, a

hospital, a school and other public buildings, with a total damage of 100·109 VND 4. Number of families already moved: 280 5. Number of families to be moved until 2005: Approx. 5.080 6. Budget for family relocation: 4,5·109 VND (≈0,9·106 VND/family) Cost of bank protection 1. Sa Dec: 50·109 VND for bank protection over a total length of 0,92 km (54,3·109 VND per

km), with an approximate depth of 25 m and a slope of 1:3; 2. Requested for new works: 100·109 VND.

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Vinh Long Province, Mekong Delta Consequences of bank erosion 1. Number of families moved between 1995 and 2002: 951 2. Land loss: 22 to 25 ha/year 3. Approximate cost of damage: 142·109 VND/year Cost of bank protection 1. 355·109 VND for a total protected length of 9,01 km (39,4·109 VND per km) on 15

locations. Ha Tay Province, Red River Consequences of bank erosion 1. Number of people already relocated: 2.000 2. Number of people still to be relocated: 2.000 Cost of bank protection 1. Annual DDMFC budget for bank protection (maintenance and new structures, distribution

depends greatly on severity of flood year): Approx. 55 109 VND for the whole Red River Delta (19 provinces);

2. Total annual DDMFC budget for Red River Delta: 160 109 VND Hung Yen Province, Red River Consequences of bank erosion since 1994 1. Land loss: 55 ha

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Appendix IX: Supplementary informations Vetiver Network Viet Nam

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For further information about VNVN, please send us email at Vetiver Network Viet Nam. We have available The green book in both Vietnamese and English. We also have pictorial brochures about vetiver, as well as CDs prepared by Paul Truong. Please email, fax, call, or write to let us know the quantity of each you would like – we'll see what we can do and let you know! Please let us know a little about yourself, your background and interests as related to vetiver, your organization (if any), and anything else you think might contribute to furthering the goal of VNVN. Or, if you wish, just write to say hello! In addition, for the names and contact information of other people working on vetiver, please go to other vetiver contacts.

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http://www.gisdevelopment.net/magazine/gisdev/2002/oct/dcrmrv.shtml

http://www.gisdevelopment.net/application/natural_hazards/floods/nhcy0009pf.htm

Using remotely sensed data to detect changes of riverbank in Mekong River, Vietnam

Pham Bach Viet, Lam Dao Nguyen and Ho Dinh Duan

Information and Remote Sensing Division - Institute of Physics, Hochiminh City

Email: vientham@hcm.vnn.vn Introduction Traditional methodologies in study of riverbank change require conventional surveys, repeated measurements to identify and to evaluate changes. Hydrology, geomorphology and geology make use of data obtained from their surveys as input in their mathematical modeling. Recent studies on Mekong River have been focused on erosion processes of shorelines at hot spots1). The common feature to all these studies is that they are localized in extent. Remote sensing techniques offer another approach to this issue - the use of satellite imagery combined with other digital data to extract information and derive certain measurements, as in an assessment of channel migration of Thu Bon River using scanned data- aerial photos and satellite imagery2). A typical study of channel migration in Yellow river (China) made use both analog and digital data with a time sequential imageries of 19 dates from 1976 to 19943). This paper presents an application of time-series satellite digital data of different sources composed of optical and radar imageries in shoreline change detection and to demonstrate a capability of remotely sensed data with digital processing and GIS analysis for river studies in a large area.

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http://www.ihe.nl/we/dicea/default.htm?/we/dicea/cress.htm

Coastal and River Engineering Support System

A cooperation project of the Netherlands Ministry of Public Works (Rijkswaterstaat), IHE-Delft and TU-Delft

Introduction This program is intended as a support for design and planning of coastal and river projects and is not intended to replace the judgement of a qualified engineer on a particular project. The contents of CRESS are not to be used for advertising, publication or promotional purposes. Citation of trade names does not constitute an official endorsement or approval of the use of such commercial products.The issuing partners do not accept liability for interpretations or implementations made by users of this program.

Nederlandse versie; voorlopig alleen RWS-Cress, met uitgebreide help-files.

English version; via this link also the French, Indonesian and Chinese version of Cress are downloadable.

Notice: At this moment there are two versions of Cress available. One version is originally developed by IHE-Delft, the other by Rijkswaterstaat. At this moment both programs are being merged. The final product wil look like the RWS-Cress version.

For Information, you may contact:

Rijkswaterstaat, Bouwdienst ir. C. Dorst Postbus 20000, 3502 LA Utrecht

UNESCO-IHE-Delft ir. M. van der Wegen P.O. Box 3015, 2601 DA Delft

TU-Delft ir. H.J. Verhagen P.O. Box 5048, 2600 GA Delft

For help use the helpfiles of the site of the Hydraulic Engineering Department of the Faculty of Civil Engineering of TU Delft

Coastal and River Engineering Support System

A cooperation project of the Netherlands Ministry of Public Works (Rijkswaterstaat), IHE-Delft and TU-Delft

Introduction Cress is available for use under DOS and under Windows (3.1, '95 and '98). The DOS version is also (partly) available in French and in Bahassa Indonesia. A windows version is available in Chinese.

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In case of problems, you may consult the faq-list

All versions are downloadable:

download RWS-Cress (English version) download RWS-Cress (Dutch version) download IHE-Cress (DOS version) download IHE-Cress (Windows version) download IHE-Cress (DOS version, French) download IHE-Cress (DOS version, Bahasa Indonesia) download IHE-Cress (Windows version, Chinese version)

After downloading, you have to decompress the files, using an unzip program (for example PKunzip).

For both versions a manual is available in HTML format:

go to IHE-Cress DOS manual

go to IHE-Cress Windows manual

In a separate file you will find the Table of contents

Reprint or republication of this program should give appropriate credit to the International Institute for Infrastructural, Hydraulic and Environmental Engineering, IHE-Delft, P.O. Box 3015, 2601 DA Delft, The Netherlands, http://www.ihe.nl

Philosophy of the package

In mathematical modelling of coastal processes there is in general a tendency to make programs more sophisticated and more advanced. The consequence of this modelling is that models become usually more specialized, and also more difficult to handle.

Although much effort is paid to the user friendliness of systems, general systems require much input, which has to be defined in some way. Most programs nowadays can be handled relatively easy only if one is familiar with the program.

On the other hand, 90 % of the problems in engineering are rather standard problems. These problems require only the application of very few formulae. Continuous research is going on to improve the quality of such formulae, although also here is a tendency to concentrate on the more exotic cases. This is very understandable, because for a researcher the challenge of such problems is much more attractive. For the design engineer, this development is not so attractive, because for his daily work he is therefore often condemned to use outdate reference material. Especially engineers working in smaller companies or agencies have difficulties is accessing the latest developments. The Shore Protection Manual is still their major source of reference information.

Because application of a dedicated program requires familiarity with the input structure, many designers having a minor problem, will not use such dedicated programs. The time they have to invest in learning how to handle the program is too

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much in comparison with the importance of the problem. So in such cases designers often go back to graphs and design manuals. To overcome this problem, IHE has developed a very simple package, called CRESS (Coastal and River Engineering Support System). In fact, CRESS is a collection of small routines, each containing a formula, or group of formulae, important in coastal and river engineering. The input and output is highly standardized, and is both available in numerical and graphical form. Working with CRESS is fast and simple, the package is designed in such a way that is works on all types of machines, even on the slow ones, and that it does not require a lot of memory. Uncompressed it still fits on a 720 Kb diskette.

