The drainage problems of an individual neighbourhood are part of a hierarchy of problems related to the drainage network of the whole city and corresponding with the hierarchy of drains which compose it. These drains range from the major canals or large sewers which collect water from large areas of the city down to the small ditches or drainpipes that run along the roadside or serve individual properties.
At the most basic position in the hierarchy is the receiving water body into which the system discharges. This may be the sea, a lake or a river. The water level in the receiving water body fixes the minimum level of the drainage channels, because the pumping of storm-water is not feasible for any but the wealthiest communities. Even if it were possible to afford pumps large enough to handle the amounts of water involved, they would not be practical because of the many difficulties of maintenance and the extent of the damage that would result from malfunction or breakdown of the pumps. The water level in the receiving water body comes very close to ground level in many flat low-income areas, which means the drains cannot be made very deep.
Next in the hierarchy is the primary drainage system, composed of main drains, sometimes called interceptor drains. These serve large areas of a city or the city as a whole, and often follow the line of natural drainage channels such as rivers or streams. The design, construction and maintenance of a city's primary drains require extensive engineering skills and a large financial base, and are well beyond the means of an individual community. These drains are not considered here.
Finally there is the secondary drainage system, a network of small drains within each neighbourhood. These are sometimes known as micro-drainage or laterals, and each serves a small catchment area, ranging from a single property to several blocks of houses. This publication deals principally with the secondary level of the drainage system. At this level, improvements can be made with modest investments, and low-cost solutions are often appropriate.
Not all the water falling as rain needs to be removed by the drainage system. Some of it will infiltrate into the ground, while some may stand in puddles and other depressions and eventually evaporate. The proportion that runs away over the ground surface and has to be carried in the drainage system is known as the runoff coefficient. In practice, there is little chance for evaporation during a rainstorm, so that the runoff coefficient to use when calculating the size of the drains required is based on the infiltration capacity of the ground. This depends mainly on soil conditions, the slope of the terrain, and on land use:
Soil conditions. Water seeps more readily into sandy soil than into clay or rocky ground.
Terrain. Water flows more rapidly down a steep slope, leaving it less time to infiltrate than when it stands or moves slowly in a flat area.
Land use. Vegetation traps much of the water and also loosens the soil, thus making infiltration easier. Roofs and paved surfaces, on the other hand, prevent infiltration.
Runoff coefficients are therefore higher in areas of clay soil or rock, on steep slopes and in densely built-up areas with little vegetation. As an example of this, the quantity of water to be drained from a high-density housing area may be 5-6 times greater than it was when the area was undeveloped and covered with vegetation.
The rate at which water enters the drainage system depends on the runoff coefficient, but also on the rate of rainfall. Of course this can vary, from a heavy downpour to a light shower, and it is hard to estimate the maximum intensity of rainfall that will occur in a given year, because of the unpredictability of the weather. However, by analysing past rainfall records, it is possible to make an estimate of the probability of any particular rate occurring. The more severe the rainstorm (i.e., the higher the rate of rainfall), the lower the probability of its occurring.
This probability is usually expressed as a return period. A rainstorm with a probability of 1 in 20 of happening in any particular year is said to have a return period of 20 years, and is called a 20-year storm. This does not mean, of course, that it happens exactly every 20 years, but that on average it will happen that often - an average of five times a century.
If a drainage system is designed for an unusually severe rainstorm with a 100-year return period, it may never be fully used within its lifetime. The money spent in constructing a system with such a large capacity might have been better spent on building smaller drains in areas that have none. Choosing the optimal return period for the design of an urban drainage system is a difficult judgement based on weighing the risk of the drains overflowing, and the damage this might cause, against the cost of building larger drains to prevent it.
A return period of five years is widely used to design primary drainage systems in tropical cities, but shorter periods (three years or less) are more suitable for micro-drainage within residential areas, where an occasional overflow is less likely to cause serious damage. In a low-income area, where the value of property liable to damage is relatively small, and only limited funds are available for drainage, the appropriate return period may be shorter still. In Mombasa, for example, a one-year return period has been adopted for all but the largest drains. In Calcutta some drains have been designed for a return period of only two months. A few inches of flooding several times each year may be a great improvement on waist-deep water for weeks on end.
