2.6 Channel design and construction

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