For a design several steps have to be taken. CRESS does not automatically transfer data from one step to the next one. In this case the user is forced to think about the input and all the intermediate results. CRESS does not prevent the user to apply a formula outside its range of application. Often in engineering it is useful to do so, however it has to be done with great care. Tendencies and sensitivities are very important. The graphical routines of CRESS are made in such a way, that the sensitivity of a given parameter can be shown easily in the diagram. The available Help routines allow the user to find some background information on applied formula, but also lists of constants, which can be entered (for example the Manning coefficients).

For more detailed background information sometimes is referred to literature, but mostly to the IHE lecture notes. We try to make more references to Dicea-files. These files are available via the web-site of IHE (www.ihe.nl) and can be downloaded if needed.

Being an educational institute, our first aim in developing CRESS was not to provide a handy tool for designers, but to develop an instrument for the training of our students. In our view a design engineer has to be able to understand the physical background of the formulae used, and has to know the sensitivity of the various input parameters. In most cases it is not necessary that a designer can derive all formulae used, neither it is necessary to know the formulae by heart. However, for training in real design, one has to apply formulae quite often, especially in order to develop a feeling for the ranges of validity and for the sensitivity. Doing this with a (programmable) pocket calculator takes a lot of time (with the risk of many data entry errors) which is in our opinion not well spent time.

In CRESS all these formulae have been placed. Applying CRESS goes very fast, and therefore all the available time can be used for evaluation of the given output. Because of the fast and flexible graphical output, students get develop a feeling for ranges in a relative short time.

It is our intention to keep CRESS on the level of the state-of-the-art in coastal and river engineering. When research results in new approaches to design problems, and in practical application this leads to acceptable results in the design process, such new developments will be implemented as soon as possible in CRESS. Examples of new developments in CRESS are the Breakwater armour unit formula of Van der Meer (as alternative of Hudson), the longshore transport formula of Queens (as alternative of Cerc), the new Delft run-up formula (as alternative of the 8Htan() formula).

It is not our intention to build out CRESS as a sophisticated tool able to solve all major problems in coastal and river engineering. For example, a very simple routine is

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available to compute a backwater curve. This routine is fast, and also the input can be generated very fast. However, this routine is not able to compute a backwater curve in a network, with a time dependent flow, etc. For such problems, one has to use a package, specially designed to solve that kind of problems.

Licensed users

Licensed users are IHE participants graduated in branches Hydraulic Engineering branch a and b in 1992 or later years. IHE participants graduated in previous years can become licensed users by writing to the program administrator. Licensed users will be informed on updates of the program. They are entitled to receive updated free of charge. In order to get an update by mail the licensed users should send a formatted diskette to the program administrator (for the Windows-version 3 diskettes). IHE will copy the update on that floppy and return it to the sender. However, a faster way is to download the program directly from the net. It is allowed to copy the program for third parties. It is appreciated when names and addresses of these third parties are send to the program administrator.

Program administrators:

ir. H. J. Verhagen, TU Delft phone: +31.15.2785067 e-mail: h.j.verhagen@ct.tudelft.nl

ir M. van der Wegen, IHE-Delft phone: +31.15.2151811 fax: +31.15.2122921 e-mail: mvw@unesco-ihe.org

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Supplement

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Supplement: Short review on bankerosion and protection 1. Causes of erosion and failure The major causes of bank erosion are: 1. Channel bed degradation; 2. Channel meander; 3. Sand-wave sediment transport; 4. Divided flow conditions; 5. Channel restrictions; 6. Varied streamflow (current-velocity-related tractive forces); 7. Water-level fluctuations (including rapid drawdown triggering slumpages); 8. Long-duration water levels and discharges; 9. Wave action (including ship-induced waves, currents and drawndowns); 10. Poor control of overbank drainage; removal of bank soil by seepage of water through zones

of low erosion resistance (piping) with slabbing and caving of overlying soils or, weather-induced spalling/cracking of upper bank surface soils, eventually in combination with surface flow;

11. Devastation of (natural) vegetation; 12. Dredging/sand mining near the bank toe and resulting (scour) holes.

Figure 1: General river flow velocity profiles

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Figure 2: Detailed river flow velocity profiles

Figure 3: Wave generation by ships

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Figure 4: Ship-induced waves The analysis of streambank changes caused by soil erosion is analogous or similar to conventional stability analysis of an excavated slope. Bank recession with time can be estimated by using a procedure as shown below. This conceptual procedure combines erosion characteristics and conventional soil parameters used in limit equilibrium slope stability analyses. Erosional changes in geometry, such as toe recession and/or bed degradation, can precipitate slope failure with resulting top retreat of the streambank. The bank recession with time is equal to the cumulative bank recession caused by erosion and slope failures.

Figure 5: Procedure for evaluating streambank stability (Us Army, 1981)

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To evaluate streambank stability, it is necessary to estimate changes in geometry due to erosion and slope movements. Bank recession or bed degradation estimated from the laboratory relationships developed for tractive (current) erosion is an approximation because it does not take into account such things as accretion along the bank, secondary currents, and bed degradation as eroded soil from upstream is deposited at the reach of the river under consideration. A sediment transport analysis which includes hydraulic sorting and armouring would be necessary to include the effects of deposition. In addition to changes in geometry due to current erosion, bank failure causes changes in geometry. Bank failures results when the induced shear stresses exceed the shear strength of the bank soils. Increases in shear stress can result from increase in slope height or steepness, increase in external loads, and raoid drawdown of the river. Decreases in shear strength of the soil can result from an increase in pore-water pressure, soil expansion, or shear movements. A general overview of typical failure modes related to bank recession is given in the figures 6 to 8.

Figure 6: Bank collapsing

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Figure 7: Series of bank slidings (opposite site of the river)

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Figure 8a: Failure modes of river banks

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Figure 8b: Failure modes of river banks

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2. Cliff erosion Shorelines are areas of (practically) unending conflict among the natural forces in wind, water and land. Due to these forces shoreline materials are washed-out. Usually, when the eroded material is replaced by an equal quantity from other areas, the shore remains (dynamically) stable. However, the local, often momentary, changes can still have catastrophic effects to the shoreline condition and adjacent properties. In case of riverbanks of natural (non-regulated) rivers, the river is in continuous process of morphological changes and a large area can be affected by seasonal changes. Erosion processes of banks and shorelines are rather complicated and are treated extensively in a number of textbooks and publications. In general, there is no one universal explanation and solution; each case needs its own analysis and treatment. Erosion problems of banks and coastal shorelines can be illustrated by a bluff shoreline where a variety of forces and processes act together (see figure 9). However, it should be remembered that it represents only one type of (combined) erosion, and that each case must be treated individually.

