Some communities suffer from drainage problems not because they have no drains, but because the existing drainage system has collapsed, become blocked, or is otherwise in need of repair and rehabilitation. Many more will find that the nearest convenient point of discharge for a new drainage system is an existing primary drainage pipe or canal that needs attention if it is to function properly.
Collapse and blockage are the principal types of drainage failure. Each of these can have several causes. Collapse of drains can occur through:
- erosion of the bottom and sides of the drain (scouring);
- excessive pressure of water in the ground beneath and beside the drain lining;
- vehicles passing over or too close beside the drains;
- root growth, especially from nearby trees;
- crown corrosion in closed drains containing sewage.
The causes of blockage can be:
- accumulation of refuse, leaves and earth in the drain;
- structures such as houses or bridge piers erected in the drain and obstructing the flow;
- excessive vegetation growing in drainage channels;
- silt deposited in low sections owing to misalignment or where the slope is insufficient and cleaning is not regular enough.
If rehabilitation of a failed system is to have a good chance of success, diagnosis and elimination of the original causes of failure are required as well as treatment of the immediate symptoms. Each possible cause of collapse has its cure.
(a) Erosion in an unlined channel is illustrated in Fig. 21 (page 39) along with the appropriate remedies. In a lined drain, erosion can mean the lining itself is not robust enough, and a more resistant lining is needed. A common weak point is at the joints between channel or pipe elements, which should be sealed with cement mortar. Where the slope is greater than 10%, baffles or steps of some kind are needed (see Fig. 6, page 17). Scouring on the outside of a channel lining can mean that water is not entering the drain but running parallel to it. If the lining rises above ground level, it needs notches in the sides so that the water can flow in. Small earth banks running diagonally across the road will also help to divert water to the drain at the side. Alternatively, scouring beside the drain can mean that it overflows during storms, indicating that more frequent cleaning, a larger drain, or more frequent turnouts are required (see Fig. 5, page 16). In closed drains that are overloaded, water can escape into the ground through the joints owing to the pressure inside the pipe. When the pressure drops, the water runs back into the drain, carrying soil with it and excavating a cavity over the drain which will eventually collapse. The solution is to seal the joints with cement grout or, preferably, to build a larger drain.
(b) Water pressure from the outside or pressure resulting from the swelling of clay can be controlled by using a sand bedding (see Fig. 15, page 29) and providing weepholes in the lining (see Fig. 11, page 25).
(c) Vehicles can easily damage open drains. If vehicle damage recurs frequently, the drains should be protected by some form of barrier such as a rail or a kerbstone. If the damage is due to vehicles attempting to cross the drain, then an adequate vehicle crossing should be built over it. Vehicle damage to covered drains indicates that they should either be laid deeper or be protected by concrete.
(d) Roots from nearby trees will tend to grow into drains, especially if they contain standing water and the linings are not impermeable. The most effective protection, if the problem persists, is to remove all trees within 5 m of the drain.
(e) Crown corrosion occurs in closed drains containing sewage, where gases from the sewage can attack and weaken cement, particularly over the crown or cover of the drain.
The cures for most of the causes of blockage are fairly easy to see: collection of refuse, removal of structures, and clearing of vegetation. If the drains have an even and adequate slope, it should not be necessary to remove silt; clearing the vegetation, whose roots hold the silt in place, should enable the next heavy flow in the drain to wash it away.
However, the slope is not always even. The drain may have been badly laid, the sand bedding beneath some sections may have been eroded causing them to sink, or the lack of weepholes or a sand bedding may have caused the lining to be lifted by the pressure of water from the surrounding ground. Uneven settlement of the ground is common in flat areas of clay soil, and is another cause of uneven slope. Damage by vehicles and earthquakes can also cause distortion, or even misalignment of sections of drainage channel or pipe, resulting in blockage by sediment or other solids. In such cases, the drains should be rebuilt to an even slope, although slight irregularities can be corrected by filling in the depressions with cement mortar.
Finally, the drain itself may be in good condition, but may fail to function correctly owing to insufficient capacity. Even a drain that was large enough when built may prove to be too small for the increases in runoff flow which accompany increased building development in the catchment area.
There are many drainage systems in urban areas that are functioning imperfectly or not at all owing to one or more of the causes of failure listed in the previous section. Before a new system is envisaged, the first step is to ascertain whether a drainage system already exists and whether it can be rehabilitated. Local residents will normally know if one exists in their area, but they may not be aware of existing main drains, especially closed drains, which are outside their neighbourhood, and into which a future local micro-drainage system could discharge.
Municipal records, including old drainage master plans, should be consulted for details of any previous drainage construction in the vicinity, and the area should be visited on foot to check their accuracy and to look for tell-tale signs such as old manholes, or pieces of pipe or concrete exposed by erosion, especially along major streets and downstream of the area where better drainage is needed.