The damage that can be done to roads by stormwater is often the major justification for drainage in low-income areas. On steep slopes, a single heavy rainstorm that makes the drains overflow can cause enormous damage by erosion, so that a longer return period may be justified than in flat areas.
Annex 2 gives further details of how to estimate stormwater flows and use them to calculate the size of drains for design purposes.
Sloping land easily suffers from erosion when the vegetation cover is damaged and when intensive land use bares the soil. It is therefore important to prevent water from rushing down in uncontrolled flows that may undermine houses and turn paths and streets into impassable gulleys. As a rule of thumb, slopes of more than 5% can be considered steep slopes.
On steep terrain, the only way to keep water in the soil is through terracing so as to reduce the slope. Various methods exist and are used to control erosion on agricultural land. However, these can be applied in an urban area only if the neighbourhood has not already been fully built up.
When the water is concentrated in a natural or artificial line of drainage running down a steep slope, it can flow at great speed and thus cause considerable damage. Various methods can be used to lead the water down gradually and in manageable quantities:
(a) Diverting the water horizontally by a bank built along the contour or by turnout drains (Fig. 5), thus reducing the speed of water flow and avoiding the accumulation of all the water from the whole slope in a single drain.
(b) Leading the water in a controlled zigzag through baffles built into the drain to slow down the flow (Fig. 6(a)).
(c) Building steps into the drain (Fig. 6(b)). The area on to which the water falls from each step is built to resist the force of the falling water. Step drains are practical if the slope exceeds 30%, but otherwise they become too expensive.
(d) Checkwalls (Fig. 6(c)) are a less expensive solution to the problem, and can be used in unlined drains. The water deposits silt behind each checkwall, gradually building up a stepped drain. The checkwalls should be set well into the ground on each side and beneath them, to ensure that the water does not cut a way past them. In particular, the foundation of each wall should not be higher than the crest of the next one downstream.
Fig. 5. Turnout drains to divert water from a steep slope
Fig. 6. Types of construction for steep drains
Checkwalls can be built of various materials besides concrete or masonry (Fig. 7). Piles of large stones help to dissipate the energy of the water as it flows through the tortuous spaces between the stones. The stones must be large enough to resist being carried downstream by the water.
In areas where rocks of sufficient size are not available, smaller rocks may be tied together in a large bundle or bale known as a gabion. A gabion is made by filling a large basket of galvanized wire mesh with stones, to make a large rectangular bundle of about 0.5-1.0 m3. These can be built up into a wall; however, it is advisable to fill them only after putting them in position. Bamboo strips may be used as a substitute for wire, although they will rot away in a few years. As the bamboo deteriorates, weak cement can be applied sparingly to the exterior of the gabion, taking care not to block completely the spaces between the rocks. When a gabion is newly placed, the rocks have to settle down; a weak concrete would easily crack whereas wire and bamboo are flexible.
Fig. 7. Types of checkwall or dissipator
In areas with a moderate ground slope of about 4-10%, drainage channels may be lined with concrete, masonry or vegetation to prevent scouring of the channel bottom. Channel linings are discussed in section 2.6.
In flat low-lying areas subject to flooding, a major problem often results from the relatively high level of the receiving water body. This limits the slope to which drains can be laid, so that water flows along them quite slowly. Together with the difficulty of digging deep drainage channels where the groundwater level is high, this means that drains have to be relatively wide in order to have sufficient capacity.
Sometimes there is no alternative to using landfill to raise the level of the ground in all or part of the neighbourhood. Landfill limited to the streets will cause increased flooding of people's plots and houses, so that adequate quantities should be provided, sufficiently close to people's houses for them to cart it away and spread it on their premises. They should be helped to judge how to place it by marks painted in advance on each house showing the level to which the ground should be raised by the landfill.
The idea of people placing rubble and soil inside their houses to raise the floor level may seem strange to some, but there are low-income urban areas whose residents have been glad of up to 50 cm of landfill placed in this way. Their houses will eventually need modification or rebuilding as a result, but the impact of landfill can so transform an area that residents often wish to build a new house more appropriate for the improved surroundings, once they are convinced that it will be safe from flood damage.