Figure 9: Physical components of bank erosion (US Army, 1981) The most prevalent causes of bank erosion and especially bluff erosion are scour at the toe (base) by currents and waves and instability of the bluff materials themselves . The erosion rate and (in-)stability depend strongly on the type and composition of the soil. Therefore the slope stability problems are difficult to analyze correctly without (local) expertise in geotechnical engineering. As figure 13 illustrates, a typical bluff often consists of different soils deposited in distinct layers, such as clay, sand, silt, etc. These soils do not permanently stand at a vertical face, but form an angled slope that varies with the soil and groundwater conditions. This slope forms following a series of failures whose nature depends on whether the soil is cohesive (clay) or

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granular (sand, silt, gravel, etc.) Cohesive soils generally slide along a circular or curved arc, and the soil moves downward and rotates along the failure surface. With granular soils, on the other hand, vertical sides blocks of soil will drop or the soil will suddenly flow down an inclined plane. Height is a factor because high bluffs impose greater stresses and are likely to suffer more severe stability problems than low bluffs. The internal strength of soils can be decreased by groundwater and seepage flows within the bank (bluff). For instance, rainwater or river water at high stages (also high tides) is naturally absorbed and seeps down to lower levels (or after a drop in water levels). In case of low banks the area can be frequently flooded providing additional source of soil saturation. The weight of saturated soil is increasing causing potential instability. Permeable soils, such as coarse sand, allow rapid and free passage of water. Impermeable soils, such as clay, do not allow the free flow of water except through cracks or other openings, and the over-pressured conditions may last for a long time leading at certain moment to the movement of a soil block (especially, in a case of rapid and large drop in the outside water levels). In figure 9, the large tree’s roots penetrate the clay layer and provide a path for seepage to the sand layer beneath. Likewise, as the clay fails, cracks form at the surface, providing a path for seepage to penetrate the soil, further weaken it, and accelerate the failure process. water can also enter the clay along the existing circular failure surface, leading to further movement. Once seepage penetrates the clay and reaches the permeable sand layer, it passes freely to the lower clay layer, where it flows along the clay’s surface and exits the bluff face. This seepage can increase the risk of a slope failure. In addition, surface flow can erode the bluff face, causing gullies and deposits of eroded material on the beach below. The seepage exiting the bluff at the clay layer can also cause surface erosion. The added weight of buildings and other structures near the top edge of the bank/bluff can increase soil stresses and contribute to slope failure. The other major cause of shoreline problems is current and wave action at the toe (including ship waves and ship currents and depressions). The eroded material is deposited at the toe and sorted by currents and /or waves. However, during severe wave activity (estuary and coastal shorelines, or river stretches with a relatively large fetch), waves can reach the bank/bluff itself and erode or undercut the toe. Also, the slope of the bottom is important to wave action on the bank. If the bottom slopes are steep, deep water is closer to shore, more severe wave activity is possible, and maintenance of a protection is more difficult. Flat bottom slopes, on the other hand, result in shallower water near the bank, which inhibits heavy wave action at the bank/toe and provides for potentially better protection conditions.

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3. Bank erosion in stable river systems [from US streambank Manual] For this discussion local instability refers to bank erosion that is not symptomatic of a dis-equilibrium condition in the watershed (i.e., system instability) but results from site-specific factors and processes. Perhaps the most common form of local instability is bank erosion along the concave bank in a meander bend which is occurring as part of the natural meander process. Local instability does not imply that bank erosion in a channel system is occurring at only one location or that the consequences of this erosion are minimal. As discussed earlier, erosion can occur along the banks of a river in dynamic equilibrium. In these instances the local erosion problems are amenable to local protection works such as bank stabilization measures. However, local instability can also exist in channels where severe system instability exists. In these situations the local erosion problems will probably be accelerated due to the system instability, and a more comprehensive treatment plan will be necessary. Overview of Meander Bend Erosion Depending upon the academic training of the individual, streambank erosion may be considered as either a hydraulic or a geotechnical process. However, in most instances the bank retreat is the result of the combination of both hydraulic and geotechnical processes. The material may be removed grain by grain if the banks are non-cohesive (sands and gravels), or in aggregates (large clumps) if the banks are composed of more cohesive material (silts and clays). This erosion of the bed and bank material increases the height and angle of the streambank which increases the susceptibility of the banks to mass failure under gravity. Once mass failure occurs, the bank material will come to rest along the bank toe. The failed bank material may be in the form of a completely disaggregated slough deposit or as an almost intact block, depending upon the type of bank material, the degree of root binding, and the type of failure (Thorne, 1982). If the failed material is not removed by subsequent flows, then it may increase the stability of the bank by forming a buttress at the bank toe. This may be thought of as a natural form of toe protection, particularly if vegetation becomes established. However, if this material is removed by the flow, then the stability of the banks will be again reduced and the failure process may be repeated. As noted above, erosion in meander bends is probably the most common process responsible for local bank retreat and, consequently, is the most frequent reason for initiating a bank stabilization program. A key element in stabilization of an eroding meander bend is an understanding of the location and severity of erosion in the bend, both of which will vary with stage and plan form geometry. As streamflow moves through a bend, the velocity (and tractive force) along the outer bank increases. In some cases, the tractive force may be twice that in a straight reach just upstream or downstream of the bend. Consequently, erosion in bends is generally much greater than in straighter reaches. The tractive force is also greater in tight bends than in longer radius bends. This was confirmed by Nanson and Hickin (1986) who studied the migration rates in a variety of streams, and found that the erosion rate of meanders increases as the radius of curvature to width ratio (r/w) decreased below a value of about 6, and reached a maximum in the r/w range of 2 to 3. Biedenharn et al. (1989) studied the effects of r/w and bank material on the erosion rates of 160 bends along the Red River in Louisiana and also found that the maximum erosion rates were observed in the r/w range of 2 to 3. However, the considerable scatter in their data indicate that other factors, particularly bank material composition, were also modifying the meander process. The severity and location of bank erosion also changes with stage. At low flows, the main thread of current tends to follow the concave bank alignment. However, as flow increases, the flow tends to cut across the convex

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bar to be concentrated against the concave bank below the apex of the bend. Friedkin (1945) documented this process in a series of laboratory tests on meandering in alluvial rivers. Because of this process, meanders tend to move in the downvalley direction, and the zone of maximum erosion is usually in the downstream portion of the bend due to the flow impingement at the higher flows. This explains why the protection of the downstream portion of the bend is so important in any bank stabilization scheme. The material eroded from the outer bank is transported downstream and is generally deposited in the next crossing or point bar. This process also results in the deposition of sediment along the upper portion of the concave bank. This depositional feature is often a good indicator of the upstream location to start a bank protection measure. Streambank Erosion and Failure Processes The terms streambank erosion and streambank failure are often used to describe the removal of bank material. Erosion generally refers to the hydraulic process where individual soil particles at the bank’s surface are carried away by the tractive force of the flowing water. The tractive force increases as the water velocity and depth of flow increase. Therefore, the erosive forces are generally greater at higher flows. Streambank failure differs from erosion in that a relatively large section of bank fails and slides into the channel. Streambank failure is often considered to be a geotechnical process. A detailed discussion of the erosion and failure processes discussed below is provided by Thorne (1993). Identifying the processes responsible for bank erosion is not an easy task and often requires some training. The primary erosion processes are parallel flow, impinging flow, piping, freeze/thaw, sheet erosion, rilling/gullying, wind waves, and vessel forces. Parallel flow erosion is the detachment and removal of intact grains or aggregates of grains from the bank face by flow along the bank. Evidence includes: observation of high flow velocities close to the bank; near-bank scouring of the bed; under-cutting of the toe/lower bank relative to the bank top; a fresh, ragged appearance to the bank face; absence of surficial bank vegetation. Impinging flow erosion is detachment and removal of grains or aggregates of grains by flow attacking the bank at a steep angle to the long-stream direction. Impinging flow occurs in braided channels where braid-bars direct the flow strongly against the bank, in tight meander bends where the radius of curvature of the outer bank is less than that of the channel centerline, and at other locations where an in-stream obstruction deflects and disrupts the orderly flow of water. Evidence includes: observation of high flow velocities approaching the bank at an acute angle to the bank; braid or other bars directing the flow towards the bank; tight meander bends; strong eddying adjacent to the bank; near-bank scouring of the bed; under-cutting of the toe/lower bank relative to the bank top; a fresh, ragged appearance to the bank face; absence of surficial bank vegetation. Piping is caused by groundwater seeping out of the bank face. Grains are detached and entrained by the seepage flow (also termed sapping) and may be transported away from the bank face by surface run-off generated by the seepage, if there is sufficient volume of flow. Piping is especially likely in high banks or banks backed by the valley side, a terrace, or some other high ground. In these locations the high head of water can cause large seepage pressures to occur. Evidence includes: pronounced seep lines, especially along sand layers or lenses in the bank; pipe shaped cavities in the bank; notches in the bank associated with seepage zones and layers; run-out deposits of eroded material on the lower bank. Note that the effects of piping erosion can easily be mistaken for those of wave and vessel force erosion (Hagerty, 1991a,b). Freeze/thaw is caused by sub-zero temperatures which promote freezing of the bank material. Ice wedging cleaves apart blocks of soil. Needle-ice formation loosens and detaches grains and

