Predictings sediment loading into Masinga reservoir and its storage capacity reduction

Abstract

It is estimated that the annual loss in storage capacity of the world’s reservoirs due to sedimentation is around 0.5 – 1.0%. For many reservoirs, however, annual depletion rates are much higher and can go up to 4% or 5%, such that they lose the majority of their capacity after only 25 – 30 years. The Masinga reservoir, one of the main reservoirs in Kenya, designed for hydropower generation, public water supply and irrigation is faced with severe sedimentation. The designed sediment load into this reservoir in 1981 was estimated to be 3.0 x 106 m3 per year (about 1% per annum reservoir reduction). By 2000, annual sediment loading had increased to over 11.0 x 106 m3 , nearly four times, thus reducing the designed capacity by more than 15%. As land degradation has become more evident with increasing land use change within Masinga catchment over the years, the operation and life span of Masinga reservoir is thus under imminent danger from erosion and sedimentation. There is need therefore to quantify spatially soil erosion and sediment yield reaching the reservoir with a view to reducing the sediment delivery. In this paper, a comprehensive procedure to predict spatial sediment yield and overall mean annual sediment volume delivered to Masinga reservoir is presented. Geographical Information System (GIS) technology as a tool to support soil erosion and sediment models is employed. Simulations of different land use and management scenarios are performed and their corresponding sediment yields estimated. Predictions show annual sediment loading into the reservoir of about 14.0 x 106 m3 for land use practices in 2003. By simulating the best feasible management practices (BMPs), the achieved results show that the sediment volume reaching the reservoir could be reduced to about 6.0 x 106 m3 per year.