urban drainage problems in the humid tropics cedo...

Hydrology of Warm Humid Resions (Proceedings of the Yokohama Symposium. July 1993). IAHSPiSl.no. 216, 1993. 377

Urban drainage problems in the humid tropics

CEDO MAKSIMOVIC, ZORICA TODOROVIC IRTCUD, Faculty of Civil Engineering, University of Belgrade, PO Box 895, 11000 Belgrade, Yugoslavia

BENEDITO P. F. BRAGA Jr IRTCUD Regional Subcentre for Humid Tropics, Centro Technolôgico de Hidraulica, University of Sâo Paulo, PO Box 11014, SP 055499, Brazil

Abstract The paper aims at giving a brief overview of the contemporary urban drainage problems and technologies by placing a strong emphasis on specific problems emerging in areas of humid tropics. Since the humid tropic areas cover a very wide range of various geographical, climate, social and cultural regions with different traditions in taking care of their water resources, this overview will provide the basics which should be relevant and applicable in all parts of the humid tropics of the globe. The need for learning on site specific solutions, in order to establish the appropriate technologies that will match the local needs in the best way, will be sought. Looking towards the future, the need for gathering appropriate and reliable local data which are invaluable for any respectable project is strongly emphasized. An integrated water management philosophy will be observed throughout the paper for determining the position of the system for urban drainage within the adjacent water related disciplines.

INTRODUCTION

All human settlements both in recent and modern civilization face a problem of protection against floods caused by local storms. Traditional storm drainage systems were conceptualized on the philosophy "fast discard of storm water". Some of the ancient civilizations practised urban drainage in a fashion which most likely needs to be reinvented and as a concept reapplied under the title "integrated water management in urban areas" in which the concept of source control play a very important role.

Modern technology for solving engineering problems based on computer techniques and a high level of information processing enables fast transition of knowledge for problem solving from one country to another, but the proper application of the knowledge available in some developed countries requires the local potentials (from the country in which it is to applied) to be activated properly. More precisely, solving the problems of integrated urban water management requires local inputs such as historical rainfall and surface water data, rainfall-runoff data, activation of traditional drainage and water re-use techniques, training of local personnel for proper maintenance and advanced management of the systems and technology, etc. Without local support the results obtained only by transfer of tools (software, technology, etc.) lead to questionable results, and often to complete failure.

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378 Cedo Maksimovic et al.

Aesthetic

Ecological

Floods i

Floods i

+ Economic

+

+

Tim

e ^

) Developm

ent p>

+ Economic

+ Aesthetic

+ Ecological

H 3 o

Values

7 Values

Fig. 1 Perceived values of water in urban areas (UNESCO, 1992).

At any rate, the recent approaches in coping with urban storm drainage problems require a special emphasis to be placed on an "integrated" approach and on the need for involving the reliable local potentials.

GENERAL OVERVIEW OF CONTEMPORARY URBAN DRAINAGE

Urban storm drainage in transition

A continuous increase of urban population and further growth of urban agglomeration is expected in the world. By the end of this century, more then 50% of the world's population will live in urban areas. The ever increasing percentage of the world's population living in urban areas urges the urban planners and the other experts dealing with water problems to re-examine the existing policies and practices in coping with various problems created by the unwanted quantity and quality of storm water and its temporal and spatial distribution. Failing to conceptualize the incorporation of storm drainage in the phase of early urban development results in a very expensive project at a later stage or even makes it impossible to implement it at all. The current policies in creating the master plans in urban water supply, sanitary and storm drainage, treatment and disposal of water into receiving bodies, have to be re-examined in the sense of the possibilities offered by new technologies and expertise. The problems of the developing countries, where most of the cities do not have an appropriate infrastructure, are of special interest. Traditionally, urban storm drainage has had relatively low priority among the other water projects in urban areas. Figure 1 presents the change of importance of the various fields of city water projects with development.

At a low level of development the highest priority is usually given to water supply. The other elements of the urban water system such as storm drainage and aesthetic,

Urban drainage problems in the humid tropics 379

or environmental aspects, have lower priority. Urban floods because of low level development cause also lower damage. As a country reaches a higher level of GNP the other aspects become more important. Consequently urban floods cause more severe damage in the highly urbanized areas in developed countries (as shown in Fig. 1).

Unfortunately some countries that have been subject to rapid development during recent decades have not developed their potential for coping with urban floods appropriately. Failing to cope with those problems could pose a serious problem for further development (Maksimovic, 1991).

The major problems of storm drainage in developing countries are caused by: — a rapid increase of population living in urban areas (Fig. 2); — a low level of awareness of the real danger and concern; — the absence of long-term strategies and master plans; — the fact that technical solutions based on local expertise are usually out of date,

or that imported "high technology" solutions are without proper involvement of the local professional community;

— the absence of nonstructural support measures; — improper maintenance and bad management of the system and of flood fighting.

In both developing and developed countries the concern for environmental protection has raised the level of awareness not only of the flood and health hazards but also of the other aspects of urban drainage such as: — flood prediction and prevention; — protection of receiving waters against pollution; — coping with the stochastic nature of flood fighting by both structural and

nonstructural measures; — incorporation of in-the-catchment quality control and runoff reduction.