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crumbs at the bank face. Freeze/thaw activity seriously weakens the bank and increases its erodibility. Evidence includes: periods of below freezing temperatures in the river valley; a loose, crumbling surface layer of soil on the bank; loosened crumbs accumulated at the foot of the bank after a frost event; jumbled blocks of loosened bank material. Sheet erosion is the removal of a surface layer of soil by non-channelized surface run-off. It results from surface water draining over the bank edge, especially where the riparian and bank vegetation has been destroyed by encroachment of human activities. Evidence includes: surface water drainage down the bank; lack of vegetation cover, fresh appearance to the soil surface; eroded debris accumulated on the lower bank/toe area. Rilling and gullying occurs when there is sufficient uncontrolled surface run-off over the bank to initialize channelized erosion. This is especially likely where flood plain drainage has been concentrated (often unintentionally) by human activity. Typical locations might be near buildings and parking lots, stock access points and along stream-side paths. Evidence includes: a corrugated appearance to the bank surface due to closely spaced rills; larger gullied channels incised into the bank face; headward erosion of small tributary gullies into the flood plain surface; and eroded material accumulated on the lower bank/toe in the form of alluvial cones and fans. Wind waves cause velocity and shear stresses to increase and generate rapid water level fluctuations at the bank. They cause measurable erosion only on large rivers with long fetches which allow the build up of significant waves. Evidence includes: a large channel width or a long, straight channel with an acute angle between eroding bank and longstream direction; a wave-cut notch just above normal low water plane; a wave-cut platform or run-up beach around normal low-water plane. Note that it is easy to mistake the notch and platform produced by piping and sapping for one cut by wave action (Hagerty, 1991a,b). Vessel Forces can generate bank erosion in a number of ways. The most obvious way is through the generation of surface waves at the bow and stern which run up against the bank in a similar fashion to wind waves. In the case of large vessels and/or high speeds these waves may be very damaging. If the size of the vessel is large compared to the dimensions of the channel hydrodynamic effects produce surges and drawdown in the flow. These rapid changes in water level can loosen and erode material on the banks through generating rapid pore water pressure fluctuations. If the vessels are relatively close to the bank, propeller wash can erode material and re-suspend sediments on the bank below the water surface. Finally, mooring vessels along the bank may involve mechanical damage by the hull. Evidence includes: use of river for navigation; large vessels moving close to the bank; high speeds and observation of significant vessel-induced waves and surges; a wave-cut notch just above the normal low-water plane; a wave-cut platform or "spending" beach around normal low-water plane. Note that it is easy to mistake the notch and platform produced by piping and sapping for one cut by vessel forces (Hagerty, 1991a,b). Ice rafting erodes the banks through mechanical damage to the banks due to the impact of ice-masses floating in the river and due to surcharging by ice cantilevers during spring thaw. Evidence includes: severe winters with river prone to icing over; gouges and disruption to the bank line; toppling and cantilever failures of bank-attached ice masses during spring break-up. Other erosion processes (trampling by stock, damage by fishermen, etc.) could be significant but it is impossible to list them all. Serious bank retreat often involves geotechnical bank failures as well as direct erosion by the flow. Such failures are often referred to as "bank sloughing" or "caving," but these terms are poorly defined and their use is to be discouraged. Examples of different modes of geotechnical stream bank failure include soil fall, rotational slip, slab failure, cantilever failure, pop-out

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failure, piping, dry granular flow, wet earth flow, and other failure modes such as cattle trampling. Each of these is discussed below. Soil/rock fall occurs only on a steep bank where grains, grain assemblages or blocks fall into the channel. Such failures are found on steep, eroding banks of low operational cohesion. Soil and rock falls often occur when a stream undercuts the toe of a sand, gravel or deeply weathered rock bank. Evidence includes: very steep banks; debris falling into the channel; failure masses broken into small blocks; no rotation or sliding failures. Shallow slide is a shallow seated failure along a plane somewhat parallel to the ground surface. Such failures are common on banks of low cohesion. Shallow slides often occur as secondary failures following rotational slips and/or slab failures. Evidence includes: weakly cohesive bank materials; thin slide layers relative to their area; planar failure surface; no rotation or toppling of failure mass. Rotational slip is the most widely recognized type of mass failure mode. A deep seated failure along a curved surface results in back-tilting of the failed mass toward the bank. Such failures are common in high, strongly cohesive banks with slope angles below about 60o .Evidence includes: banks formed in cohesive soils; high, but not especially steep, banks; deep seated, curved failure scars; back-tilting of the top of failure blocks towards intact bank; arcuate shape to intact bank line behind failure mass. Slab-type block failure is sliding and forward toppling of a deep seated mass into the channel. Often there are deep tension cracks in the bank behind the failure block. Slab failures occur in cohesive banks with steep bank angles greater than about 60o. Such banks are often the result of toe scour and under-cutting of the bank by parallel and impinging flow erosion. Evidence includes: cohesive bank materials; steep bank angles; deep seated failure surface with a planar lower slope and nearly vertical upper slope; deep tension cracks behind the bank-line; forward tilting of failure mass into channel; planar shape to intact bank-line behind failure mass. Cantilever failure is the collapse of an overhanging block into the channel. Such failures occur in composite and layered banks where a strongly cohesive layer is underlain by a less resistant one. Under-mining by flow erosion, piping, wave action and/or pop-out failure leaves an overhang which collapses by a beam, shear or tensile failure. Often the upper layer is held together by plant roots. Evidence includes: composite or layered bank stratigraphy; cohesive layer underlain by less resistant layer; under-mining; overhanging bank blocks; failed blocks on the lower bank and at the bank toe. Pop-out failure results from saturation and strong seepage in the lower half of a steep, cohesive bank. A slab of material in the lower half of the steep bank face falls out, leaving an alcove-shaped cavity. The over-hanging roof of the alcove subsequently collapses as a cantilever failure. Evidence includes: cohesive bank materials; steep bank face with seepage area low in the bank; alcove shaped cavities in bank face. Piping failure is the collapse of part of the bank due to high groundwater seepage pressures and rates of flow. Such failures are an extension of the piping erosion process described previously, to the point that there is complete loss of strength in the seepage layer. Sections of bank disintegrate and are entrained by the seepage flow (sapping). They may be transported away from the bank face by surface run-off generated by the seepage, if there is sufficient volume of flow. Evidence includes: pronounced seep lines, especially along sand layers or lenses in the bank; pipe shaped cavities in the bank; notches in the bank associated with seepage zones; run-out deposits of eroded material on the lower bank or beach. Note that the effects of piping failure can easily be mistaken for those of wave and vessel force erosion. Dry granular flow describes the flow-type failure of a dry, granular bank material. Other terms for the same mode of failure are ravelling and soil avalanche. Such failures occur when a

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noncohesive bank at close to the angle of repose is undercut, increasing the local bank angle above the friction angle. A carpet of grains rolls, slides and bounces down the bank in a layer up to a few grains thick. Evidence includes: noncohesive bank materials; bank angle close to the angle of repose; undercutting; toe accumulation of loose grains in cones and fans. Wet earth flow failure is the loss of strength of a section of bank due to saturation. Such failures occur when water-logging of the bank increases its weight and decreases its strength to the point that the soil flows as a highly viscous liquid. This may occur following heavy and prolonged precipitation, snow-melt or rapid drawdown in the channel. Evidence includes: sections of bank which have failed at very low angles; areas of formerly flowing soil that have been preserved when the soil dried out; basal accumulations of soil showing delta-like patterns and structures. Other failure modes could be significant, but it is impossible to list them all. Cattle trampling is just one example of a common failure mode.