The water level in the receiving water body often fluctuates, owing to tidal effects or the flow of water into it from other catchment areas. These variations in level can be analysed in terms of their return period when a decision is made as to the depth of landfill required.
Alternatively, tidal variations in level can be turned to advantage by installing a sluice gate at the outlet from the drainage system which is opened at low tide and closed when the level rises. The need for landfill can also be avoided by building a large embankment or dike along the bank of a river liable to flood, or right around the residential area creating a polder (Fig. 8). Of course, some installation such as a sluice gate is needed to allow a way out for water drained from the area. However, no such arrangement should be considered without very thorough study by an engineer and a guarantee of reliable operation and maintenance. A dike that overflows or a sluice gate that fails to function could do enormous damage.
Fig. 8. The polder system
Another difficulty in the drainage of flat areas is the deposition of sediment in the drains, owing to the slow speed of flow of the water. Where possible, drainage systems should be designed to produce a minimum self-cleansing speed of flow, at least when the drains are running full, so that water will carry the sediment along with it. In a drainage channel with a rectangular cross-section, the water will flow slowly in a thin layer on the bottom after light or moderate rain. Moreover, any irregularities in the flat bottom will create puddles in which mosquitos can breed. Building a drainage channel with sloping sides and a narrow bottom helps to maintain a steady flow speed whatever the water level in the channel. A refinement of this principle is to build a channel with a composite section (Fig. 9). The central channel with a narrow bottom is to carry the flow in dry weather and moderate rain, while the outer channel is for the occasional heavy flood flow. The outer channel floor should preferably slope gently down to the central channel or cunette.
Fig. 9. Cross-sections of typical composite drainage channels
A self-cleansing speed of flow also requires a minimum slope, which is greater for small drains than for large ones. Roughly speaking, a channel 10-15 cm wide will need a minimum slope of about 1 % to achieve a self-cleansing speed of flow. A channel twice the size needs roughly half the slope. Such minimum slopes are not always achievable, though, as there may not be a sufficient drop in level from the street to the receiving water body. However well the system is designed, some sediment is bound to be deposited, so that regular cleaning is essential to keep the drains working.
Engineers and administrators often have a preference for closed drains rather than open channels, probably because they are more accustomed to them. Yet closed drains have several disadvantages:
(a) They cost more to build, because they require deeper excavation, must withstand heavy loads on the street overhead, and also require expensive additional works such as manholes and inlets.
(b) Construction defects, deterioration and accumulation of debris or sediment are more difficult to monitor than in an open drain.
(c) The design, construction and maintenance of closed drains require more sophisticated engineering techniques.
(d) Since closed drains are laid beneath the ground, a smaller drop in level to the receiving water body is available to obtain a sufficient minimum slope to ensure self-cleaning flow speeds.
(e) Mosquito breeding in closed drains is more difficult to control.
(f) Slowly-moving sewage produces gases that can attack cement and concrete in a closed drain if it is not well ventilated.
The main advantage of closed drains is that they do not take up surface space. They also reduce the risk of children playing in or falling into polluted water, and the possibility of vehicles damaging the drains or falling into them. It is nevertheless a fact that open drainage channels are used and maintained in good hygienic and aesthetic conditions in sophisticated cities such as Amsterdam and Singapore. Closed drains should be built in low-income tropical areas only after very careful consideration of the other options.
If open channels are built, careful thought should be given to the question of access bridges across them to adjoining properties, for people and vehicles. Without such provision, residents are likely to place stepping stones in the drains, fill them with earth or obstruct them in other ways. The worst option is a drainage system that is partly open and partly closed, so that rubbish thrown into the open section blocks the closed sections, where it is harder to remove. Water dammed up behind the blockage provides a shady stretch of polluted standing water in which mosquitos can breed prolifically.
Some short covered sections are almost inevitable, however, at road crossings and under access bridges. An iron grille should be placed at the upstream end of each such section to keep out solids. If these are made as shown in Fig. 10, then it will be easier to remove the accumulated debris by pulling it up the bars with a rake.
The bottom level of a covered section should not be any lower than the bottom of the channel downstream of it. Otherwise water will stand in it, enabling mosquitos to breed, and it will also be likely to become blocked with silt. If the pipe is of large diameter and if protecting it from damage by traffic would entail burying it at a level lower than that of the channel downstream, then an alternative is needed, such as a wide, shallow culvert (e.g., a reinforced pipe) protected with a concrete slab cover.
