Water from Small Dams

Chapter 7.Construction of a valley dam7.1 Check list before starting construction workBefore construction work begins it is important to check that the following criteria havebeen met and relevant procedures followed according to the sequence listed below:1) A suitable site for the dam has been identified and its feasibility investigated interms of the issues highlighted in Chapter 2.2) A written agreement on the ownership of the dam site, an access road, usage ofwater from the dam and conservation of the catchment has been completed.3) Design drawings and bill of quantities for the dam are ready.4) A decision regarding the method of excavation of soil works whether manual labour,draught power or machinery. Quotations for purchases and hiring of labour ormachinery or prior agreement with community regarding free labour inputs.5) Any legal requirements have been addressed.6) Funds for survey, design and construction of the dam have been secured.7) Construction of valley dams should only be done during dry seasons where there isvery little risk of heavy rainfall because a dam under construction can easily beswept away by a thunderstorm or heavy shower.7.2 The keyThe key under the dam wall prevents water from seeping through layers of sand orsandy soil under a dam wall. The two ends of a key should extend to the maximum levelof water in a reservoir.Place two pegs at both ends of the lowernylon string to show the width of the key,which should have a width of 2.5 metres.Attach a nylon string between these 4 pegsto mark the full extent of the key.When a key has been excavated to a depthof 60 cm below any layer of sand or sandysoil, the vertical sides of the key are cut toa slope of 45 degrees for stabilising theexcavation.45