In modern civilizations one can find technologically advanced countries suffering from both insufficient or improper drainage systems, and also lacking the methodology for coping with the problems (analysis, design, rehabilitation, maintenance, etc.) which is very much out of date. On the other hand there is a lot to be learned from some areas in which simple and reliable systems are based on classical approaches and on the appropriate resources and techniques locally available especially for nonstructural measures (Fig. 3). The concept of urban storm drainage itself has changed from purely collection and fast disposal of surface water to an important part of an integrated water

1965 1970 1980 i 1990 2000

I—i. projected Fig. 2 Increase of population living in urban areas (UNESCO, 1992).


Management measure

Purpose Quantity Quality Control Control

Reservation of land for recreation and other compatible open space use Control of land use outside flood-prone areas Flood insurance Floodproofing of existing or proposed structures and facilities Relocation or demolition of structures and facilities Slope and swale treatment Community utility and service policies Inspection and maintenance program Emergency action program Surface water management manual Education and training

X X

X X X X X

X

X

X X X

X X

Fig. 3 Nonstructural measures available for controlling the quantity and quality of urban surface water.

management system in an urban area. The first status mentioned above can be called a "single purpose - single means" approach (Walesh, 1989).

It should also be noted that good results can only be achieved with the proper balance of investment in both structural and nonstructural measures at the right period (before big damage occurs). For example in Fig. 4 the increase in the performance of storm runoff is shown in combination with the developments achieved by structural and nonstructural measures. In some countries (Brazil for example) the role of nonstructural measurements is strongly emphasized. As for the protection of receiving

Storm drainage peak flow (or runoff volume)

Capacity of the system sufficient

Period of the insufficient capacity of the system

T O T A L

Total yield of the catchment for a given return period

STRUCTURAL COVERAGE

S NONSTRUCTURAL

COVERAGE 4

Time

Period of investment in structural measures

Period of investment in nonstructural measures

Fig. 4 Meeting the growing needs of storm drainage by combination of structural and nonstructural measures.


waters, significant changes are occurring which affect both the concept of design of urban storm drainage systems and the concept of its interaction with the other physical processes in the catchment. Large metropolitan areas in humid tropical climates have additional specific problems. Because of high temperatures, the growth of bacteria in receiving waters is more intensive and faster. This poses an additional threat to the humans as well as to other living creatures. This threat multiplies during flood periods.

The earlier philosophy of meeting certain criteria of dilution in receiving water bodies is being replaced by the concept of integrated water quality management in cities and in catchments. Emphasis is being placed on the source control and in many cases treatment of the most polluted part of the storm water. It has been realized especially in large metropolitan areas that by only building the drainage network storm drainage problems cannot be solved efficiently, or that this approach leads to prohibitively expensive systems.

The alternative solution, such as those shown on the right-hand side of Fig. 5, have been applied for efficient source and quantity control. The new storm drainage systems which are either (partly) designed and constructed from the beginning (new cities) or developed by the rehabilitation of the existing ones, will have to take into consideration the surface water quality control measures and to incorporate them into the system for water quality control in the catchment and in the receiving water bodies.

Some examples applied in new cities (Kohoko New City, Japan, and the rehabilitation of the Emscher River Basin (1989) are very good illustrations. They document the large potentials as well as impose the possible failures that have to be prevented.

Fig. 5 The concept of integrated water quality management in cities and in the catchments.


Phases in development of urban storm drainage systems

In the gradual conversion of the human settlements from rural and small urban to large urban (and even to megalopolis) areas, urban storm drainage is also subject to gradual conversion from almost natural streams to advanced systems based on computerized forecasts and both quality and quantity control. This transition is shown in Fig. 6.

Unfortunately, the need for treating the urban storm drainage system with all its complexity has not yet been fully realized in many countries. In the years to come water quality aspects will have to be integrated into storm drainage systems in a way similar to sanitary sewers. Many cities in humid tropical areas already exist in heavily polluted environments, and have heavily polluted receiving waters; and many other cities are threatened by this problem in the very near future.

U R B A N I Z A T I O N

Natural catchment

• natural channels streams

• water courses

Partly sewered

• man-made ditches open channels

• sewers

Combined sewer systems

open channels combined sewers overflows treatment facilities

Fully separated sewer systems

separate and trunk sewers treatment facilities outfalls

Advanced drainage systems

separate sewers trunk sewers infiltration ponds storage basins etc.

Fig. 6 Different phases in development of urban drainage systems.

HUMID TROPICS - WHAT IS DIFFERENT?

Geographical position and general characteristics

From the viewpoint of climatology, the humid and subhumid tropics have high absorption of heat from the sun and low emanation compared with other climate regimes. Abundant annual rainfall and high mean monthly temperatures make up the principal climate features, although seasonal and spatial variations are present in various regions. Solar energy as the driving force of atmospheric circulation influences the hydrological cycle more strongly then in other regions on the earth. Various definitions of the humid and subhumid tropics have been developed has served as the basis from which other classifications have evolved. Thornthwaite (1948) proposed the concept of water balance and potential évapotranspiration in his classification.


