Sediment Yield Analysis of the Mwache River Basin and Its Impacts on the Mwache Multi-purpose Dam in Kwale County, Kenya.

Abstract

Water reservoirs in the form of dams and water pans face the challenge of excessive accumulation of sediments leading to reservoir siltation and reduced lifespan. Sedimentation is exacerbated by the ever-changing, retrogressive and environmental degrading land use practices particularly in most arid and semi-arid lands. The objectives of this study were to establish the sediment yield from Mwache river basin, to assess the comparative contribution of land use-land cover types on sedimentation and to estimate the life expectancy of the Mwache multi-purpose dam located within the river basin. Regular gauging stations (RGS) corresponding to three diverse land-use-land-cover types were selected; Chigutu stream RGS on shrub land, Maji ya Chumvi stream RGS on grass land and Mwache dam site stream RGS on annual crops. Stream cross-section area and flow rate measurements from each of the three streams were taken and water samples collected using equal discharge increment (EDI) procedure. The water samples were analyzed for total suspended solids using APHA 2540 analytical method of the year 2005. Analysis of the data collected was done using simple linear regression model and hypothesis tested using the student’s t-test statistical analysis to establish whether there were significant variations in sediment yield from the different land use-land cover types in Mwache River basin. The estimated volume of sediments from Chigutu, Maji ya Chumvi and Mwache dam-site streams were 20,235m3, 7,948.571m3 and 35,510m3 corresponding to shrub land, grass land and annual crops respectively. On the best sediment yield reliability test by simple linear regression model, shrub land had a coefficient of determination r2 of 92.65%, grass land 72.13%, and annual crops 66.86%. The estimated annual sediment yield was 46.038 tons/year and the estimated life expectancy for the Mwache multi-purpose dam was 202.8 years implying that the dam construction was viable similar to results from the project design report which estimated the lifespan at 291.3 years in the year 2014. However, with the r2 values for all the three land-use-land-cover types being strongly positive, sedimentation is still an issue albeit the measures which have been put in place to address it. This study therefore recommends to the Water Resources Authority, Coast Development Authority and other involved stakeholders to sustain the periodic sediment yield assessments at the various gauging stations within the Mwache basin, and to intensify the frequency of monitoring of the sediment yield mitigation measures put in place under the various land use-land cover types, priority order being annual crops which had the most yield, followed by shrub land which had the second most yield and finally grass land which had the least.