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Modelling Water and Sediment Fluxes in Steep River Channels: Case of Awash Basin

By

S. Golla1 H. EL-Sersawy2 A. A. Ahmed3 V. Vanacker4

1- Theme Researcher of STWMC, P.O. Box 5673, Addis Ababa Semunesh_golla@yahoo.com

2- Nile Research Institure, Elqanater El-Khiria, P.O. Box 13621, Egypt

Osma_elsersawy@yahoo.com

4- Postdoctoral Researcher, Flemish Counterpart of STWMC, Leuven, Belgium vanacker@geog.ucl.ac.be

Abstract Numerical models have become popular technological tools that may be useful in assessing quantitatively the river morphology in terms of its hydrodynamic and sediment fluxes. Rivers characterized by steep gradients present numerous challenges with respect to numerical modeling for a variety of reasons such as complex topography and the overall complexity patterns. In this paper, a new methodology is proposed for modeling the water and sediment fluxes in steep rivers utilizing the best available data. The methodology proposed utilizes a 2-D numerical model to determine the velocity and sediment distribution in the longitudinal and transverse directions. The methodology is applied to a case study, the Awash River in Ethiopia. The sediment carried by Awash River results from the degradation of its surrounding watershed area, causing sediment deposition in the Koka reservoir. In additions, sediment deposition along the river reach may cause flooding in some parts. The characteristics of torrential or mountainous rivers are different from alluvial rivers with respect to their irregular geometry such as width and slope, their non-uniform grain size distribution, etc. Unsteady hydrology such as flash floods and direct sediment input from mass movements make that the water flow is basically controlled by the section geometry and that sediment transport is basically controlled by the sediment input. In this regard, due to the steepness of Mountain Rivers and insufficient data, hydro dynamical models, which do not deal with supercritical flow conditions, have a limited application in mountain areas.

1 Ministry of Water Resources, Ethiopia. 2 National Research Institute, Egypt. 3 UNESCO Chair in Water Resources, SUDAN. 4 Katholieke University, Belgium.

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The paper presents the application of a hydro-dynamical flow and sediment transport model in the upstream part of the Awash River. The limitations and constraints of the model application for solving sediment transport in steep river channels are presented as well as some technical ways to improve the application of the model in the river. Key Words: Awash River Basin, Steep River, two- Dimensional Model and Sediment Transport

Introduction Land degradation in the watershed results in high erosion problems. This is causing increased sediment loads in the river system and accelerated deposition of sediment in reservoirs and irrigation channels. This result in great socio -economic and environmental losses, both on-site due to a decrease in soil fertility and off-site due to increased maintenance costs in hydropower plants and irrigation systems and sediment deposition along the river channel causing flooding in the surroundings. It is clear that research of sediment transport and watershed management is a key to improve the management of the entire Nile basin. Therefore, data on sediment production and transport are necessary to improve our understanding of the river system. The processes of erosion, entrainment, transportation and deposition in a river catchment are complex. The detachment of particles in the erosion process occurs through the kine tic energy of raindrop impact, or by the forces generated by flowing water. Once a particle has been detached, it must be entrained before it can be transported away. Both entrainment and transport depend on the shape, size and weight of the particle and the forces exerted on the particle by the flow. When these forces are diminished to the extent that the transport rate is reduced or transport is no longer possible, deposition occurs. Sediment is transported in suspension, as bed load rolling or sliding along the bed and interchangeably by suspension and bed load. The nature of movement depends on the particle size, shape, and specific gravity in respect to the associated velocity and turbulence. Under some conditions of high velocity and turbulence, e.g. high flows in steep-gradient mountain streams; cobbles are carried intermittently in suspension. Conversely, silt size particles may move as bed load in low-gradient, low-velocity channels, e.g. drainage ditches. Even in transport, whether as bed load or in suspension, sediment may cause problems. The products of erosion may be deposited immediately below their sources, or may be transported considerable distances to be deposited in channels, on flood plains, or in lakes, reservoirs, estuaries, and oceans. When stream flow enters a natural lake or reservoir, its velocity and transport capacity is reduced and its sediment load is deposited. In natural lakes that have no outlets the total incoming sediment load is deposited. In artificial lakes with outlets, e.g., reservoirs, the amount deposited depends on the detention storage time, the shape of the reservoir, operation procedures, and other factors. As stated by Brune, 1953, in most storage reservoirs of modern design more than 90% of the incoming load is usually trapped (ASCE, 1977). Sediment may cause severe damages depending on the amount, character, and place of deposition. Deposits that occur on floodplains create numerous types of damages to crops and developments. The deposition of sediment in drainage ditches, irrigation canals, and in navigation and natural stream channels creates serious problems in loss of services and cleanout costs. The deposition of sediment in our natural stream channels has greatly aggravated floodwater damages. The deposition of sediment in channels decreases the