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4. Bank erosion and planform changes 4.1 Overview In this chapter planform changes are discussed. The emphasis on natural changes. Broadly speaking a distinction is made to gradual and sudden changes in channel planform. Gradual changes are usually linked to bank erosion. Sudden changes occur when cutoffs develop. Both changes are discussed in this chapter. In this chapter causes and rates of bank erosion and some prediction methods are discussed. Section 2 is mainly dealing with bank erosion along meandering rivers. Section 3 discusses cutoffs of meandering rivers, but the analysis can be applied to braided rivers as well. Next planform changes of braided rivers are reviewed. The main difference between the two is that bank erosion along a meandering river can be predicted fairly well, while bank erosion along braided rivers is much more subject to stochastic behaviour. Also other planform changes in braided rivers are reviewed. Section 5 discusses the predictability of planform changes. 4.2 Bank erosion of meandering rivers General Bank erosion is a common feature of all alluvial rivers. Rates of lateral erosion for various rivers are greatly different due to variations in geological structure and sedimentological composition of the valley material. Rates of lateral migration vary from a few meters per year (Brice 1984, USA rivers) via some tens of meters per year (Mahakam River, Kalimantan, Indonesia) and 50-75 m/year (Missouri, Mississippi) to values of 30 to 750 m/year for the Brahmaputra River, occuring mainly during the flood season (Coleman, 1969; Klaassen & Masselink, 1992). Bank erosion is most prominent in river bends due to the increase of velocities in the outer bend and the spiral flow which tends to deepen the outer bend. The rate of bank erosion is determined by the strength of the bank on the one hand and the fluid forces on the other hand. Often a distinction is made between noncohesive and cohesive banks. Bank erosion usually tends to increase the length of the river, especially if the erosion is directed in lateral direction and not so much in downstream direction. Cutoffs are the typical phenomenon balancing this gradual lengthening. Bank erosion processes Bank erosion can be due to fluvial entrainment of individual particles (mainly by large velocities and shear stress on the banks), undermining of the toe of the bank and subsequent soil-mechanical failure or liquefaction by overpressure in fine sand during falling water levels. Coleman (1969) observed that for the Branhmaputra River the majority of the failures was due to current undermining and subsequent failure of the levee deposits. For the toe erosion it holds that the material which enters into the river has to be eroded. Hence the bank erosion is controlled on the one hand by the instability of the banks and on the other hand by the sediment transport capacity of the flow near the outer bend. For more details on bank erosion processes see the article by Osman and Thorne (1988). The article of Hagerty (1991) draws attention to the possible existance of horizontal layers layers of different permeability in the banks and their effects.

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Bank erosion rates There is not much theoretical or even empirical work related to quantitative prediction of bank erosion rates. Brice (1984) found that bank erosion rate increased linearly with drainage area for a number of USA rivers. Here two different approaches are discussed. One approach deals with estimates of yearly erosion and was developed by Hickin and Nanson (1984) for rivers in Canada. Klaassen and Masselink used the same approach for the Brahmaputra River in Bangladesh. The other approach, included in models in which the bank erosion is simulated, links the momentary bank erosion to the local conditions (either flow or bank height). Hickin and Nanson (1984) did an extensive study using photographs of sand and of the radius in Western Canada. They found that erosion rate was a function of the radius of curvature to width ratio R/W, with a maximum at R/W = 2.5. This can be written as: M(R/B) = M2.5 . f(R/B) (1) For f(R/W) an empirical relation was derived: for 1 <R/B < 2.5 f(R/B) = 2/3 (R/B-1) (2a) for R/B > 2.5 f(R/B) = 2.5 B/R (2b) The maximum erosion rate occurs for R/B = 2.5 and is defined as M2.5 (in m/year) and this maximum rate is proportional to total streampower Ù, which is defined as: Ω = Q5 τ h-1 = ρ g Q5 i (3) where Ω = total stream power (in Watt/m'), Q5 = discharge exceeded once in 5 years (m3/s). M2.5 is inversely proportional to a bank-strength parameter YB (dimension N/m2) which is a function of bed material size:

2.5B

= M h.YΩ (4)

Example: Given: Q5 = 800 m3/s i = 10-3 h = 5 m Dbank = 1 mm R/B = 5 Solution: Dbank = 1 mm -> YB = 80 N/m2 R/W = 5 -> f(R/W) = 0.5 Ω = ρg i Q5 = 8000 Watt/m' M2.5 = Ω. h-1YB

-1 = 20 m/year M = M2.5 . f(R/W) = 10 m/year. The use of this relation in other parts of the world must be done with care but can give some idea of possible bank erosion rates. The other approach uses local conditions like the local shear stress, the local velocity the local sediment transport and/or the local bank height for estimating the bank erosion rates. Typically

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also here calibration coefficients have to be introduced which represent a.o. the bank properties. Althought the approach may be more generally applicable, these calibration coefficients have to be determined for each river separately. Prediction methods which take into account the local conditions have been proposed by e.g. Ariathurai and Arulanandan (1978), Crosato (1990), Mosselman (1992), DHI (1996) and Shishikura (1996). A typical example of such a predictor is:

B ca

c

-n = Etδ τ τδ τ

(5)

where nB = bank line position and in the right side the local shear stress and the criritical shear stress are present. The coefficient Ea has to be calibrated to the local conditions. 4.3 Cutoffs in meandering rivers Natural cutoffs occur when channels start to develop that cut short a river bend. Cutoffs occur both in meandering channels and in braided channels. For the development of such a cutoff channel there are two criteria should be applied: (1) The shear stress should exceed the critical shear stress. (2) The sediment entering the potential cutoff channel should be less than the sediment

transport capacity of the cutoff channel. In meandering rivers cutoff channels develop usually in the floodplains. These floodplains are often characterized by cohesive soils and vegetation. Both aspects have a pronounced influence on the resistance to erodibility. Hence often those cutoffs do not develop easily. If the dimensions of a cutoff are characterized by the cutoff ratio ë (see Klaassen & van Zanten, 1989), then typical ratio's of ë for meandering rivers are 5 to 30. For a particular river, often use can be made of "scars" in the terrain to study old natural cutoffs and hence to decide on a typical cutoff ratio for a particular river. Artificial cutoffs can be made by making use of the analysis of these natural cutoffs. Such artificial cutoffs are succesfull even for smaller ë if the depth during flood is sufficiently large. The shear stress should be larger than the critical shear stress. Often does the critical shear stress become smaller in vertical direction because of the decreasing influence of cohesion (the lower strata contain usually less fine sediments). The pilot channel should have sufficient width. Initially the scouring process will promote vertical erosion, only later will the pilot channel widen. In braided sand-bed rivers cutoffs occur frequently. Cutoff ratio's between 1.0 and 1.5 have been observed (see Klaassen & Masselink,1992). This is due to the fact that during flood the critical shear stress is exceeded everywhere. Also on the bars that are present in areas where potential cutoffs may occur. Hence whether a cutoff occurs is dependent on the ratio of the transport capacity in the potential cutoff and the quantitites of sediment entering at the upstream bifurcation. As shown by Mosselman et al (1993) in particular the angle of the upstream angle is an important parameter for the occurrence of a cutoff, implying that the sediment distribution at the bifurcation (which is very sensitive to the geometry of the bifurcation) plays a key role. For more details on the occurrence of cutoffs in braided sand bed rivers see Klaassen & van Zanten (1989).

