Many low-income communities in developing countries consider stormwater drainage to be their most urgent need as far as urban infrastructure is concerned. This is partly because their houses are often built on unsuitable land. In areas sufficiently close to the city centre for the journey to work to be affordable, land prices tend to be beyond their means. The only land they can afford, or on which the owners will allow them to stay as squatters, is land that is unsuitable for other purposes. This is often on steep hillsides subject to erosion and landslides, or it is low-lying, marshy land often subject to flooding.
Historically, most of the major cities of the developing world arose along the coast as ports, often on the estuaries of rivers which served as commercial arteries for the transport of goods to and from the hinterland. It is the coastal regions of the world that have the highest average rainfall (Fig. 1), but the flat estuarine terrain and often impermeable alluvial soils make drainage difficult.
Even in the arid areas where average rainfall is low, tropical rainfall - when it comes - is more intense than in temperate climates, and the lack of vegetation and of adequate drainage means that torrents of water can form in minutes, causing damage to homes and property which will take years to repair. Rainwater is not the only problem. Leaking water mains, wastewater from washing and bathing, and the sewage from overflowing septic tanks and blocked sewers constitute health hazards, damage buildings, and can cause flooding if an adequate drainage system does not exist.
The lack of drainage is especially serious where the ground is either steeply sloping or very flat. On very steep sites, as in parts of Luanda, Rio de Janeiro and Hong Kong, stormwater flows fast and violently, damaging buildings, eroding the land and sometimes causing landslides. Soil eroded from a hillside is usually deposited at the foot of the slope; the soil eroded in a single rainstorm has been known to bury houses completely in this way. In other cases the land is flat; in cities such as Bangkok, Calcutta, Colombo, Dar es Salaam, Jakarta, Guayaquil, Lagos, Manila and Recife, many neighbourhoods are flooded at least once or twice a year, and people have to learn to cope with water inside their dwellings. Sometimes people build their houses on stilts and connect them by elevated pathways. However, their construction is rickety, and it is very easy to lose one's balance and fall into the muddy, polluted water underneath.
Fig. 1. Areas of the world with more than 1.5 m average annual rainfall
Deaths due to drowning in floods or burial beneath landslides or collapsing homes are perhaps the most dramatic signs of the suffering that drainage can help to alleviate. Less noticeable to an outsider, but of greater impact on the residents' lives in a poor community, is the steady toll of disease, disability and death taken by standing water.
First in public health importance are the many faecal-oral infections acquired by consumption of contaminated food and drink. The microscopic pathogens that cause them are found in the excreta of infected people or animals. Surface water becomes contaminated with these pathogens from sources such as blocked sewers and overflowing septic tanks, and often from defecation in the open by livestock and by people who have no toilet. This contaminated surface water can then infect people in many ways. It can contaminate their hands, their utensils or their drinking-water supply (Fig. 2). Children are particularly exposed to infection when playing or bathing in surface water.
Fig. 2. Stagnant water and disease transmission - the health consequences of poor drainage
The faecal-oral diseases include the well known water-related diseases that are often fatal, such as cholera and typhoid fever, but also the many common diarrhoeal diseases that particularly affect young children in developing countries, contributing to malnutrition and death. In fact, these diarrhoeal diseases are often responsible for more child mortality than any other cause of death. Important measures for their control are an improved water supply and better sanitation, but these are almost impossible to install in areas subject to frequent flooding.
In countries where schistosomiasis is endemic, poorly drained urban areas present ample opportunities for transmission of the disease (Fig. 2). Contamination of standing water with the faeces of infected persons (or, for one form of the disease, with their urine) enables the schistosomes, the microscopic parasites that cause this infection, to reach the small aquatic snails in whose bodies they multiply. From every infected snail, thousands of schistosomes emerge and swim in the water. Local residents become infected when they enter the water and the schistosomes penetrate their skin.
Schistosomiasis may sometimes be thought of as a rural disease, but it is often no less prevalent in urban areas where drainage is lacking. Some of the species of snail host thrive and breed rapidly in the heavily polluted stagnant water which often accumulates there. Moreover, a single infected person in an urban area can cause sufficient contamination to infect very large numbers of the people living in his or her crowded neighbourhood, because even a small number of snails, once infected, can produce many thousands of schistosomes over a long period of time.
A public water tap in a poorly drained low-income community in Pondicherry, India. Sanitary services such as water supply and excreta disposal cannot function hygienically without adequate surface water drainage.
Photo: S. Cairncross
Another important group of diseases related to poor drainage is transmitted by mosquitos. Different diseases are transmitted by different species of mosquito, and each chooses different bodies of water in which to breed. Some prefer water that is heavily polluted, some prefer it clean; some breed in flooded areas, some in the drains themselves if they are blocked by rubbish or vegetation or are laid unevenly so that there is standing water in them.