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channel capacity and the flood-carrying capacity. This results in higher and more frequent overflows. This study focuses at sediment transport in steep river channels. The characteristics of Mountain Rivers are different from alluvial channel: they are characterized by an irregular geometry, non-uniform grain size and unsteady flow conditions. The sediment transport in these channels is mainly controlled by direct sediment input e.g. from gullies or mass movements. The Awash River catchment, located in the Ethiopian, was selected as a case study within the FRIEND/NILE project and various studies related to rainfall/runoff modeling, drought and high flow regimes are being carried out in this catchment. Regarding the steepness of the river channel, a short natural reach of approximately 1 km was selected between Hombole station and the Koka reservoir. However, the model can later be extended to other watersheds or other river sections. The objective of this study is to study (i) sediment transport along the river reach upstream part of the Koka reservoir, and (ii) bed channel changes due to sediment deposition. A two-dimensional hydraulic model was selected and applied for the selected study reach. Materials and Methods Study Area The Awash basin is bordered by the catchment of the Wabi Shebelle River to the South, the catchment of the Blue Nile to the west, the inland depressions of the Dankail desert to the north, and Somalia to the east, Fig.(1). The Awash River originates from the high plateau some 150 km west of Addis Ababa, at an altitude of about 3000m. It then flows eastwards through the Becho plain areas and is joined by several small tributaries before entering the Koka reservoir, which is considered as the downstream limit of the upper Awash basin. The basin extends as middle and lower Awash with a total catchment area of 110,000km 2. The study area is above Koka reservoir that encompasses an area including the capital Addis Ababa, other medium and minor towns, agricultural and grazing land and also swamps and flood plains entering the Koka reservoir at an elevation of 1500 m.a.s.l. Land use in the catchment area is mainly agricultural land used for rain fed crops and grazing lands. There are some plantations scattered in the catchment. The rainfall pattern is bimodal with two rainy seasons each year. The first short rainy season is from March to May, and the second main rainy season starts in July and lasts till September. During the dry season, i.e. from October till February, the prevailing winds are anticyclone winds, mainly blowing from the northeast. At other times of the year, winds are variable in direction and strength, but are in general upper rain-bearing air currents coming from the southwest.

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Fig. (1) Location map of the study area

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Data Acquisition In order to estimate sediment transport in the upper basin part of Awash River, topographic, bathymetric, hydrological, catchment and channel characteristic data were collected. Table (1) gives an overview of the data that were used in this case study, and indicates the source of the data.

Table (1) Summary of the Utilized Data

Input Data Utilization Source Remark

Topographic and bathymetric Data

Topography Creation of finite element network

1/50,000 Topographic Map UTM 37, Clarke 1880

Bathymetry

Creation of finite element network

Cross-sectional data (distance vs.

elevation)

Additional data were taken from the field to supply the original bathymetric data

(river width and elevation), and some interpolation was done between the measured cross-sections to create the

finite element network.

Hydrological Data

Discharge

Upstream boundary conditions for flow module, calibration

of the model

Flow data at Hombole Station

(1968-2003)

Water Surface Elevation

Downstream boundary

conditions, calibration of the

flow model

Calculation of the water

surface elevation is based on slope-velocity profiles

Stream velocity Calibration of the model coefficients

Stream velocity measurements at Hombole Station

Sediment loads

Upstream boundary conditions for the sediment transport

model

Sediment rating curve based on sediment samples taken at Hombole Station

Channel Characteristics

.

Evaluation of bed friction coefficients and eddy viscosity.

Field measurements, evaluation of the

riverbed characteristics

Meteorological conditions

Calculation of wind stresses at the water

surface

Weather station at surrounding area

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River cross -sections were surveyed in 1997 for the entire reach from Hombole Station to the Koka reservoir, and the data were measured as distance vs. elevation. A photomap of the cross section of Awash River at Hombole station is presented in Fig. (2). Hydrological data are used for defining the upstream and downstream boundary conditions for the application of the hydrodynamic flow and sediment transport Model. Flow and sediment load data were extracted from flow and sediment measurements of the Awash River at the Hombole Station, Fig.(3).