Fig. 10. A type of grille which can be cleaned easily with a rake
It is sometimes a conventional practice to build a small basin, called a silt trap, at the entrance to a closed section. However, in most low-income areas these fill very quickly with sand or rubbish, so that they are of little use in practice. Because they are also breeding sites for mosquitos, they should be avoided.
The cheapest drains of all are unlined channels, which can be cut along the roadside with a road grader. The sides of an unlined drain should not slope by more than 1 in 2 to ensure that they will be stable. If the slope along the drain is greater than about 1%, the drain may be damaged by scouring, and some lining will usually be required to protect the channel bottom from the fast-flowing water. For slopes of 1-5%, partial lining is likely to be sufficient and will cost less than complete lining (Fig. 11). In a partially lined drain, special protection is needed at the most vulnerable points, such as culverts, drain junctions, sharp bends, and steep sections.
Another cheap measure, especially suitable for the upper part of a partially lined channel, is to lay turf or sow grass, whose roots will help to hold the soil in place. The most satisfactory grasses are those that spread sideways and cover the surface of the soil. Their rapid growth can be encouraged with fertilizer, by laying topsoil, and by building temporary checkwalls to cause silt to be deposited.
For relatively gentle slopes, the lining does not have to be of solid concrete or masonry. Compacted gravel or stone will be sufficient. Various types of permanent and temporary lining are shown in Fig. 12. Drains with vertical sides always need a lining to support the sides. As this type of channel is used only when space is in short supply and when the drains have to pass close to houses, the lining must be strong enough to protect adjacent house foundations.
Lined drainage channels often fail because the lining does not allow water to enter from the ground at either side. Either this causes water pressure to build up and overturn the linings, or the water runs alongside the drain, cutting a parallel channel. The solution is to provide weepholes, about 10 mm in diameter, in the lining at the sides. This can be done with short lengths of pipe running horizontally through the masonry and embedded in the mortar, spaced at intervals of not more than 1 m.
Fig. 11. Cross-sections of typical unlined, partially lined and lined drains
In very narrow streets where heavy vehicles do not pass and space is at a premium, the road itself may be designed to function as a drain (Fig. 13). This is possible only if the slope is less than 5% and if the road has a surface such as compacted gravel or stone to protect it from erosion. Alternatively, drainage channels may be provided with removable covers (Fig. 13), which should have holes or notches in them to enable water to enter and make it easier to remove them to clean the drain beneath. The latter approach can also be used on very steep sections, with a series of prefabricated channel elements laid as a stepped drain beneath a pedestrian stairway. Fig. 14 shows a design of this kind used in the city of Salvador, Brazil.
Fig. 12. Types of drainage channel lining
The smallest channels, less than 300 mm deep, do not need weepholes, and can conveniently be lined with brick or with precast concrete elements (Fig. 15). Elements should weigh less than 50 kg, so that they can be carried and laid in place by two persons without machinery. Precast channels should preferably be laid on a bed of compacted sand, 50 mm thick. A single channel size can be adapted for larger flows by laying it deeper and building up the sides with masonry.
Fig. 13. Combined drains and pathways
Prefabricated elements have the advantage over masonry or in situ concrete linings in that they can be laid relatively quickly. Masonry drains take a long time to build, and concrete poured in place requires several days to set. Meanwhile, local traffic is disrupted, and the fresh masonry or concrete can be ruined by a sudden downpour of rain. If the drains are built in the dry season to avoid an unexpected rainstorm, there may be a shortage of water to cure the concrete in place. In a covered workshop, elements are protected from the sun and rain, water for curing can be made available, and quality control is easier and better than in conventional construction work.
Fig. 14. Combined footpath and drain made of prefabricated elements, as used in Salvador, Brazil
A common type of closed drain is constructed from prefabricated sections of cement pipe, typically 1 m long and 50 mm in diameter. A tongue on the end of each section fits into the next, ensuring that they are properly aligned (Fig. 16(a)). If closed drains are used to convey sewage as well as rainwater the drainage system is known as a combined sewer system, and another kind of pipe connection is normally used to prevent the sewage from leaking out and contaminating the groundwater (Fig. 16(b)).
