Water from Small Dams

Water drawn directly from a dam reservoir is not suitable for domestic use, unless thewater is boiled, or sterilised by the sun’s ultraviolet rays in a transparent bottle (theSODIS method). However, clean water can nearly always be drawn from a hand-dug wellsunk into a seepage line somewhere downstream of an earth dam.7.5 SpillwaysA spillway should be sited at a distance of at least 10 metres from either end of a damwall to avoid floodwater eroding the dam wall. The depth of the spillway is measuredfrom the line of maximum water level, which is equal to the lower line of the freeboard.The floor of a spillway is made level at the centre line. From there the floor should slope3 cm for every 100 cm towards its upstream and downstream edge. The gradient can befound using a simple tool consisting of a spirit level placed on a 3 metre long timber thathas a 9 cm long leg at one of its ends. When the spirit level shows it is horizontal, the legwill be situated 9 cm lower than the other end of the timber, thus marking the requiredslope of 3 cm per 100 cm. (as 3 cm per 100 cm = 9 cm per 300 cm.)A spirit level placed horizontally on a 300 cm long piece of timber having a 9 cm longleg at one end gives a gradient of 3:100, which is required for the floor of spillways.Where the floor of the spillway does not consist of weathered rock, then sills(small walls) of stone-masonry should be constructed across the width of the spillway todistribute an even flow of water in the spillway and prevent erosion.Further protection from erosion can be achieved by building a low wall of large stones setin mortar along the side of the spillway next to the dam wall.To minimise the risk of a thunderstorm flooding a reservoir and destroying an incompletedam wall, it is important to excavate a part of the spillway to its final depth before majorconstruction work on the dam wall begins, so that water has an outlet, if needed.Soil taken from the spillways should be used for the construction of the dam wall. If thevolume of soil is insufficient, soil has to be taken from either widening the spillway orfrom a borrow pit in the reservoir. For safety reasons it is wiser to widen the spillwaythan to enlarge the borrow pit.48