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4.4 Planform changes of braiding rivers See the articles of Klaassen and Masselink (1992) and Klaassen et al (1993). 4.5 On predictability and modelling of bank erosion and planform changes Traditionally river planforms have been classified as alternate bars, meandering and braiding (see Leopold et al, 1964). As far as bank erosion is concerned, a major difference exists between braided rivers on the one hand and straight rivers with alternate bars and meandering rivers on the other hand. It seems that for straight rivers with alternate bars and for meandering rivers the future planform changes can be predicted with some accuracy. Bank erosion occurs mainly along the outer bend, where the velocities are largest. Recently this has also been applied in the development of mathematical models for the prediction of the planform changes along these meandering channels (Crosato, 1990; Mosselman, 1992). It is of course obvious that the duration of the flood is important input parameter. This is not fully taken into account in the prediction method of Hickin & Nanson (1984). Two types of mathematical models are presently available to model bank erosion. MEANDER of WL/Delft Hydraulics is specifically useful for the simulation and prediction of meandering for decades ahead. The prediction is essentially deterministic (given a certain discharge hydrograph), but it has been applied in a stochastic way by varying the input discharge hydrographs and assuming the occurrence of cutoffs depending on the extremity of the flood. Calibration of the model on field data (changes of meandering patterns over the past decades, e.g. from satellite imagery) is required. The model has been applied in several WL/Delft Hydraulics studies. In MIKE21 of DHI a bank erosion module is coupled to a 2D flow, sediment transport and morphology module. In practice this model, which has been used for predicting planform changes along the Brahmaputra River, can only be used a few years ahead, because the required computer capacity is still quite high. For braided river systems the predictability is substantially less. Clearly some chaotic behaviour is apparent, not because of the discharge hydrograph but mainly because of the highly non-linear character of the system. It seems that small differences in location of bars in the system can induce the rapid formation of cutoffs and the closing of existing channels. This holds especially for braided sand bed rivers like the Lower Brahmaputra River in Bangladesh. See Klaassen & Masselink (1992).Although some planform elements remain to be present over a long period, in particular severe bank erosion shifts rapidly form one place to another in a few years. For the time being mathematical models for planform changes for this type of rivers are not advocated. See Klaassen et al (1994) and Jagers (2003).

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5. Survey and Data Collection Preliminary Analysis for Bank Erosion Reconnaissance Country: Place: Date: Problem definition Qualitative description of occured damage and expected future damage due to bank erosion on specific locations. Damage may be material or non-material. Documentary Survey Documents and general information about the river system to be gathered and consulted before a field visit: 1. Both recent and old topographical maps at various scales 2. Morphological maps 3. Geological maps 4. Aerial photographs and satellite data 5. Small and large scale nautical river charts (bathymetry) 6. Longitudinal and tranversal river profiles 7. Tidal tables for the lower river reaches 8. Meteorological and hydrological data 9. Statistic data about flooding 10. Old newspapers and other historical documents about flooding and bank erosion 11. Technical reports of existing or planned protection structures along the river Detailed information 1. Land features (sources: maps, photographs) a. Land use near the river banks: Recreation/nature conservation & national parks/agriculture/

fish farming/housing/infrastructure/industries b. Elevation of the land behind the banks c. Widths of land threatened by inundation or erosion d. Presence of flood protection structures: Levees and dikes 2. Bank features (sources: maps, photographs, transversal cross-sections) a. Bank profiles: Steep/smooth b. Bank protection structures: Revetments/groins 3. River features and bathymetry (sources: maps, photographs, charts, technical reports) a. Type of river: Meandering/braiding b. River mouth configuration: Single or multiple delta outlets c. Stability of the river bed: Stable/degrading d. Location of important tributaries e. Large scale human interference affecting hydraulic and morphologic conditions:

channelization work/changes in upstream reservoir operation/changes in land use f. Specific river data: Flow velocities, flood discharges, bed gradients, depth contours, currents,

sediment loads of the river and its main tributaries (m3/year), flooding frequencies

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4. Tides in the lower river reaches (source: tidal tables) a. Mean high water level: MSL + ... m b. Mean low water level: MSL - ... m c. Mean tidal range: ... m d. Mean high water springs: MSL + ... m e. Mean low water springs: MSL - ... m f. Mean spring range: ... m g. Mean high water neaps: MSL + ... m h. Mean low water neaps: MSL - ... m i. Mean neap range: ... m j. Mean tidal current: ... m/s 5. Waves (source: technical reports) a. Amount of ship traffic, type and speed of ships b. Fetch lengths, depth and wind characteristics c. Spectrum of wave heights d. Spectrum of wave periods e. Spectrum of wave directions 6. Bank erosion data (source: technical reports) a. Eroded volumes (m3/year) b. Eroded widths (m/year) c. Eroded lengths (m) 7. Soil data a. Profiles from CPT's and borings b. Soil characteristics: sieve curves, d10, d50, d90, angle of internal friction, cohesion, etc.

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Field inspection Province: Community: Location (km): Date: Land features a. Land use near the river bank: Recreation/nature conservation & national parks/agriculture/

fish farming/housing/infrastructure/industries b. Elevation of the land behind the bank: Low/high, flat/rising c. Evidence of frequent overbank flows: Sand splays/overbank erosion/crop damage d. Width of land threatened by inundation or erosion e. Presence of flood protection structures: Levees and dikes Bank features a. Bank outline: Straight/wide curve/narrow curve b. Bank heights and angles c. Bank failures mechanisms: Slab/rotational failures d. Bank stratigraphy: Material and thickness of the observed layers e. Vegetation: Distribution, size, type, and approximate age on the bank and in the channel f. Signs of erosion and bank instability: Tension cracks, collapsed banks, gullies g. State of erosion: Little/significant/severe/very severe h. Highwater and flood marks (debris trapped in vegetation) i. Presence and type of (nearby) bank protection structures: Groins/revetments j. Condition of bank protection structures: Good/reasonable/eroded in one or more places/being

undermined/destroyed River features a. Characterization of river reach: Lower reach/delta/middle reach/upper reach/meandering/

braiding b. Presence of terraces (inactive floodplains) and berms (active floodplains) c. Width and depth of river channel at the level of the floodplain, the top bank, berms and

terraces (if present) at about every 15 to 20 channel widths along the channel d. Nearby presence and significance of tributaries e. Flow velocity: High/medium/low f. Presence of strong currents, turbulent flow, eddies g. Approximate Manning roughness for the active channel, berms and floodplain h. Sediment load: High/medium/low i. Amount and size of sediment deposited at and just downstream of tributary confluences j. Bed material: Gravel/coarse sand/fine sand/silt/clay k. Bed stability of the river reach: Stable/degrading, presence of knickpoints & knickzones l. Condition (scour) and influence of nearby river structures (bank protection, bridges, drop

inlet structures, culverts, grade control structures, water intakes, pipelines) on the river flow regime and morphology