More recently Chang & Lau (1983) reviewed definition schemes using wet month and hydrological growing season criteria. Various criteria are used to distinguish between the humid tropical regions and the others. The definition scheme of Chang & Lau (1983) based on the thermal, wet month and hydrological growing season criteria is shown in Fig. 7. According to Chang & Lau's (1983) classification, the humid and subhumid tropics are geographically located between the tropics of Cancer and Capricorn. The region extends approximately between 10°N and 10°S of the equator. It includes the Amazon basin, central America, southern Mexico and the Caribbean islands, the Congo basin, Mozambique, Madagascar, Kenya, Guinea coast of Africa, the peninsulas and large islands of southeast Asia, part of India, northeast Australia, New Guinea and Pacific islands from Hawaii to Caledonia. According to this classification the region is further divided into four climate subregions based on the duration of the wet months. The considerations applied in this paper will be also applicable in the neighbouring regions which have similar climate or more often similar other conditions (cultural, economical, etc.) that create similar storm drainage problems in the urban areas (Hufschmidt & Kindler, 1991).

Fig. 7 Climate definition scheme developed by Chang & Lau (1989).

It is not easy to distinguish precisely between different climate regimes in the tropics. No single classification is both general and specific enough to serve all purposes, and any defined boundaries of the tropics are prone to fluctuation. For convenience in discussing water resources and hydrological aspects, three subdivisions of the tropics proposed by Barrow (1987) are adopted in this paper. They are (1) the humid (wet or equatorial) tropics; (2) the subhumid (subhumid or wet and dry) tropics; and (3) the tropical drylands (savannas, semiarid and arid tropics).

The humid tropics are characterized by a wet climate for more then seven months in a year. They are mainly found within a narrow band between 5° and 10° poleward on either side of the equator. In some east coast areas, it may extend as much as 20° or even 25° from the equator. On average, annual rainfall exceeds 2000 mm (and even 10 000 mm in some regions), and, at elevations below 900 m, year-round temperatures are about 27°C. Rainfall varies little from one month to another. Convection is the main rain-generating mechanism. The infrequent windy periods are characterized by low wind speed. Precipitation nearly always equals or exceeds potential évapotranspiration, and there is no marked dry season in the year. A dense network of rivers with stable flow exists in the humid tropics. The upper Amazon basin of


South America, the northern Congo basin of Africa, and the southern Malaysia Peninsula and Indonesian islands of southeast Asia are representative of the humid tropics in the world (Hufschmidt & Kindler, 1991).

The subhumid tropics may be described as the seasonally wet and dry tropics, where there is a wet season of 4.5 to 7 months in a year. During the wet season, precipitation often exceeds évapotranspiration. Subhumid tropics are most extensive on the windward coasts of south and southeastern Asia (primarily in India, Bangladesh, Burma, Thailand, and the Philippines) but also occur in coastal regions of western Africa and northeastern South America. In southeastern Asia, seasonal reversals of winds and topography create favourable rainfall conditions. These "monsoon islands" receive much of their rainfall from the summer monsoon. The main hydrological feature of the subhumid tropics is the presence of many permanent rivers with significant seasonal variations due to snowmelt from higher elevations, and interannual flow variation. Hydrological components vary markedly from one season to another. The Irrawaddy and Mekong rivers are typical subhumid tropical rivers in Asia. In this area, large rivers like the Irrawaddy and the Mekong have gentle floods, while small rivers have flash floods. A crop of rice is grown during the wet monsoon in the gently flooding water, which does not exceed 5 to 10 cm a day. However, severe floods are often a threat in theses regions, as experienced in the 1987 and 1988 flood disasters in Bangladesh.

Tropical cyclones (hurricanes or typhoons) occur frequently in some areas of the subhumid tropics such as in the eastern Philippines, the Pacific islands, Bengal Bay and Caribbean islands. The distinctive hydrological feature related to cyclones is the frequent occurrence of tropical storms and violent floods which exceed the magnitudes and rates of increase of floods generated by other types of precipitation. They multiply the increase of the standard storm drainage systems.

In general, water is more likely to be abundant in humid and subhumid tropics then in other climate areas and some of the world's largest rivers are located in the humid and subhumid tropics.

However, the different rain generating mechanisms - convection, convergence, and tropical cyclones, or their combinations produce complex rainfall and runoff conditions. Spatial and temporal variations in rainfall are pronounced in the subhumid areas. Frequency distributions of daily runoff are highly skewed. Rainfall intensities and associated runoff in tropics always tend to be high and, accordingly, much of the rainfall does not become available to agriculture especially where soil infiltration rates are low. Over much of the subhumid tropics, rainless periods of sufficient length to hinder crop growth exist, while for the rest of the time disposal of excess precipitation is a problem. The high rates of evaporation and transpiration in the humid tropics cause soils to dry out soon after rainfall. High temperature tends to produce "hungry" soils with less nutrients. Tropical storm water runs rapidly over the ground surface causing soil erosion and flooding. As for the groundwater resources in the humid tropics, because of the prevailing abundant rainwater and surface water, they are underdeveloped compared with those in temperate regions. Data on groundwater resources in the humid tropics are scant. Concerns have been expressed about the often-contaminated surface water quality in the humid and subhumid tropics. Stronger photosynthesis and bacterial activities and higher temperatures in the larger and slower moving rivers are distinctive factors affecting tropical river quality.


