Sand dams: Source of water in arid and semi-arid lands of Kenya

Rural people in arid and semi-arid lands (ASALs) in Kenya collect their water from small non-perennial streams when they carry water in the wet season and from holes dug in the sand in the shallow riverbed during the dry periods. To ensure sustainable supply of water during the entire season, a weir called SAND DAM is constructed by erecting a concrete/stone masonry wall on a natural rock outcrop across a riverbed at a suitable site. The dam is constructed with the assumption that there will be sufficient amount of sediment eroded from the watershed, to be trapped by the dam. Water will be impounded in the pores of the accumulated sediments for use during the dry season. This paper presents an assessment carried out in 1999 of the water resources available in some existing sand dams in the West Pokot district and their potential adequacy to match the demand. The amount of drainable water in the sand dams ranged from just over 44 m 3 to over 5500 m 3 . The highest specific storage (defined as the volume of drainable water per unit area of dammed channel cross section, m /m ) in sand dams is apparently obtained from river slopes between 1 and 2%. Other channel characteristics varied greatly from one river channel to another and even along a specific channel. Average channel – side slopes of 2:1 and 3:1 were adopted. The lowest dam had a height of 0.5 m while the highest was 3 m. The reach lengths of the reservoirs ranged from 40 to 500 m while channel widths ranged from 6.5 to over 50 m. The paper also highlights the acute shortage of hydro-geological data that form the basis for planning and development of the sand dams and points to the urgent need to install a satisfactory gauging network in ASALs for monitoring rainfall and runoff. In particular, bedload transport is crucial in relation to the design of sand dams and therefore requires exhaustive research in the arid and semiarid regions.