Sediment a. Sediment sources: banks/land behind the banks/watershed (upland) erosion b. Sediment carriers: tributaries/gullies/drainage ditches/river bed c. Sediment samples from low-lying hinterland d. Sediment samples from each stratigraphic bank layer e. Sediment samples from thalweg and some other points in a cross section f. Sediment samples from tributary mouth bars

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Questionnaire for local authorities and people (i.e. fishermen) Province: Community: Location (km): Date: Flooding a. How often does flooding occur? b. Are there very big differences between the heights and duration of the floods? c. What is the worst flooding in memory? Bank erosion a. How much bank erosion has occured in the last year? b. Since when does the bank erosion occur? c. Does the amount of erosion greatly vary from one year to another? d. How much damage has been caused by bank erosion in the last 10 years? River profile a. Has the river depth in front of the bank greatly changed in the last few years? b. Are there scour holes in front of the bank? c. Is the river depth in front of the bank subject to seasonal changes? d. Has the river bed been degrading over the last 10 years or is the river bed relatively stable? Currents a. How strong is the current near the bank? b. How often do turbulent flows and eddies occur near the bank? Waves a. What are the maximum wave heights which occur on the river? b. Are these waves caused by ships or do they occur during storms? c. How often do these waves occur? d. What are the wave directions? Winds a. When do the strongest winds occur? b. What is the direction and speed of these winds? c. What is the prevailing wind direction throughout the year? Other remarks Collect a. Maps b. Documents and interviews c. Dated pictures of erosion d. Drawings: cross-sections, protection structures e. Historical information (changes in time) f. Soil samples

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6. Types of bank protection The choice of viable alternatives from a seemingly endless variety of methods to protect a streambank against any of several causes of damage and failure is a difficult task. Therefore, as much knowledge, experience, and guidance as possible should be utilized from the efforts of others (past and present), the results of previous investigative work, and design directives (as described in the number of textbooks and design Manuals and as available from other sources). The following categories of streambank protection techniques, from which more specific items can be derived, are not mutually exclusive and often could be used in combination: 1. Direct bank protection with materials more resistant to erosion than the underlying soil due

to greater density, mat-type construction, or reinforcement of the bank soils (as with vegetation).

2. Spur Dikes (Groynes) or Hard Points or other devices to slow the flow velocities along the bank.

3. Groynes or other techniques to shape the channel alignment locally or extensively so as to direct the flow away from the bank or reduce sharp curvature of the channel.

4. Protection of the toe of the bank to prevent undercutting. 5. Grade control of the channel bottom. 6. Improving the structural stability of the soil mass of the streambank. 7. Controlling flow of water entering channel over its bank. Any substantial changes affecting the whole channel should be given very careful consideration so as not to initiate a "domino effect" of new problems resulting from an improper action to solve the original problem. If whole-channel improvements are necessary, the best scheme usually interferes the least with the natural stream condition. As much of the natural channel as possible should be used, and the water and sediment flow characteristics through the reach should be changed as little as possible. A wide variety of both natural and man-made materials are currently available to control bank erosion. These include rock riprap, concrete blocks in various configurations, concrete mats, vegetation schemes, etc.. All of these materials gave unique advantages and disadvantages depending upon the size of the area to be protected, the cause of the bank instability, the magnitude of hydraulic loading, the availability of the material, and the cost. Bank protection materials in high-energy environments (turbulent flow, high velocity, waves) must be placed on appropriate granular or fabric filters (geotextile) to prevent the loss of bank material to the penetrating currents and waves. Usually, in low-energy environment, quarry-run riprap (wide-graded rock from the quarry) of sufficient size and thickness might perform well without filter layer underneath.

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Figure 10: Failure mechanisms related to slope protection Rock is the most used material for protection against bank erosion, although the methods of application and design vary widely. It will likely continue to be the first choice of bank protection materials where material of sufficient size is available and affordable, because of durability, flexibility, easy repair, and other advantages. • A riprap blanket is flexible and is neither impaired nor weakened by slight movement of the

bank resulting from settlement or other minor adjustments. • Local damage or loss is easily repaired by the placement of more rock. • Construction is (usually) not complicated and so special equipment or construction practice

is not necessary (besides the sites with large depth and heavy currents). • Riprap is usually durable and recoverable and may be stockpiled for future use. • The cost-effectiveness of locally available riprap provides a viable alternative to many other

types of bank protection. • Riprap stability increases with increasing thickness as more material is available to move to

damaged areas and more energy is dissipated before it reaches the filter and streambank.

Figure 11: Design components of typical revetment

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Costs of protective schemes vary widely, depending upon the extent of the problem to be solved, the availability of locally available materials, and the size of the problem (project). The cost-effectiveness of riprap from a local source remains strongly competitive with other long-term protection types, and it is usually a very effective erosion protection device. In addition to riprap, the rock-dominated methods also afford some promise toward effectively controlling streambank erosion. For example, rock toes with suitable upslope vegetation function well in some situations. Similarly, the techniques of using rock hard points, jetties, and windrow provide adequate protection when properly designed, but some initial erosion should be anticipated before the units become effective. Gabions offer an effective bank protection technique where suitable riprap is not available.

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7. Techniques of bank protection Note: Usually, before placing a revetment the bank should be previously graded to a stable slope. Quarry-run rock protection consist of wide-graded-stone material with maximum size limited to about 0.5 m and relatively large percentage of fines; the recommended thickness is usually much larger than the traditional riprap to allow washing out of the fines and creating a natural (poor) filter. Riprap consist of durable, relatively small-graded stone material (D85/D15<2) with average size (diameter) defined by the local hydraulic loading, and usually placed in two layers on granular filter of geotextile (eventually with a cushion layer inbetween).

Figure 12: Application of riprap for bank protection Gabion protection consists of wire cages filled with small stone or waste brick material. Usually, filter material (or geotextile) is placed between the gabions and the original bank.

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Stone mattresses/Reno mattresses/Crushed stone and wire mat protection consist of natural or crushed stone (about 0.10m max. diameter) placed in a flat wire-basket., or protected by a weir mat (on the top) anchored to the underlying soil. Riprap-filled cells or grates consist of a cellular-type containment with bottom and top opening that can be square, hexagonal, etc, filled with fine riprap. Waste materials (minestone, slags, silex, building waste, etc.) are often locally available and (mostely) inexpensive waste products from the industry. They are placed in a number of layers to obtain the prescribed thickness. Possible environmental impact must be taken into consideration.

Figure 13: Transformation from riprap to stone pitching

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Concrete blocks and slabs Concrete mats Concrete-filled geo-mattresses (Fabriform) consist of a fabric envelope filled with pumpable sand and cement grout. A geotextile or a bedding layer are usually provided under the mat.

Figure 14: Block mats Soil-cement Asphalt (bituminous revetments) Windrow revetment consists of a mound of stone placed on the ground, or partially or totally buried, immediately adjacent and parallel to the general alignment of the eroding bank. As bank-line caving reaches the windrow, the stone is undercut, thereby falling down the bank and protecting the bank the bank against further erosion (see figure 15).

