Water from Small Dams

9.3 Washed-out spillwaysSpillways can be washed-out to such depths that they drain all floodwater out of theirdam reservoirs either due to erosion caused by excessive floodwater, or because the floorof a spillway was not made to withstand erosion, as recommended below.Recommendations to prevent washed-out spillways:• A huge volume of rainwater runs off a large catchment during heavy rains. If asmaller dam is built with a large catchment, the reservoir will fill up with waterquickly and a huge surplus of water will pass over the spillway, where it may causeerosion.• It is therefore recommended that smaller valley dams (storage capacities below10,000 m 3 .) should not be constructed in valleys with catchments larger than about400 hectares (1,000 acres).• Where the floors of spillways consist of sandy soil, they may be washed-out bysurplus water from even small catchments. This can be prevented by either:a) Planting drought-resistant and short perennial grasses with runners(Star Grass or Kikuyu Grass) in contour lines spaced about 30 cmacross the floor of spillways.b) Cover the floor of spillways with stones packed closely together and interplantedwith the type of grass mentioned above.c) Construct low walls of stone-masonry as horizontal steps across the floorof spillways, where they will function as a staircase and reduce the speedof the overflowing water. The walls should be built in keys 30 cm deepand spaced about 2 metres apart.d) The slope of the spillway floor should not exceed 3 cm in 100 cm.9.4 Silted-up dam reservoirsRainwater transports topsoil and other light surface particles from a catchment to a damreservoir where some of it settles to the floor of the reservoir as a layer of silt.A layer of silt being only a few centimetres thick is beneficial to a dam reservoir becauseit reduces seepage, but thicker layers of silt decrease the storage capacity, reducing thevolume of water and the lifespan of dam reservoir.Catchments without soil conservation, and earth dams without silt traps, may result indam reservoirs that cannot store any water at all within less than 10 years.57