Some hydrological investigations toward the design of sand dams in Kenya

Sand dams are unconventional, low technology option for water supply used in arid and semiarid lands (ASALs) in Kenya. The main feature of a sand dam is a concrete weir constructed across an ephemeral stream. Design approaches vary from place to place with equally varied results. However, it is apparent that storage capacities of sand dams are dependent on hydrologic and topographical factors of the catchment. This paper outlines the results of a field study carried out in 1997–1999 in West Pokot district of Kenya to investigate the role of hydrological factors in the design of sand dams. Rainfall and stage of flow at an ephemeral stream in a typical ASAL area were monitored. Stream discharge ranged from zero to 29 m 3 /s. Suspended sediment concentrations were found to be as high as 24200 mg/l and consisted of around 30% clay. A maximum bedload flux of 2.02 kg/m/s was observed at a modest discharge of 2.55 m 3 /s. Gradual construction of the weir in small step heights was shown to result in the deposition of sand that has a better grading than would be attained in single stage construction.