The role of sand dams in water supply in arid areas (Machakos, district - Kenya)
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In the arid and semi-arid areas of Kenya, inadequate water supply is a majbr constraint to development and various methods have been attempted to increase the water supply. In this project, a case study was carried out to evaluate the role of sand dams in water supply for domestic and livestock use. The study was carried out in part of an arid and semi-arid district (Machakos). A catchment of 67 was selected where 12 sand dams have been constructed. Sand dams are stone/concrete weirs constructed across the riverbeds where sand accumulates during the rainy season and water is stored in the pore spaces of sand. Detailed investigations were carried out on one sand dam which included: i) Volume of sand deposited ii) Maximum water storage capacity iii) Extractable volume of water from sand iv) Water level fluctuations due to evaporation losses, recharge from upstream runoff and extraction by people and livestock v) Water utilization by people and livestock The results obtained showed that the quantity of extractable water stored in the sand dams is small, but the reduced evaporation losses allows for longer period of storage and makes the water available for extraction in the middle of the dry season when it is required most. Large pore spaces other than total porosity determines the potential water holding capacity of sand. Coarse sand and fine gravel will hold more water than fine sand. This means that sand dams are applicable only to the areas which are rich in sand and gravel deposits.