Remove cover slabs from drainage channels, taking care that there is no risk of pedestrians' or vehicles' falling into them accidentally. Starting at the downstream end, remove silt and solids. Special tools for cleaning channels are described in section 3.3. After clearing, flush the drainage line with water. The fire services may be able to help with this.
Inspection of closed drains
The inspection of closed drains is more difficult and dangerous, and should be carried out under expert supervision. The first step is to draw a sketch map of the system, if record drawings are not available. The map should show all existing manholes, inlets and other drainage structures. If the gap between any two manholes is very long compared to that between most others, it is likely that one or two other manholes have been buried or destroyed between them. From the regular manhole spacing, it should be possible to calculate the most likely location of a missing manhole. Local residents, who may know of buried manholes, should also be consulted. The probable sites of missing manholes should be excavated, to uncover them.
No one should enter any manhole until it has been adequately ventilated. As a precaution, the manholes upstream and downstream of the section to be inspected should be opened at least two hours beforehand. To save time, a number of manholes can be opened simultaneously. Further ventilation can be achieved by introducing the air hose from a compressor, if one is available. Inspection should start as far downstream as possible, and work upstream. Water in flooded manholes should be pumped out to the next manhole downstream using a sump pump, of the type used by construction firms for excavations. Alternatively, the water could be baled out with buckets or removed with a siphon, but this is likely to take a very long time.
Once the manhole has been ventilated, a further safety check is necessary to ensure that it is safe to enter. A lighted candle or a miner's safety lamp is lowered into the manhole. If the flame dies, it means that there is insufficient oxygen inside and that anyone entering the manhole could be suffocated by the gases produced by sewage and sediment. However, no naked light should be used until the manhole has been ventilated, as it could cause those gases to explode.
A final safety precaution, no less necessary, is that no one should enter a manhole without a lifeline. A spare lifeline should be ready for use if necessary. Access steps in an old manhole are liable to be seriously corroded and much less secure than they appear. They are often slippery. At least two people should remain above ground to pull out the third member of the party in case of emergency. They should never follow the third member into the manhole, even in an emergency, as they could all be killed. Even if the manhole is properly vented, the person inspecting it should disturb the settled sludge and silt as little as possible. These sometimes contain poisonous gases, which could be released when the sediment is agitated. If a drain is completely blocked so that it cannot be inspected, material should be removed only from the upstream end. These safety precautions are illustrated in Fig. 22.
The alignment of a closed drain can be checked by two people in consecutive manholes using a flashlight and a mirror, as shown in Fig. 23. First the flashlight and the mirror are held within 5-10 cm of the bottom of the pipe (known as the invert), and then they are both raised to just below the crown. If there is any irregularity in the vertical alignment of the drain pipe, it will be detected in one of these positions, since it will obscure the flashlight beam. This procedure will also make minor defects and obstructions visible.
The most likely place to find cracks and misalignment is immediately adjacent to the manhole, owing to uneven settlement of the ground after pipe-laying (Fig. 24). Another problem to look for is crown corrosion, which can easily be discovered by attempting to dig into the pipe material at the sides and top with a penknife or a large nail.
Drainage pipes of less than 1 m in diameter cannot be entered safely, and great care should be taken in entering larger drains. The dangers include poisonous gas, cave-ins, sudden rushes of water from clogged sections or from storms, and even wild animals. Naked lights such as matches or candles should not be used in a closed sewer or within 3 m of any open manhole. A miner's safety lamp is preferable to a flashlight, to avoid the risk of explosions.
Fig. 22. Safety in entering closed drains
Fig. 23. Checking the alignment of closed drains
Some sections may require complete rebuilding, but others may only need to be cleared of obstructions and flushed with water. Deteriorated or cracked concrete or masonry should be made good, care being taken to avoid major irregularities, especially at joints, which may hold back solid objects and cause blockage. The surface to be repaired should be roughened by hitting it with a sledge hammer, and then plastered with good quality cement mortar. If plastering is needed on the bottom of the drain, first divert the water flow away from the working area by building a small dam of earth or sandbags and digging a temporary parallel channel or by pumping.
Fig. 24. Common failure sites in closed drains
Some fittings may be damaged or have disappeared, especially metallic ones such as manhole covers, inlet screens and grilles, which may have been stolen and sold as scrap metal. The community may decide to replace these with concrete equivalents, or to fix metal screens into concrete. This makes drain maintenance a little less easy, but minimizes theft if it is a problem. If closed sections are frequently blocked by refuse, additional screens should be installed at the upstream end to keep the refuse out (see section 2.5). Existing covers and inlets should be cleaned, and repaired or replaced if necessary, and steel fittings painted with two coats of tar or primer paint.
If the drainage system has sluice gates (Fig. 25), the handle, plates and guide channels in the frame should be checked. Rust and old paint should be removed with a steel brush. Any holes should be patched by welding a steel plate over them. The gate and frame should be painted with three or four coats of an epoxy or other equally durable type of paint. The stem and guide plates should be well greased.