Fig. 15. Cross-sections of three types of small open drain
The trenches in which pipes are laid are normally dug at least 0.5 m wider than the pipe diameter, and deep enough for the top of the pipes to be covered with at least 1 m of soil. Before the pipe is laid, a 50-mm thick bed of sand is placed in the trench, compacted and carefully levelled to give an even slope. The pipes are laid over the sand and a spirit level is placed on each pipe to check the evenness of the slope. Then more sand is added and compacted beneath and around each pipe until it is half buried. Finally the excavated soil is replaced in layers 150 mm thick, each of which is compacted carefully. The purpose of the sand bedding and the minimum depth of 1 m is to protect the pipes from damage by heavy vehicles passing over them. In areas with only very light vehicular traffic the sand bed can be omitted and the pipes laid at shallower depth.
Fig. 16. Jointing of reinforced concrete pipes
A closed drainage system must be provided with inlets for the water to reach it from the road surface. These should be covered with a grille to prevent leaves and other coarse solids from entering and blocking the system. One inlet is usually provided every 30-50 m along the road, depending on the slope and rainfall intensity. For the same reason that silt traps are not recommended (see section 2.5 above), gully pots should not be provided at drain inlets.
To facilitate cleaning and maintenance, manholes should be provided at intervals of 120-150 m for pipes of more than 0.6 m in diameter, and 70-100 m for smaller pipes. They are also required wherever the pipes change in diameter or direction, and at intersections. Further details on the design and construction of closed drains are given in standard works on sewerage.
Building a drainage system requires skilled supervision, but many tasks can be done by the community. They include:
· excavation work (digging);
· transport of soil, water, sand and cement;
· compacting of soil or sand in the drain foundation;
· prefabrication of drain elements;
· watering and curing of drain elements;
· transport and storage of drain elements;
· accounting for drain elements and guarding them;
· finishing and planting of embankments;
· providing food for volunteer workers.
Most of these tasks require little special skill and can be done after an hour of instruction. The only task requiring real training is the production of prefabricated drain elements.
The prefabrication of drain elements can easily be learnt by semiskilled community members, for whom it may also prove to be an attractive method of income generation. In many cities there is a ready market for prefabricated elements of this type.
Two simple prefabrication methods are shown in Fig. 17. The first was developed by the National Housing Authority of Thailand and offered to small contractors. The other type of element was designed by the Roorkee Research Centre in India and was intended to form the bottom part of shallow, partially lined drains, although it can be used as a small drain on its own, as shown in Fig. 15. The mould is covered by plastic sheeting or oiled newspapers before the reinforcement and cement mortar are placed on it. A wooden board with a semicircle cut out of it is moved along the channel to ensure that the mortar has the correct thickness.
The purpose of the reinforcement is to prevent breakage during transport of the elements, so that if they are not to be taken long distances the chicken wire can be replaced with cheaper alternatives such as sisal or coconut fibre. This makes the element very flexible while the mortar is still wet, so that a third casting method can be used. The mortar is placed on a plastic sheet while it lies on a flat surface. A rectangular wooden frame lying on the sheet is used to give the layer of mortar the right length and width. The vegetable fibres are spread on the mortar when it has been placed to half the final thickness. When all the mortar is in place, a straightedge is used across the frame to ensure that it has an even thickness - the depth of the wooden frame. Finally, the frame is removed and the plastic sheet is lifted up by two wooden battens previously fixed along the sides, and draped over a semicircular concrete channel used as a concave mould.1
1 HILLMAN. E. Pre-fabricated fibre-reinforced cement irrigation channels, Waterlines, 4 (4): 22-25 (1986).
Fig. 17. Methods of casting prefabricated drainage channel elements
After two days of setting, the elements are removed from their moulds and cured for 5-14 days so as to strengthen the concrete. As the objective of curing is to prevent rapid evaporation of water from the surface, curing is best done by placing the elements in a tank filled with water. However, it is also acceptable to cover the elements with mats or fabric which are then sprayed every evening with water, or otherwise kept wet, for at least five days.