Malaria is the best known of the mosquito-borne infections, and is transmitted by Anopheles species, many of which bite animals as well as humans. Transmission can be particularly intense in urban areas where there are relatively few animals to divert the vector species of mosquitos from human blood meals. Anopheline mosquitos do not usually breed in heavily polluted water, but can multiply in swamps, pools, puddles, and also in streams and stormwater canals in which there is standing water. Anopheline mosquitos breeding in poorly drained areas can transmit malaria to adjacent parts of town. A particular danger in a city is the significant amount of international travel to and from it, which increases the risk of importation of new and possibly drug-resistant strains of the malaria parasite.
Another family of mosquitos, the Aedes species, can transmit several viruses, such as those that cause dengue and yellow fever. Urban epidemics may result. In recent years, a more virulent form of dengue has been observed, known as dengue haemorrhagic fever (DHF). Both DHF and yellow fever are often fatal. Aedes mosquitos usually breed in clear water, for instance in domestic storage vessels, but they have also been found to multiply in swampy and flooded areas, and in open drains and stormwater canals.
Finally, there is the particularly urban problem of bancroftian filariasis, which can cause elephantiasis (irreversible swelling of the legs) as well as other disabling symptoms. Although transmitted in rural areas by Anopheles species, which appear to be the original vectors of the disease, it seems to have adapted to transmission in urban areas by the Culex pipiens group (Fig. 3), which generally multiply in heavily polluted bodies of water. Transmission of the disease is a relatively inefficient process, so that many years of exposure to intense night-time mosquito biting are needed for the average case to develop. Nevertheless, more than 80 million people in the developing world are infected. In many countries, such as India, it is especially prevalent in urban areas. Filariasis transmission by Culex pipiens mosquitos is now common in Asia, is occurring in cities on the eastern coasts of Africa and South America, and may soon begin in the large poorly drained urban areas of West Africa where both the disease and the vector mosquito already exist.
Fig. 3. Geographical distribution of Culex pipiens mosquitos and bancroftian filariasis (from: Curtis, C. F. & Feachem, R. G. Sanitation and Culex pipiens mosquitoes: a brief review. Journal of tropical medicine and hygiene, 84: 17-25 (1981)).
Drainage construction is an effective mosquito control measure. It is cheaper than application of insecticides and does not have to be repeated regularly; in many cases, it costs less than a year's supply of insecticide. Unlike insecticides, it can have no detrimental effect on the environment; on the contrary, it constitutes an environmental improvement. Moreover, the danger of mosquitos' developing resistance, as they have been known to do to insecticides, does not apply.
The urban poor may often build on land with drainage problems, but good urban planning can help to avoid making those problems worse.
One of the simplest planning measures is to set out regular plots before house building starts in an area, leaving space for well-aligned roads. Adequate road width and alignment will make it much easier to build drains when they are needed later. Site-and-service schemes are expensive and take a long time to plan and implement, but such a site only scheme should be within the means of any municipality. Once the overall layout of a neighbourhood has been planned, residents (or future residents) can be shown how to set out individual rectangular plots with nothing more sophisticated than a tape measure, or even a piece of string with knots at regular intervals. Some degree of discipline over house building is necessary, to ensure that plot boundaries are observed, and to prevent houses from obstructing existing drainage paths or from occupying land needed for future drainage works. The residents themselves are in the best position to enforce this discipline.
The development of residential areas can increase drainage problems in other ways. As vegetation is removed, the capacity of the ground to retain water and resist erosion is reduced. The increasing area covered by roofs and road surfaces diminishes the area of ground into which water can infiltrate, leaving a greater volume of water to be removed by drainage. Low-lying areas subject to flooding play a role in storing the water from sudden rainstorms until it can drain away gradually; when these are filled in for housing, the result may be flooding in other areas.
Roads must be built above the flood level, and the resulting embankments can obstruct natural lines of drainage, or can channel water alongside them causing erosion. In some cases, as in parts of Bangkok, roads have been built by filling in existing channels, causing serious flooding. Where the natural drainage channels are not filled in or obstructed by buildings, they often become blocked by domestic refuse.
On the other hand, drainage improvements in one area are closely linked with drainage problems elsewhere, and are best planned in the context of the city as a whole, or at least of a whole catchment area. Better drainage in one neighbourhood means that surface water flows away faster, imposing a greater burden on the capacity of the system downstream. At the same time, drainage improvements at a local level may be of little use if water still backs up because the downstream capacity is insufficient. This has been a serious problem in Jakarta, where improved local drains were often submerged by water held back by constrictions in the city's major canals.
Of course, it is possible for a community to make local improvements, even without the full involvement of the city planning authorities, but at least some consideration should be given to the body of water into which a new drainage system will discharge. Whether this is a main sewer, river, lake or sea, the maximum level to which it floods will normally set the minimum level for the drainage system. The discharge of drainage water also affects the quality of the receiving water into which it flows, especially when sewage or septic tank effluent is released into the drains. In Bangalore, for example, the discharge of sewage into several dams in the city led to intense breeding of mosquitos until measures were taken to breach or bypass them.
Drainage improvements are not only a job for a drainage engineer. They involve several professions and need the cooperation of several sectors if they are to succeed. Drainage is of great concern to town planners and, if some houses have to be relocated to make room for new drains, architects and builders may also be involved. Drains are usually built beside roads, and the roads department will have an interest because good drainage is essential to protect the road surface.