Fig. 25. Requirements for maintenance of sluice gates
The most important maintenance task is to remove refuse, silt and other solid material from the drains. All drains should be cleaned at least twice a year, preferably at the start and end of the rainy season. Some drains, especially the secondary drains and house connections, will need to be cleared more frequently. Small open channels in flat areas are likely to require cleaning on a weekly basis. Unlined channels need to be regularly cleared of vegetation.
It is important to establish the cleaning of drains as a routine activity at regular intervals, and not wait until the system fails as a result of blockage. Repairing the damage done when the system fails, including damage to the drains themselves, can cost far more than regular preventive maintenance.
Drain clearing must be coordinated with the collection and disposal of solid waste, so that solid material removed from the drains will not be left where rain can wash it back or where it can be a nuisance and a health hazard, encouraging the breeding of rats and flies.
Cleaning of open channels is usually done manually, with the help of spades, hoes, shovels and scoops. It can be disagreeable and strenuous work if it is done with the wrong tools, especially if the drains are deep. It is worthwhile having some special tools that can clean the drains over their whole length, such as shovels that just fit into the drain.
One tool that has proved to be useful for cleaning deep and narrow drains is an agricultural hoe with an extra-long handle (Fig. 26).
Fig. 26. Tools for cleaning drains
Another tool, also shown in Fig. 26, is the Ahmed-Davis shovel. This was developed in Tunisia, where it was found to reduce cleaning time by 30%. One person pushes the shovel deep into the drain using the handle, and then the other pulls it forward and upward using the steel wires attached to the front end. The size and shape of the shovel are determined by the size and shape of the drains. It may help to pierce several small holes in the bottom of the shovel so that water in the solids from the drain can run out when the shovel is lifted.
The responsibility for maintenance of a system of drainage channels is often divided between several residents, neighbourhoods or work teams, each responsible for a particular section. If so, it is advisable to install grilles across the channels at the downstream end of each section. This ensures that solids are not carried along to the next section, imposing an excessive burden on that section's team.
A grille in a small drainage channel. Grilles such as this retain rubbish near the point where it enters the channel, so that the responsibility for cleaning each section can be fairly divided.
Photo: S. Cairncross
In the cleaning of closed drains, all the safety precautions described in section 3.2 should be scrupulously observed.
The most common tool used in removing silt and solids is a bucket tied in the middle of a steel cable (Fig. 27). The cable should be at least twice as long as the longest distance between manholes. One end is threaded into the drainage line with the bucket facing downstream, and wound on to a windlass on the ground beside the next manhole. As the bucket is pulled down the line, it scoops the solids and silt. The bucket diameter must be at least 5 cm smaller than the internal diameter of the drainage pipe to allow excess solids to pass around it, and ensure that the bucket does not become jammed against obstructions.
Excessive force should not be used to pull the bucket if it sticks, as this may compress the solids, making them still more difficult to remove. Instead, it should be winched back and a smaller bucket or an auger (described below) used for the first pass. When the bucket reaches the downstream manhole, it should be removed with the solids and detached from the cable. The cable is wound back and the bucket reattached. The procedure is repeated until the drain is cleaned.
If the drainage line is blocked, or the solids are too stiff to be removed using the bucket, the line can be cleared by an auger (Fig. 28). The auger is like a large drill bit, and is rotated by means of a lever inserted into one of a chain of connected driving rods. The rods are normally 10-15 mm in diameter and made of stainless steel. Water trapped upstream of the blockage does not have to be removed. When the auger penetrates the solids obstructing the line, this water will help to flush the solids away.
Fig. 27. Cleaning of closed drains using a bucket
Fig. 28. Cleaning of closed drains using an auger
The need to coordinate drainage maintenance with solid waste disposal has already been mentioned. Coordination is necessary for two reasons. First, the solids removed from the drain must be adequately disposed of. Second, the drains cannot be kept clear without effective solid waste disposal. If solid wastes are not collected regularly, residents will have little choice but to throw their rubbish into the drainage channels, or to dump it in the streets and open spaces where it will be dispersed by stormwater, wind and animals, much of it eventually reaching the drains. The most effective way to ensure good coordination between drainage maintenance and refuse disposal is for both these activities to be the responsibility of the same municipal department or neighbourhood committee.
Maintenance, including the inspection, cleaning and repair of the drainage system, must be institutionalized if it is to be kept up throughout the life of the system. For this reason, the ultimate responsibility should preferably be with the municipality, which has paid staff who can carry out the work. It is much more difficult to mobilize a community on a voluntary basis to carry out a routine task, year after year, than to win their active participation for the limited period required for construction. Nevertheless, there is ample scope for participation by the community in drainage maintenance. Section 4 describes how this can be organized.
Safety in sewers and at sewage works. London, Institution of Civil Engineers, 1967.
FLINTOFF, F. Management of solid wastes in developing countries. New Delhi, WHO Regional Office for South-East Asia, 1984 (WHO Regional Publications, South-East Asia Series No. 1).