Construction should always start from the downstream end. This keeps the working area dry and makes it easier to check the slope. Checking the slope by eye is risky. Simply checking that water flows away down the newly-laid drain will ensure that it slopes the right way, but in flat areas it may lead to the drain being laid at too steep a slope, making it impossible to maintain an adequate slope further upstream.
If surveying equipment is not available, a simple alternative is a long plastic hose filled with water (Fig. 18). Make sure there are no bubbles in it, and lift up both ends. The water level at each end of the hose will be the same. If a slope of, for instance, 0.4% (0.004 m/m) is desired, this means a difference in level along a 10-m length of drain of 0.004 × 10=0.04 m. If the hose is held so as to keep the water level 1.00 m above the bottom of the trench at the downstream end, the bottom should therefore be 0.96 m (1.00 - 0.04) beneath the water level at a point 10 m upstream. The same method can be used to check the level of the drain elements when they are laid, using a piece of string to check their horizontal alignment. A spirit level should be used to check the slope of individual elements.
Fig. 18. Laying drains to an even slope using a water hose level
It is not advisable to use such simple methods when laying closed drains. A trained surveyor with proper surveying equipment is required.
The community's role in drainage construction can be considerable (Fig. 19), but municipal authorities cannot rely on such community participation to work smoothly in all cases, especially if unpaid work is involved. If a contractor has been engaged for the construction work, he or she may prefer to use his or her own staff so as to maintain control of the construction schedule. If the contractor has to wait for the community to get organized, the municipality may be charged for overheads and staff salaries incurred during the delay. It is often preferable to write into the contract an obligation to give priority to hiring local people and to training a local maintenance team.
If the community is to participate in construction, substantial effort must be devoted beforehand to mobilizing community members and organizing their contribution, and their advice and agreement must be sought from the beginning of the planning stage.
Fig. 19. Tasks in drainage construction
A drainage scheme which is not accepted by the community is sure to fail. Community participation is discussed in further detail in section 4.
Ideally, a community drainage scheme should be developed with a local authority or other body which has the capacity to provide engineering expertise. However, some communities may wish to undertake some improvements if they prove unable to obtain such technical assistance, or while waiting for it to materialize. This section suggests how they might do so, taking advantage of the residents' ability to monitor the results of their work and make progressive modifications in subsequent years.
The following initial steps (a)-(g) are recommended to any group or individual wishing to plan a drainage scheme. No specialized training is needed to carry them out. Engineers carrying out initial studies may also find them useful as suggestions. For steps (h)-(m), it is desirable to have the help of an engineer.
(a) Before you start your survey, it will be wise to obtain a map of the area. Maps are usually obtainable from the city planning department, the land registry or the national survey department. If no up-to-date map is available, you can use aerial photographs, or a tracing made from an original photograph. Quite suitable photographs for this purpose can be taken with an ordinary camera from a small aeroplane, flying at a height of about 600 m. At this low altitude, survey flights can be done under any reasonable weather conditions. The shadows cast by clouds will not interfere with interpretation of the photographs. As a last resort a sketch map can be made; the section on plane table methods in a book on surveying will describe how this can be done without sophisticated equipment.
The map should be of a convenient scale, preferably at least 1:5000. A scale of 1:1000 is best. A map that is of too small a scale can be expanded with a pantograph, or by dividing it into squares and copying it on to larger squares. You can then sketch in details, visiting the area to see if any features are missing or need to be changed.
(b) Walk around the area. Ask the residents about the probable causes of recent flooding or landslides. Residents may not have a technical background, but they can usually identify the source of the water that caused the problem. Long-standing residents may even be able to relate flood or landslide problems to particular events in the past, especially to major civil works carried out in the vicinity, such as the building of road or railway embankments or cuttings, or the filling in of depressions.
(c) Try to establish the water levels reached during a specific flood. Residents can describe the depth in terms of their anatomy (ankles, knees, waist) or by pointing to parts of their fence or house. Marks from a flood may still be visible on walls. Use a tape measure to express this as a depth in centimetres, and write each measurement on your map of the area at the appropriate point. Since water finds its own level, these measurements will give you a good idea of the topography of the area; the greatest depths will be in the lowest-lying areas.