Urban areas (and especially the large cities) in these regions, are characterized by air temperatures that are much higher then in the adjacent suburban and rural areas. These "thermal peaks" could act as the orographic barriers that could enhance rain extraction from clouds and heavy storms. There is some evidence of this phenomenon but further studies are needed for its full understanding (Monteiro, 1986).

From the point of view of urban storm drainage there are many differences between the regions with temperate climates (which are major source of data and technology in which these problems are being coped with properly during recent decades) and the humid tropics. The major differences are shown in Fig. 8, together with the relevant effects, and are as follows: (a) Meteorological conditions (microclimate, temperature, daily variations, etc.).

Difference:

Meteorological conditions in large metropolitan areas

Storm characteristics - season, duration, depth,

areal distribution, temporal distribution

Vegetation cover and seasonal variation

Building practice Retention capabilities

Interaction with solid wastes

Existing systems, inadequate or outdated

Economical potentials low

Lack of qualitied staff and' reliiable data

Lack of all kinds of data

Uncontroled construction poor soil protection after deforestation cause severe sedimentation problems

public awareness very low Priority in investment in these project low as well

Cultural, educational, traditional, hygienic conditions

Effect:

- Conditions for formation development and profile of storms are different

- Design criteria have to be established

- Response of the system different - Hurricanes can occur

- Separate modules for infiltration and more rigorous calibration

- Initial losses vary - Surface depression different - Surface runoff altered

- Carrying capacity of receiving waters reduced

- Pollution hazard increased

- Need for integrated approach - Low priority

- Need for establishment of pilot study areas and continuous training

- Design is unreliable - Need for systematic and organized data collection

• Appropriate methods have to be sought

- Appropriate priority measures have to be invested

- Sedimentation has to be prevented

- Public awareness needs to be raised

Fig. 8 Major differences between humid tropics and regions with temperate climate pertinent to urban storm drainage.


(b) Storm characteristics, seasonal variations, duration, rainfall depth, temporal and areal distribution etc. (see the next section).

(c) Vegetational land cover (absence of seasonal variations modifies the surface detention capacity. Soil characteristics are different as well and they affect also the difference in the infiltration processes.)

(d) Building practices in many cases modify surface retention. (e) In significant parts of many cities there is no solid waste collection system. Wastes

are dumped into streets and in open channels and usually lifted during heavy storms. High contents of floating objects block bridges and additional adverse flooding spreads (Jakarta, Rio-de Janeiro).

(f) Existing systems (if any) are usually designed for much smaller areas and are usually inadequate or outdated (Jakarta, etc.).

(g) The economical potential is not high and priority in investing in these systems is very low.

(h) General lack of any kind of appropriate data. (i) Lack of qualified staff and expertise in consulting and contracting companies. (j) Soil erosion problems from bare and poorly protected soils after deforestation, or

by careless construction. (k) Significant parts of cities are occupied by slums that introduce a range of problems

to city planners. Figure 9 shows a typical situation in the region. Some of the above factors require more attention because they play a crucial role.

(a) (b)

Fig. 9 (a) Surface drainage channels used for waste dumping, (b) Blocking of bridges and overflows causes floods in upstream areas.


Climate conditions and relevant storm pattern

General climate conditions in the four subregions (shown in Fig. 7) are well documented (e.g. Hufschmidt & Kindler, 1991; Landsberg, 1986). According to these reports the annual average of rainfalls are shown in Table 1.

Unfortunately, the annual average and the relevant statistical distribution data do not provide sufficient information for the reliable design and management of storm drainage systems. These systems are usually designed to carry the runoff caused by 2-5 years return period storms of short duration (10-200 minutes). The types of rainfall data and analysis are shown in Fig. 10. For both design and real time control of advanced systems the time resolution of rainfall data needed are of the order of one (at most five) minutes. Data of this quality that enable the information needed such as shown in Fig. 11 are rarely found in humid tropical countries. It has been proved by many studies that these data are very much site specific and transfer of data from similar regions or application of regional analysis is not a very suitable method for obtaining them. They even vary within one particular city. Information on seasonal variations (Fig. 12) are of some use, but they are still insufficient.

The only reliable method is the collection of storm data at sufficient time resolution and at several micro locations, especially in big cities. Modern raingauges (tipping bucket or similar) that incorporate a memory unit are available at affordable prices for these purposes.

Tropical storms (hurricanes) are also specific features of some areas in those climate zones. No reliable storm drainage can be designed to sustain a hurricane. Flooding is to be expected, but the consequences can be decreased by a properly designed system and by the application of flood warning and storm control measures.

In Fig. 13 the coastal areas effected by tropical storms are shown (Landsberg, 1986) whereas the monsoon affected land areas are shown in Fig. 14.

Urban storm drainage systems are normally designed for storms of 2-5 years return period and serious flooding problems are to be expected in the shaded areas once the cities become dense enough to generate high runoff peaks (Fig. 15). Although the high runoff peaks cannot be conveyed by standard underground system design for 2-5 years return periods, it is important to analyse the consequences of storms that are much stronger. The philosophy of dual drainage can provide significant relief. Although one should again underline that the success of their application is bound by the availability of reliable local data for both input to models and for their calibration, the modern storm drainage simulation packages can perform this kind of analysis.