Fig (2) Cross -sectional PHOTO of Awash River at Hombole station

0

50

100

150

200

250

300

350

400

Jan

- 68D N O S A J J M A M F

Jan

- 79D N O S A J J M A M F

Jan

- 90D N O S A J J M A M F

Jan

- 01D N O

Time in Months and Years

Flo

w in

m3/ s

Average Flow Data

Fig. (3) Mean monthly discharge of Awash River at Hombole Station (1968-2003) A relationship between flow discharge and suspended sediment concentration was established to obtain a time series of suspended sediment databased on the existing time series of flow rate data. The sediment-rating curve is given in Fig.(4).

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Hydro Dynamical and Sediment Models The SMS program was used to solve numerically water flow and sediment transport equations. This program has different interfaces and uses hydro-dynamical modeling for the computation of sediment loadings .

y = 87.573x1.3484

R2 = 0.746

0

100000

200000

300000

400000

500000

600000

1 10 100 1000Discharge in m3/s

Sed

imen

t con

cent

ratio

n to

n/da

y

Fig (4) Sediment Concentrations Rating Curve at Hombole Station A two-dimensional finite element network was created in GFGEN. The RMA2 component was used for the simulation of the surface flow, and models sub-critical, depth-averaged flow for both steady state and transient hydraulic states. SED2D was used to compute sediment loadings and bed elevation changes using the input of the hydrodynamic solutions computed by RMA2 (Finite Element Surface Modeling System, User’s Manual). The governing equations are based on the continuity and the momentum laws, which represent mass conservation and momentum balance in the two, coordinate directions respectively. The flow continuity and momentum equations are used by RMA2 model to solve the depth-integrated calculation of water flow. Networking and Mesh Generation of the Modeled Area A preliminary finite element mesh was developed, using the Map and Mesh Module of the SMS 8.0 software package. A network of finite element mesh with quadrilateral elements constructed from nodes. The construction of the mesh is based on the image map, the bathymetric and surveyed map of the area. Using the SMS Model, the finite element mesh generated and the bathymetry data interpolated onto the mesh. Accordingly, the mesh contains elements and nodes with an elevation variation of 1686.5 – 1689.5 (m.a.s.l.). The finite element mesh including the elevation data (m.a.s.l.) is presented in Fig. (5).

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elevation

1686.70

1687.26

1687.82

1688.38

1688.94

1689.50

Generated Mesh

Fig (5) Generated Mesh for the modeled area Model Calibration and verification The model (Both a hydrodynamic and a sediment transport) was applied on a small reach of about 1km, where the sediment concentration and bed changes are significant. Thus, the hydrodynamic modeling simulates the water surface elevation, the water depth and the velocity in the reach at different time steps. The sediment transport model simulates sediment concentration, water depth, and shear stress and bed changes due to the sediment inflow. We run both models using real hydrographic data for a time period of one month, where sediment transport is expected to be very high. A time step of half a day was chosen for the simulations.

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It is clear that the modeling has been constrained by the limited availability of topographic, bathymetric and hydrological data. In this study, we combined several methods to get long with the limited data available (Table 1). Hydrodynamic Model Calibration The measured inflow discharge data, water elevations data, and velocity of flow were inspected for selecting a time period for the model calibration. The data was used at the boundary condition of the study reach for the calibration process of the hydrodynamic (RMA2) condition. It covers the period from 1998 to 1999. The data collected in the period 2003 was used for model verification, in particular that recorded in August during the maximum discharge time. Table (2) shows the comparison of the computed and measured results at the calibration stage of the model for the years 1998 and 1999.

Table (2) Comparison of measured and simulated data

Measured data

Comparison of Measured and Simulated data

Date of Measurement

Depth Simulated Measured Difference

Remark

Aug. 14, 1998 6.54 6.336 6.54 0.204 The measured data is on the higher side

Aug. 13, 1999 6.67 6.47 6.67 0.20 “ Hydrodynamic Modeling Results Figs (6) show the results of the hydrodynamic model runs for the date Aug 13 (time step 48hr) and 19/2000 (time step 192hr using the data available. Fig 7 shows the comparison of measured and simulated water depth from August 11-21/2000. The outputs are the mean water depth and the average velocity in the selected reach of Awash River.

