* The method depends on the type of dam and the type of ground. The construction of sand dams in cascades improves total storage and efficiency and minimizes seepage losses. Dams made of concrete, stone-masonry (cheapest and easiest) and brickwork require skilled labour for construction, but are stronger and have a longer lifespan.
* Key into banks (sub-surface dams) or construct wing walls to avoid erosion around edges of the sand dam. Where wing walls are built, a good technique is to start from the wing walls and work inwards to the centre, since community enthusiasm lags by the time wing walls are constructed (if not built first), yet they are essential to proper functioning. Length of the wing wall varies according to bank characteristic: loose riverbanks, 7 metres; hard soils, 5 metres; hard & impermeable soils or rocks, wing wall is not needed. Planting napier grass along upstream riverbanks controls erosion and fixes the course of the river in a flood.
* The height of wall built before each flood event should not exceed accumulation rate of coarse to medium sand during that flood event, otherwise ponding & silt deposition will occur, which can lower specific yield and higher capillarity, then lead to limited extraction rates in wells upstream and more water lost to evaporation. Dams at 1.3m depth showed that where finer material content (0.063 mm or less) is increased, specific yield is known to decrease remarkably. Accumulation rate and therefore height varies according to location and should be adjusted at each site after the first flood event demonstrates the rate of accumulation. Height per stage will probably be between 0.3 metre and 1 metre per stage according to experience from past projects. Some silt deposition will always occur as velocities decrease toward the end of the flood event; the idea is to limit its quantity in final sand volume.* Avoid downstream erosion problems by making a protective slab (stilling basin) at the base of large stones set in concrete. Dimensions to be designed, but is not necessary where there is exposed rock bar downstream.
* In upstream parts of a catchment it is recommended that sand dams are always built in stages, since the availability of coarse material is generally limited and base flow is small or absent (base flow = sub-surface flow which aids surface flow so that silt/clay can still be flushed away once rainfall stops, rather than sinking directly into the sand bed). It has also been suggested that to build in stages over several years is also more beneficial for functioning of dam committees. Build them within a spillway, which was part of the first stage & wing wall construction.