Maintenance of the drainage system depends on an efficient service for collection of solid waste, as without one the drains will soon fill with rubbish. Moreover, the street-cleaning and solid-waste collection service will often be the most suitable municipal department to clean the drains regularly, as it will have the necessary vehicles to remove the solid materials such as silt, vegetation and refuse that will accumulate in them. The health department will be concerned to ensure that the cleaning is done well and regularly, and that the drains are not built in such a way as to make this difficult or to promote disease transmission. This in turn involves several specialities, such as medical entomology.
The community has a key role to play. Whether or not local residents participate in construction, their cooperation is needed in obtaining the necessary land. Some people may have to sacrifice part of their premises, or agree to relocate their houses, to make room for the new drains. Whether or not the community takes responsibility for maintaining the system, a responsible attitude on their part will be a great help towards its upkeep, reducing the amount of rubbish thrown into the drains, and damage done to them by vehicles, building work or vandalism. A single uncooperative resident who blocks the water flow, or neglects to clean his or her section of the drainage line, can harm the interests of the whole community. Proper drainage therefore calls for the close cooperation of the community and its leaders, and also of those who work with the community, such as educators and health workers.
A cooperative attitude is not enough, however. Effective collaboration between municipal departments and involvement of the community have institutional implications. At the level of local government, the most fundamental consideration is that some department at least must have the primary responsibility for urban drainage. In many cities there is no clear definition of who is responsible for cleaning and maintaining the natural and man-made drainage system, and in some it is not even clear who is to build it, or which national government department is to finance major drainage works. An example of the absurd situations that can arise in such cases is for one department to remove rubbish from the drains, for the solid-waste collection service to refuse to collect the rubbish from where it is dumped on the adjoining roads, and for the roads department to sweep it back into the drains again!
Ideally, the regular cleaning of urban drains should be the job of the street-cleaning and solid-waste collection service. However, other sectors will usually be responsible for drainage construction and repairs, so that several sectors are inevitably involved. Some arrangement for regular liaison meetings should therefore be set up, and a single department should be responsible for convening them. The health department should be represented.
Some institutional arrangements are also needed in the community, to mobilize and coordinate the community's contribution and to ensure that it is not undermined by the antisocial behaviour of a minority. If possible, it is best to build on existing community institutions, although these may already be fully occupied with other day-to-day tasks. In many cases, a useful initial step will be to form a drainage committee to organize the community's contribution to planning, implementation and maintenance of drainage improvements. Community institutions are discussed further in section 4.
A typical neighbourhood drainage improvement programme passes through four principal phases:
- construction, and
The first two of these are fundamental, as they determine all that follows.
Initiation of a programme may arise from the community's own realization of the need for better drainage, possibly after experiencing a particularly serious flood or seeing drainage improvements in other neighbourhoods. In many cases, however, it is catalysed by some external agency, such as the municipality, a political party or nongovernmental organization, or by a concerned individual such as a teacher or health worker. This phase involves identification of the need for drainage, formation of a consensus regarding the scope of the problem and the desired solution, and establishment of a drainage committee, at least on an interim basis. Where the initiative comes from outside, it is also likely to include a certain amount of work in the community to develop awareness of the problem and mobilize support for a drainage programme.
Planning is the most important phase of all, as it involves the most fundamental decisions. The more decisions that can be taken at the planning stage, the better it is for the future of the programme.
The most basic decision for the community is whether to implement the drainage programme on a formal basis through the local authorities, or to attempt a do-it-yourself project on its own. Formal drainage projects tend to be expensive, so that the first task of the drainage committee is to lobby and persuade the local authority to agree to support the programme (unless of course the authority itself initiated the scheme). The authority will usually have to obtain finance from some other agency, which is likely to require a feasibility study and design by a consulting engineer before a contractor is given the job of building the new drainage system. All this takes time - typically three to ten years - and members of the community may prefer to carry out some do-it-yourself interim measures themselves, while they wait (Fig. 4).
Fig. 4. Possible sequences of events in solving local drainage problems
Whichever approach is followed, it is important to define not only the layout and design of the new drainage system but also the community's role in the construction and maintenance phases, and how its contribution is to be organized.
Construction and maintenance are discussed in detail in the remainder of this book. The following description of how the sections are arranged may help to guide the reader.
The technical aspects of drainage design and construction are considered in section 2. Much of this information will also be of interest to nontechnical readers. Sections 2.1 and 2.2 are especially important as they explain basic concepts in lay language. Section 2.9 describes how a community can plan its own drainage improvements without external support, and will also be of interest to an engineer making a first approach to a local drainage problem.
Section 3 discusses rehabilitation and maintenance. Most of the chapter is technical, but lay readers will not find it difficult to follow. Institutional aspects of maintenance are discussed in section 3.4.
Section 4 considers participation by the community in drainage schemes, a subject whose importance tends to be underrated. The reader should at least look at section 4.1 before deciding whether to read the rest!
A glossary of terms is provided in Annex 1, and other annexes cover design calculations, terms of reference for consultants, and resources for the orientation of the drainage committee.
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