Soil, vegetation and traditional methods of urbanization

Soils in the humid tropics are highly variable as they are a function of the many different microclimates and associated weathering processes. According to Balek (1983), soils in most parts of the tropics are generally infertile and seriously limit the development of tropical agriculture. Despite the rich vegetative cover in many humid and subhumid parts of the tropics, the humus content of tropical soils is low. Also, as Balek (1983) points out, many tropical rivers do not create extensive and rich alluvial plains. Even the Niger inlands delta and the Amazon lowlands are not very suitable for crop production. In contrast, major streams in south and southeast Asia, including

Table 1 Tropical metropolitan areas with over 1 million inhabitants (as of 1981) - source: Landsberg (1986).

City Country Population Annual rainfall (X 106) (mm, rounded)

Abidjan0

Accra0

Addis Ababa Ahmadabad0

Bandung0

Bangkok0

Belo Horizonte Bogota Bombay0

Brazilia Calcutta0

Caracas0

Chittagong0

Dacca Delhi/New Delhi Fortaleza0

Giza* Guadalajara Guangzhu Guatemala City Guayaquil0

Hanoi0

Harbin Havana0

Ho-Chi Min City0

Hongkong0

Jakarta0

Johannesburg* Karachi*0

Kinshasa Kuala Lumpur0

Lagos0

Lima0

Madras0

Manila0

Mazatlan Mexico City Monterrey Nanking Nova Iquacu Porto Alegre*0

Quezon City Rangoon0

Recife0

Rio de Janeiro Riyadh* Salvador0

San Juan0

Santo Domingo0

Sâo Paulo Singapore0

Surabaya0

Taipei0

Ivory Coast Ghana Ethiopia India Indonesia Thailand Brazil Colombia India Brazil India Venezuela Bangladesh Bangladesh India Brazil Egypt Mexico China Guatemala Ecuador Vietnam china Cuba Vietnam

Indonesia S. Africa Pakistan Zaire Malaysia Nigeria Peru India Philippines Mexico Mexico Mexico China Brazil Brazil Philippines Burma Brazil Brazil Saudi Arabia Brazil Puerto Rico (US) Dominican Republic Brazil

Indonesia Taiwan

1.6 1.0 1.2 1.7 1.7 5.2 2.4 4.3 8.2 1.1 9.1 3.0 1.1 3.0 5.2 1.3 1.3 2.3 2.9 1.6 1.1 2.6 3.0 2.0 3.4 5.1 6.5 1.3 8.6 2.2 1.0 1.7 4.9 4.3 5.5 2.1

14.0 1.9 2.4 1.1 2.3 1.0 2.7 2.3 5.1 1.3 1.8 1.1 1.0 8.3 2.4 2.4 3.5

1980 790+

1070 820+

1900 1440+ 1060+ 1470+ 2080+ 1560 1600+

840+ 2730+ 2010+

670+ 1370+

60 900+

1720 1310+ 1100 1800+ 580

1230 1980+ 2270 1800+ 710 200+

1390+ 2410 1830+

50 1210 2070

810 750+ 580

1320+ n.

1330 n.

2620+ 1610+ 1080+ 150

1900 1630 1420 1430 2420 1780 2180+

Source: National Geographic Atlas of the World, 5th edition, Washington DC, 1981, and World Weather Records.

0 indicates coastal city. * areas outside the tropics but below 30° latitude and having tropical climates. + rainfall shows a very pronounced annual (monsoonal) variation.


1. Annual total rainfall depth

85 86 87 88 89 90 91 92 93 94 95 96

2. Seasonal monthly distribution

VrTK

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

3. Intensity duration frequency (i-d-J)

5 years *i

2 years S r e t u r n

[ period 1 year J

/< «ft

4. Storm profiles (for a given storm duration)

-*d

5. Event records (historical time series)

Time resolution 1 min

6. Spatial storm pattern (radar image and similar)

100 120 HOkm

• of little significance for urban storm drainage projects

• significant

- very important

• very useful

- essential for experimental catchments

• very useful for design

- essential for flood warning and real time control

Fig. 10 Types of data on rainfall and their usefulness in storm drainage practice.

the Ganges, Brahmaputra, Irrawaddy, Chao Phrya, Mekong, and the Javanese rivers, are capable of creating fertile plains and deltas (Balek, 1983). The above information provides general guidelines for the possibility of applying source control measures for urban storm drainage. Soils in tropical areas are known to be subject to heavy erosion due to both their structure and the impact of raindrops that are bigger in tropical zones. Intensive erosion combined with the absence of solid waste collection is the major decrease of carrying capacity of urban streams which require frequent and

Cedo Maksimovic et al.

5 6 7 8 10 20 30 50 70 100 15 20 30 45 60

Storm duration (min) Storm duration (min)

Fig. 11 Intensity-duration rainfall curves.

expensive drainage. In urban storm drainage soil type plays an important role in the assessment of infiltration. A specific feature of the humid tropics is the absence of seasonal variation of vegetation cover that offers both infiltration and surface depression (Hufschmidt & Kindler, 1991).