(d) Note the natural direction of flow of wastewater from houses, and of surface water from rainstorms. Mark existing lines of drainage on the map, including both natural streams and man-made channels, noting problems such as stretches of channel blocked by garbage, eroded sections, areas of standing water, and landslides. Make a note of structures built along the waterways which could obstruct drainage or prevent future widening.
(e) Note the discharge point and the water level in the receiving stream, river or sea. Ask the residents about water level fluctuations in the receiving water body, particularly the maximum water levels reached in the last few years and, if possible, the dates on which they occurred. If the receiving water body is a river, the water resources or hydrology department may be able to help, particularly by giving you the dates and probable return periods of major floods, which you can then compare with witnesses' accounts of which areas were flooded on those occasions. If the receiving water is the sea, the local port authority can provide tidal tables. The highest tides normally occur in March/April and September/October.
(f) Prepare a sketch of the drainage improvements most urgently needed, showing where expansion of existing channels is required, where new drains will have to be dug, and where protection against erosion is to be provided in steep areas. Drains should as far as possible follow existing or planned road and path alignments. Nevertheless, some existing structures may have to be moved, and these should be marked on the sketch map. Fig. 20 shows the result of steps (a)-(f) in a particular community.
(g) Call a meeting of the residents and present the proposed scheme to them for their suggestions and approval. Their intimate knowledge of the area puts them in a good position to offer practical advice.
(h) Design the initial improvements. In the absence of the information or technical assistance needed to make detailed design calculations, it is best to start with unlined channels for flood control and boulder checkwalls (or dissipators) to control erosion.
Unlined channels 0.3 m wide are a useful size for the small branches along individual streets and alleys; if they prove to be too narrow, they can be widened at a later date. A close look at existing lines of drainage, including natural channels and especially constrictions which cause water to back up behind them during storms, will give a good idea of the best size for the major drains. If in doubt, channels 1 m wide would be a convenient size to start with.
For steep areas, large boulders, preferably at least 30 cm in diameter, can be used to build dissipator checkwalls held in place by wooden stakes (see Fig. 7, page 18). If the boulders are washed away, it may be possible to collect them and pack them in gabions. The spaces between the stones should not be blocked with mortar, as this will only deflect the direction of the water flow, causing erosion at another point. It is better for the water to flow between the stones, dissipating its energy as it does so.
(i) Excavate the channels, starting from the downstream end and working upstream. To drain flat areas, fix the downstream end of the system at the lowest level possible without its becoming submerged by a typical flood of the receiving water body, even if this means water may flow some distance up the channels at high tide. The slope should be carefully controlled using a water hose (see section 2.8). As a start, a rise of 0.2 m for every 100 m would be a reasonable minimum slope for a channel that is 1 m wide, and 0.5 m for every 100 m in a channel 0.3 m wide. If the slope of the ground allows it, a greater slope is better. If the slope is greater than 5 m in every 100 m, place checkwalls as soon as possible after excavating the channel, one for every 1-m drop in level.
(j) Once the channel has been excavated, visit after rain has fallen and note which sections contain standing water. These are low points. Either raise the bottom level by backfilling with gravel or soil, or deepen the bottom downstream so that the water can flow away. Look also for early signs of future erosion, such as water bypassing the checkwalls or running parallel to the channel.
(k) Add a temporary lining if desired, such as the wood or bamboo lining shown in Fig. 12, page 26. Temporary screens to hold back rubbish can be installed across the smaller channels using wood or bamboo stakes, with gaps of 20-50 mm between them. Allocate responsibilities for weekly cleaning of each screen and organize a monthly work party to clear the major channels.
(l) Monitor the system's functioning during at least one wet season, noting areas of scouring, overflow, stagnant water, silt deposition, or other problems. Water will tend to erode and deepen a channel if it is too narrow and steep, and to deposit silt if it is too wide and the slope is insufficient. Fig. 21 shows some of the changes that can occur in a typical unlined channel, and how they can be used to diagnose the need for suitable measures.
(m) After one or two years, the channel sizes and slopes should be more or less established. The community may now decide to line the channels, build permanent bar screens, road crossings, checkwalls, etc.
Fig. 20. Sketch for planning a drainage system
Fig. 21. Troubleshooting - diagnosing problems in an unlined drainage canal
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