Sediment Yield Variability at Various Spatial Scales and Its Hydrological and Geomorphological Impacts on Dam-catchments in the Ethiopian Highlands

Abstract

This chapter analyzes the spatial variability, impacts, and factors of sediment yield (SY) and reservoir sedimentation rates (SRs) in the upper Blue Nile River Basin. SY data collected using reservoir sediment surveys or runoff and suspended sediment concentration (SSC) measurements at river gauging stations in the framework of different programs were compiled, screened, and used for our analysis. A large spatial variation in area-specific SY (SSY), ranging between 4 and 4,935 t km−2 year−1, was observed among catchments. This variation is attributed to both human and environmental factors. The high SY values have drastic consequences for the life expectancy of many reservoirs in the Ethiopian highlands: 50 % of reservoirs risk losing their economic life within half of the design period. Moreover, the high trapping efficiency of the reservoirs for flow and sediment led to selective deposition of sediment fractions within the reservoir and channel stabilization and vegetation regrowth in the downstream river reaches. Unfortunately, the availability and reliability of SY data for this region is poor by international standards. The SY assessment initiatives taken through institutional collaboration projects is a good start; however, such projects have limited capacity and a short life span, so they cannot produce a sustainable solution for this important data gap. Hence, concerted efforts on the maintenance and monitoring of existing gauging stations on top of establishing new ones are needed to better understand the different eco-hydrological environments in the basin.