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Figure 15: Windrow revetment, definition sketch (US Corps, 1981)

Figure 16a: Reinforced revetment

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Figure 16b: Reinforced revetment (US Corps 1981) Reinforced revetment has a bank-line toe of erosion-resistant material placed riverward of the high bank, reinforced intermittently by stone-filled tiebacks extending landward from the toe into the riverbank (see figure 16). Earth core dikes are mounds of sand fill (or clay) extending riverward of the bank line and protected on the upstream face by a stone toe and covered by a (relatively) thin layer of stone. Composite revetment has a bank-line toe of erosion-resistant material, an upper bank treatment covering the zone of normal seasonal fluctuations, and a freeboard zone that is generally vegetated (see figure 17).

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Figure 17: Composite revetment Grout-filled paper bags consist of nylon reinforced bags filled with sand and cement grout and placed individually on a prepared slope. Transverse spur dikes (river groynes) is a standard protective technique in a concave bend of a meandering stream with (usually) noncohesive banks and insignificant suspended load. Several design attention points can be mentioned: • Spur dikes can reduce near-bank velocities to one-half of those that occur without a dike

field; • Spacing-to-length ratios as high as three may be effective in protecting concave banks with

spur dikes; however, the ratio was found to vary with discharge. Spacing-to-length ratio for specific projects can be determined by previous experiments in similar circumstances or site-specific model studies (or prototype pilot project).

• Speu dike root (section extending landward into bank) should be protected from scour caused by vortices set up along the upstream and downstream faces.

• The spur dike should be (usually) aligned perpendicular to the bank or current. • Aprons (bottom armour layer, usually fine rock) are effective in limiting the depth of scour

at the spur dike’s toe ( the point of maximum scour will move more downstream from the toe of the spur dike, improving the structural integrity of the spur dike).

• Existing equations for prediction of scour at spur dikes in concave bends are questionable and should be interpreted as indicative only.

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Figure 18: River groins Permeable spur dikes (or fences, vanes) The advantages and dis-advantages of permeable groins compared with solid groins or spur dikes can be summarized as below: 1. The vulnerability for floating debris and ice (United Nations, 1953, Central Board of

Irrigation and Power, 1989 en Federal Highway Administration, 1995); 2. Danger for the inland navigation when the groins are submerged. This holds also for

impermeable groins. (United Nations,1953 en Central Board of Irrigation and Power, 1989); 3. Less effectiveness to guide the flow then impermeable groins (Federal Highway

Administration, 1995). The authors have different opinions on the degree of bank protection provided by permeable groins compared to the effectiveness of impermeable groins. The effectiveness of permeable groins depends probably significantly on the sediment transport in the river. The most important advantages of permeable groins are: 1. Less deep scour holes, according to the Federal Highway Administration even no scouring at

all if the permeability exceeds 70 % (United Nations, 1953, Central Board of Irrigation and Power, 1989 en Federal Highway Administration, 1995);

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2. The flow pattern has less variations, and this benefits the navigation (United Nations, 1953, Central Board of Irrigation and Power, 1989 en Federal Highway Administration, 1995);

3. Less erosion at the connection of a groin and the bank. (Federal Highway Administration, 1995);

4. Permeable groins are cheaper to construct then impermeable groins (Central Board of Irrigation and Power, 1989);

5. Permeable groins have a wider construction window (Central Board of Irrigation and Power, 1989);

6. A short construction period (Central Board of Irrigation and Power, 1989). Hard points consist of two components: a short spur of erosion-resistant material extending from the bank riverward, and a root of stone placed in a trench excavated landward from the bank line to preclude flanking (see figure 19). The structure protrude only a short distances into the river channel. The structures are especially adaptable in long, straight (or slightly concave) reaches not subjected to high direct attack.

Figure 19: Hard point with section detail Refusals consist of erosion-resistant material placed in a trench excavated landward at the upstream end of each revetment segment to prevent flanking. Breakwaters are structures whose primary purpose is to protect the banks from any erosion that may be caused by wave action. Parallel spurdikes are longitudinally placed structures for guiding the stream; however, they may also fulfil the same function as breakwaters (reduction of wave action).

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Used automobile tires can be installed to provide protection in the form of either a wall (bulkhead) filled with sand and capped with concrete, or of a mattress placed on the bank, which was previously graded to a stable slope. Geobags filled with sand or clay

Figure 20: Geobags before and after collapse

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Figure 21: Concrete geomattresses before and after collapse Geosystems (geotubes, geocontainers, etc.) - geotube protection is a fabric tube placed parallel to the direction of the flow and filled with either sand or gravel (or sand-cement). Various sizes are available. When exposed, UV protection is needed. Note: geotubes can also be applied for construction of spur dikes and breakwaters. Tree retard systems generally consist of groups of trees cabled together, placed perpendicular to the bank line, and anchored in place using cables with fabricated weights. A small stone root is constructed into the bank line to anchor the landward end of the tree and protect the landward end of each retard from flanking by overtopping flows. Vane dikes are low-elevation, within-the-channel fills of stone or lower grade material that hold the high-velocity erosive flows away from the banks and encourage the accumulation of sediment on the landward side. The flow is allowed to course both ends and overtop the structure to create and preserve environmentally desirable shallow, braided channels.

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Figure 22a: Stream guiding by bandals

Figure 22b: Stream guiding by fences and/or vanes

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Fencing (or vane screens) of various configuration Sheet-piles and/or bulkheads Vegetative protection, consisting of either grass or shrubbery, is often provided in conjunction with some of the methods described previously. Protection consisting only of vegetation can be applied at low hydraulic loadings. In general, most bank protection techniques are not economically justified from the cost-benefit point of view. The choice of protection is usually dictated by the importance of problem and the financial ability. Considering optimisation, one option may be to perform only minimal protection first, then repair as necessary, e,g., windrow revetment, low-elevation structures, intermittent bank-line revetment, or hard points. Also, low-grade or waste materials may be satisfactory at certain conditions, e.g., minestone, slags or poor quality rock. However, nearly in all situations, the most important conclusion is to provide effective protection at the toe of the bank. Prediction of when, where, and extent of bank erosion and/or bank instability remains a difficult matter. The forces contributing to bank instability are generally (theoretically) known and (partly) understood; however, application of the theoretical principles to the real world (practice) are complicated by the many processes acting simultaneously throughout a given river reach. Streams displaying very active tendencies to erode their banks often seem to reverse themselves and display periods of relative stability. These processes will continue to make the prediction of erosion indeterminate, and most efforts to control the erosion will be based on after-the-fact information (systematic monitoring and analyse).

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Figure 23: Alternative toe protections

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28. Howard, A.D. (1992), Modeling channel migration and floodplain sedimentation in meandering streams, In: Carling, P.A. & Petts, G.E. (Ed.), Lowland floodplain rivers, Geomorphological perspectives, Wiley, pp.1-41

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The Hydraulic Engineering Department of the Road and Hydraulic Engineering Institute (DWW) of the Directorate-General of Transport, Public Works and Water Management (Rijkswaterstaat) is the advisory division for technique and environment for road and hydraulic engineering, which does research, advises and transfers knowledge on nature and environmental engineering of the physical infrastructure, water and water defence systems, and supply of raw construction materials, including environmental aspects. Rijkswaterstaat, Road and Hydraulic Engineering Division Van der Burghweg 1, P.O. Box 5044, 2600 GA Delft, The Netherlands Tel. +31-15-2518518 Fax +31-15-2518555/568 E-mail: dwwmail@dww.rws.minvenw.nl tawsecr@dww.rws.minvenw.nl K.W.Pilarczyk@dww.rws.minvenw.nl k.pilarczyk@planet.nl Internet: www.minvenw.nl/rws/dww/home/ www.tawinfo.nl (English, PUBLICATIONS downloads)