Modelling of Suspended Sediment Discharge for Masinga Catchment Reservoir in Kenya

Abstract

Masinga reservoir is experiencing high rates of siltation due to accelerated soil erosion in the catchment occasioned by rapid deforestation. There is need, therefore, to devise sediment management strategies both for the reservoir and the catchment to minimize the siltation rate. To give meaningful suggestions on techniques for sediment management, the priority is to model the sedimentation process in the reservoir. Sedimentation models however, require suspended load as the basic input data. For Masinga reservoir, there is no reliable suspended sediment data. Only scanty information on suspended solids is available. Because of this data scarcity, an integrated approach was used to model both the soil erosion from the catchment and the sedimentation process in the reservoir. The Water Erosion Prediction Project (WEPP) model was applied to Masinga catchment to estimate soil loss due to surface runoff. Modelling of the sedimentation process in the reservoir was accomplished by using a semi-two-dimensional model for water and sediment routing, GSTARS 2.1. Through erosion modelling, Sediment-rating curves for both Tana and Thika arms of the reservoir were obtained and used as input into the sedimentation model. The output from the sedimentation model showed preferential siltation in the old river channels, at the two mouths of the reservoir, at the confluence, and near the dam wall. Based on this spatial sediment distribution information, sediment management strategies are proposed. Dredging out fine sediments, construction of sedimentation basins at the two mouths of the reservoir and flushing out sediments through bottom outlet gates are suggested to provide solution to Masinga's myriad siltation problems.


(Journal of Civil Engineering, JKUAT: 2002 8: 89-98)