Sediment accumulation in Roseires Reservoir

Sedimentation is a serious problem faced by natural and man-made reservoirs. It is a major problem which endangers and threatens the performance and sustainability of reservoirs. It reduces the effective flood control volume, presents hazards to navigation, changes water stage and groundwater conditions, affects operation of low-level outlet gates and valves, and reduces stability, water quality, and recreational benefits. Reservoirs are often threatened by loss of capacity due to sedimentation. While there being many causes of reservoir sedimentation watershed, sediment and river characteristics are among the main natural contributing factors. Other important factors are reservoir size, shape and reservoir operation strategy. Man-made activities also play a significant role particularly in land use patterns. This paper is an attempt to assess sediment accumulation as well as the rate of sedimentation in the Roseires Reservoir. The basis for the study is the previous bathymetric surveys carried out on the reservoir in the years 1976, 1981, 1985, 1992, 2005 and 2007. Analysis and comparative studies were carried out between the different surveys to quantify the amount of sediment deposited as well the rate at which sedimentation took place. The design storage capacity of 1967 for the different reservoir levels was taken as a baseline. The sediment accumulation rates for the different bathymetric surveys are obtained as the difference between baseline capacity and the computed capacity at the respective levels during the specific survey. It was found that sedimentation in the Roseires Reservoir resulted in the reduction of the reservoir capacity from design storage of 3.0 Bm3 in 1966 to 1.9 Bm3 in 2007, i.e., a loss of approximately 1.1 Bm3 during 41 years of operation. The sedimentation rate varies with both time and levels in the reservoir.