Singapore (1°N, 106°E)

°C 30

25 : •

Rainfall 500 [mm]

m J FMA M J J A S O N D

°c 30 20 -10

Rainfall 500 [mm

Calcuta(23°N,86°E)


Monrovia (6°N, 11° W)

Rainfall 900 r-

500

kffl

°c 30 25


Santarem (2°N, 55°W)

L Rainfall 500 r-

tbdf] JFMAMJ J A S ON D

Fig. 12 Seasonal distribution of rainfall depth for several cities in the humid tropics (Lindh & Niemczynowicz, 1991).


Fig. 13 Coastal areas affected by tropical storms (Hufschmidt & Kindler, 1991).

Fig. 14 Monsoon-affected land areas (Hufschmidt & Kindler, 1991).

Conclusion

As a conclusion to this section one can only state that the differences are significant to such a level that it prevents the modern technology from the developed countries to be simply copied and transferred. Learning about local conditions is a "must". Best results are obtained by proper interaction.

SITE SPECIFIC PROBLEMS REQUIRE SPECIFIC SOLUTIONS

The lack of financial resources is not the major reason why modern technology applied in developed countries cannot simply be copied, because it would not work properly or not at all. First of all, for the diagnosis of the status of the existing system and for the implementation of advanced models (rainfall-runoff, water quality) and for the assessment of the effect on receiving water body, local (site specific data) have to be collected and matched to the models. On the other hand, modern concepts of integrated water management in urban areas require the real "integrated" approach to be implemented (Lindh & Niemczynowicz, 1991). The range of possible solutions require for example the incorporation of solid waste problem solution concurrently with storm drainage. Otherwise ambitious programmes without a proper interdisciplinary approach (Jakarta, Bangkok, Sâo Paulo, etc.) are being delayed for decades with results that are more than modest.

For example in storm drainage practice a decentralized solution based on a series of sources control measures (Fig. 16) combined with proper interaction with the other elements of urban water interaction systems can generate much better results than construction of a gigantic water treatment facility. Numerous unsuccessful projects in the cities of the humid tropics are good examples that the technology cannot simply

Ûedo Maksimovic et al.

a) Before urbanization

Fig. 15 Effect of urbanization of the peak flow (Ômax) and f ' m e °f tne P e a^ (-Q from

a unit area (Source: Japanese Ministry of Construction, modified, 1990).

be copied and transferred. A careful study of the local culture, tradition in coping with natural hazards and mitigation processes are needed. An example of a trial to incorporate a traditional urban drainage system in the city of Tehran into a newly designed system is described by Assefi (1991). The network of the traditional existing systems of open channel (jues) is shown in Fig. 17.

The conclusions drawn by Monteiro (1986) on the urban climatology: "It was realized that the large amount of foreign literature demanded special perusal because simple transfer of methodologies into this very different geographical reality was not possible" can be repeated for the urban drainage as well.

On the other hand the particularities of the humid tropics should not be mystified. What is different can be obtained by field surveying and studies. General procedures in data collection, analysis, modelling and real time operation are applicable throughout different climate conditions provided that the local conditions are incorporated.


Balance of solid and gasseous phase

r44r-Balance of liquid and vapour

phase with atmosphere

Transboundary balance of water

Water intake and purification

Meteorological & similar services

- Radars - Weather forecast etc.

Transboundary balance of other

Technology transfer -research training

Other interaction - migrations - transportation - trade - culture & tradition

Socio economical aspects

Interactions with financial institutions

&i> - Transport processes - Chemical and biological proicesses (oxygen, deplition, etc.) - Multiuser conflicts - Upstream downstream interactions

Receiving water body

Fig. 16 An interactive storm drainage system.

DATA NEEDS AND POSSIBLE SOURCES

General aspects

It has been pointed out many times (Dunne, 1986; Maksimovic, 1992) that the lack of reliable and accurate data is one of the major obstacles for the correct application of appropriate technologies in modern urban drainage systems not only in the humid tropics. In his paper Dunne points out correctly that the efforts for obtaining the field data needed for urban waters are often much smaller than expected. Data can be obtained locally with little effort.

The data needs will be analysed for three major phases in implementation of an urban drainage project. The three phases are: — planning and design, — operation and real time control, — reconstruction and rehabilitation.

In addition to the above three groups of activities data that are needed for any city or site of potential construction of a storm drainage system, slightly more data are needed by the dedicated research and development bodies such as Regional IRTCUD Subcentres for Humid Tropics.

394 Cedo Maksimovié et al.

Fig. 17 Network of open channels: part of the traditional urban drainage systems in the city of Tehran.

The additional data are needed for analysis, design and management. In both groups of activities the data are the following: — catchment boundaries defining the areas from which the runoff water is drained

to the system under consideration, — land use pattern, discriminating between pervious and impervious areas, roofs,

paved and less pervious zones, — possible flow pattern for the delineation of the catchment into subcatchments, — soil types and the characteristics that affect infiltration, — existing drainage system and its status (diagnosis), — storm characteristics, — water quality on the surface and in the underground system of storm sewers, — existence and characteristics of auxiliary structures (overflow, sluice gates,

pumping stations, etc.), — status and carrying capacity of the receiving water bodies, — quality of runoff waters.