Water Depth (m) at Time step 192hr

0.72

1.34

1.96

2.58

3.20

3.82

Fig (6a) Water Depth in the selected reach of Awash River

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Comaprision of Measured and Simulated Water Depth

3

3.2

3.4

3.6

3.8

4

4.2

4.4

4.6

4.8

8/11/04 8/12/04 8/13/04 8/14/04 8/15/04 8/16/04 8/17/04 8/18/04 8/19/04 8/20/04 8/21/04

Date in Days

Water D

epth (m

)

Measured Water Depth

Simulated Water Depth

Water Velocity (m/s) at Time step 192hr

0.29

0.69

1.09

1.48

1.88

2.27

Fig.(6b) Average Velocity in selected reach of Awash River

Water Depth (m) at Time step 48hr

1.76

2.34

2.92

3.51

4.09

4.67

Fig (6aa) Water Depth in the selected reach of Awash River

Water Velocity (m/s) at Time step 48hr

0.56

0.90

1.24

1.58

1.91

2.25

Fig. (6bb) Average Velocity in selected reach of Awash River

Fig. (7) Comparison of measured and Depth

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Sediment Modeling In this part of the paper the SMS model was used to predict the sedimentation situation i.e. bed change at the channel and bed shear stress in the selected Reach of the Awash River. The results for time step 48hr and 192hr are presented in Figs.(8a,8b,8aa,8bb).

bed change : 192.000

-1.01

-0.76

-0.51

-0.26

-0.01

0.25

Fig.(8a) Bed Change in the selected reach of Awash River

Fig. (8b) Bed shear in the selected reach of Awash River

bed change : 48.000

-0.507

-0.390

-0.273

-0.156

-0.039

0.078

Fig. (8aa) Bed Change in the selected reach of Awash River

bed shear : 192.000

0.6

9.2

17.9

26.6

35.2

43.9

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The results of the model runs for August 2000 are given in Figure 6 and 7 and 8. They clearly indicate that the stream velocity decreases in the downstream direction. The riverbed changes significantly during the flood season. Bed level changes are largest in the downstream section, as the slope of the river channel decreases downstream. The Bed level changes along the curved part of the river channel indicate that erosion occurs at the outer bend of the river channel and that deposition is occurring at the inner bend.

bed shear : 48.000

1.7

8.3

14.9

21.5

28.1

34.7

Fig.(8bb) Bed Shear in the selected reach of Awash River

Conclusions and Recommendations In this paper, we presented the application of a water and sediment transport model in a river channel in the Ethiopian Highlands. This case study of the Awash River in Ethiopia indicates that the quality of the model results of the two-dimensional hydrodynamic flow and sediment transport model highly depends on the quality of the Topographic, bathymetric and hydrological data. However, even with limited data available, the combination of expert knowledge and basic hydrological assumptions allowed us to apply an advanced two-dimensional hydrodynamic flow and sediment model in a complex topographic setting. The comparison of simulated and measured data indicates that the simulations are representative for the water and sediment transport during the flood season. Acknowledgements This paper was prepared based on the research activities of the FRIEND/Nile Project which is funded by the Flemish Government of Belgium through the Flanders-UNESCO Science Trust Fund cooperation and executed by UNESCO Cairo Office. The authors would like to express their great appreciation to the Flemish Government of Belgium, the Flemish experts and universities for their financial and technical support to the project. The authors are indebted to UNESCO Cairo Office, the FRIEND/Nile Project management team, overall coordinator, thematic coordinators, themes researchers and the implementing institutes in the Nile countries for the successful execution and smooth implementation of the project. Thanks are also due to UNESCO Offices in Nairobi, Dar Es Salaam and Addis Ababa for their efforts to facilitate the implementation of the FRIEND/Nile activities.

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REFERENCEES ASCE, 1977. “Sedimentation Engineering, Manuals and Reports on Engineering Practice”, 54, 9-83 & 317-349. Chow, V. T. 1964. “Handbook of Applied Hydrology”, McGraw-Hill Book Company, New York. Chow, V. T. 1983, “Open Channel Hydraulics”. McGraw-Hill, London, 680 pp. 2005, Sediment Transport Technology. EVDSA, Dec., 1989, “Master Plan for The Development of Surface water resources in the Awash Basin”, Vol 4, Annex A and Vol 6, Annex E Finite Element Surface Modeling System, “Two-Dimensional Flows in a Horizontal Plane”, User’s Manual, Highway Research Center, 6300 Georgetown Pike, McLean, Virginia 22101-2296.