Data on storm characteristics

The types of data needed and the possible series of the analysis have been discussed and are shown in Figs 10-12. The spatial and time resolution of storm data for urban drainage projects is much different to that for other water related projects. As a rule


of thumb the principle 1-1-0.1 (Niemczynowicz, 1984, 1991) can be used, meaning: 1 raingauge for 1 km (for experimental catchments), 1 minute time resolution and, 0.1 mm of rainfall depth as a basic unit.

The data at this resolution are desirable for model calibration and verification. For the extrapolation to full scale projects on the catchments where no data are available, one should aim at applying the data from at least the same city.

Land use and catchment characteristics

Such data are rarely found in humid tropical countries. One of the major problem in large urban cities in the humid tropics is the assessment and update of the realistic land use pattern. Recently developed tools for handling various sources of information and for matching them with commercially available GIS packages and simulation software provide an efficient means of obtaining this information in computer readable formats. Figure 18 presents an approach in the development of so collected gluing routines. An example of the application of one of these technologies is given in Fig. 19.

It is worth mentioning that under the conditions of rapid urbanization and the changes that affect surface runoff one of the fastest ways in obtaining the land use pattern and digital terrain modelling is the application of photogrammetry. This should normally be available in many countries and if not photogrammetrical images can be easily obtained by special flights organized for this purpose. In the humid tropics very useful results can be obtained by using infrared images.

The above data should be combined with the maps and plans available from ordnance surveying and city planning institutions.

Sources of information

Photogrametry images

Scanned maps

Commercial GIS packages

Commercial simulation (runoff) models

Automatic field

surveying

Paper maps etc.

-X_ _A- _A_ :̂;;.;a?Hi0SBN;:GlRï;o;:ù ;T i N E S SM% wmétmâ

Y ~r~ r

ARC - INFO INTERGRAPH MAP SOFT GRASS etc.

_A_ _A_ _A_ l ^ ï M I ^ f i J â ^ M R S Q : UlTi l N ESS - Creation of i r i p u H i i

~Y~

MOUSE Hystem Extran WALLRUS BEMUS SWMM

etc.

o_ IMflflllflff^

Fig. 18 Matching sources of information with commercial GIS and rainfall-runoff simulation packages (Elgy et al., 1993).


ipH ink , /Aim / 1M

Fig. 19 Application of photogrammetric maps for creation of land use and digital terrain (DTM) mapping (Elgy et al., 1993).

Runoff quantity and quality

Runoff from selected sites (experimental basins) or within the existing storm sewer should be recorded depending on the scope of measurements (forming data base,


model calibration, capacity verification of the existing systems, etc.). It goes without saying that the data on the underground structure (network, utilities etc.) should be made available.

The problems associated with runoff quantity measurements are presented in details in Maksimovic (1993). The types of measuring stations are shown in Fig. 20.

The most frequent methods applied in flow measurements and data acquisition are shown in Fig. 21.

As pointed out by Maksimovic (1993) runoff measurements can result in big errors. Possible sources of errors have been analysed in Maksimovic & Radojkovic (1986) and are shown in Fig. 22. The errors are normally encountered where either a flow measuring structure is not properly designed and constructed for the range of flow stations that can occur in operation or when it is not regularly and properly inspected and maintained. Experience has shown (Hemain, 1990) that frequent inspection and analysis soon after data collection is crucial for the success of the measurements.

Measurements and interpretation of runoff quality is important for design of treatment plants and for the protection of receiving water bodies. More details on measurements can be found in Marsalek (1993) and Geiger et al. (1987).

It is unrealistic to expect the measurements to be organized on a citywide scale and to last for long periods.

As pointed by Dunne (1986) it is essential to organize measurements for a particular city even with cheap and available methodologies and instrumentation. Training of local staff to perform measurements in a reliable manner has proved to be possible even in countries and institutions in which technical and professional level are not high.

For the large developments and important rehabilitations, special projects of measurements and data processing have to be elaborated. The lack of reliable data can result in serious misinvestments and failures. Examples are numerous.

Group T y p e

I

II

III

Permanent flow gauging system

Temporary gauging stations

Single shor t term or ad-hoe stat ions or measurements location

St ruc tura l features

- Flumes and other flow measuring s t ruc tu res

- Permanent fitting into systems; stilling well

- Permanent access and protection

- Minor modifications of the system

- Portable flumes. - Portable s u p p o r t s .

- No s t ruc tu ra l changes - Improvised suppor ts - Full mobility

Instrumentation

- Fixed ins t ruments - Telemetric data

transmission e tc .

Portable or semi-fixed equipment

Portable of hand held ( insert ion) meters and portable data record ing

Fig. 20 Types of measuring stations used in runoff data collection for urban storm drainage projects (Maksimovic, 1993).


Wet fall sampler (open position)

Bulk precipitation / sampler Rain

m 0

Stepping motor

Distributor

dg V / ^ N

6 channel recorder Freezer

rryiW pagFggBB Flume Sampler

a) Unsubmerged (modular) flow

Clamping band

- x /

Flume Sampler

b) Submerged flow

Manhole cover

^¥

_ Cable (secured)

- Housing

- Data logger memory & battery (environmentally proff)

" " JUItrasonic velocity ë=£> and depth sensor p l o w direction

Fig. 21 Permanent and temporary monitoring stations.

Small islands

Problems of small islands in the humid tropics are specific and also require particular local data. Many tropical islands drainage designs still use the colonial practice introduced by British, Dutch and German engineers of a covered roadside ditch in addition to the gutter. This practice works well in Europe with relatively cool temperatures and moderate rainfall. But in tropical areas, ditches are perfect places for collection of streets wastes, serving as a breeding ground for bacteria and undesirable odours.

The potential water quality impact of tropical island urbanization should not be neglected. Because of the heavy rainfall it is not likely that all the storm runoff could be treated. For developing islands probably no treatment will be provided for storm runoff for many years. The deteriorating biological and chemical quality of storm water because of urbanization may cause irreversible damage to receiving waters, particularly to groundwater of small islands. For coastal urban areas the decision on the use of a single drainage network, or a number of smaller networks for different areas discharging into different receiving waters is a matter of optimal decision making (Yen & Yen, 1991).

Urban drainage problems in the humid tropics

a) Weir al) Correct (assumed)

a2) Incorrect (usually occurs during peak flow)

Hydraulic jump not fully formed

b) Flume

bl) Assumed

Jump

Assumed discliargc g , QT

Real discharge (errorneus)

b2)Inccorect Jump not fully formed (ondulatiagjump)

W

Supercritical

Assumed (bl)

/ 4 »

AQ

"^71 ' Real (b2)

a) Flow in the upstream section during standard-average conditions

al) Weir

b) Irregular flow conditions during peak flow

v-Assumed(theoretical)

c) Stage - discharge functions

Assumed

V- "" '^Real

*) Irregular level caused by high approach velocity of insuficient capacity of weir

a2) Flumes

Horizontal N bottom

Horizontal P bottom

Fig. 22 Erroneous measurements of flow in weirs and flumes.

UNESCO/ERTCUD activities

Realizing the needs for coping with the integrated water resources problems in urban areas, UNESCO has initiated several long-term actions and short-term projects such as the international IHP IV project M.3.3. "Integrated Water Management in Urban Areas", which deals with urban storm drainage, which consists of two subprojects:

M. 3. 3a: Use, Planning and Treatment of Water and Wastewater in Urban Areas; M.3.3b: Runoff and Drainage in Different Climates: Tropical, Arid or Semiarid

and Cold.


Aiming to cover urban drainage and related fields (precipitation, receiving waters, networks, etc.) and to cover the relevant research and training needs at the global level, a specific centre named IRTCUD (International Research and Training Centre on Urban Drainage) has been established in Belgrade with regional subcentres in Brazil and in Norway. It covers a wide range of activities starting with the formation of a bibliographic UDBB and factual data UDM bank, collection and dissemination of knowledge and educational tools, undertaking joint research and consultancy projects and ending up with the organization of training courses, conferences and other related events. An important activity of IRTCUD is the development of educational tools and its dissemination and use for training (Maksimovic, 1991b).

The Centre is open for cooperation, putting a strong emphasis on the assistance to professionals and researchers to develop their own capacities. Of special interest are the initiatives to establish experimental catchments and the other facilities which contribute to better implementation of the products of higher level of information processing.

Major sources of bibliographic information in urban storm drainage are international conferences, journals and research reports but the factual information (data) have to be gathered from local sources. The art of collecting and handling local data has been mastered in many countries. This information cannot be imported.

As for humid tropical areas an important role is to be played by the IRTCUD regional subcentre in Sâo Paulo, Brazil (IRTCUD/RSCB). The RSCB has initiated a range of activities and projects that should fill the gap between what is needed and what is available. The programme of activities and project proposals are described in the project documents and studies (Tucci, 1991; Braga, 1991) and could serve as a good starting point for future activities. Significant efforts have to be made in order to attract the financing of activities of this nature which are essential for the success of the projects of full scale.

CONCLUDING REMARKS

— Urban storm drainage is an important element of an urban water system and of a system of urban infrastructure.

— Traditional methods for the design and management of these systems and failing to make a balanced interaction with the other elements of the system (foul sewers, treatment, receiving waters, water supply, etc.) usually results in an improper or unfavourably expensive system that is rarely accomplished.

— The humid tropics have particular failures that have to be analysed (primarily through the data required locally) and the selective combination of the modern technologies combined with local data have to be implemented in design, construction and management.

— Recently created international networks of institutions and experts under UNESCO's umbrella (including IRTCUD/RSCB) can provide assistance and information to support future projects.

— The conference on particular problems of storm drainage in the humid tropics (UDHT) that is planned in the near future should provide a forum for further consideration of problems and actions.


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Balek, J. (1983) Hydrology and Water Resources in Tropical Areas. Developments in Water Science no. 18, Elsevier.

Barrow, C. (1987) Water Resources and Agricultural Development in the Tropics. Longman Scientific and Technical, London.

Braga, B. P. F., Jr (1991) Hydrology of small urban basins in the humid tropics. Project proposal. Chang, J. H. &Lau, L. S. (1983) Definitions of the humid tropics. Paper presented at the IAHS meeting, Hamburg. Dunne, T. (1986) Urban hydrology in the tropics: problems, solutions, data collection and analysis. In: Urban

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