DEALING WITH SEDIMENTATION ISSUES IN SPATE IRRIGATION SYSTEMS

With growing water variability and scarcity in Africa a reality, a renewed interest in spate irrigation systems, which harness water in ephemeral environments, has emerged. Spate irrigation is seen as practical solution to supplement food production and improve food security in areas where water supply is scarce or highly variable. The political will in support of the system is evident in countries like Ethiopia, where significant investment is currently underway to upgrade the system. While the potential exists, the actual contribution of spate irrigation systems to food security has, so far, not been significant. One of the major reasons is the very high sedimentation rates of canals and irrigated fields. In spate irrigation systems sediment is a double-edged sword. Fine sediment brings fertility and makes it possible to build up well-structured soils, while on the other hand coarser sediment reduces spate flow diversion efficiency, raises the bed level and removes fields from the irrigation command. As is the case in several other countries, the spate irrigation systems in Ethiopia that were modernized through conventional civil engineering practices, lack effective technical and operational measures that maximize the benefits of sediments. To address this particular problem, a study was conducted in the Fokisa Spate Irrigation System in Ethiopia. The study employed field observations and simulations with various one-dimensional hydraulic models. Primarily based on the study, but also drawing from experience in other spate irrigation systems, this paper analyses the effectiveness of settling basins and canal designs with non-uniform cross sections and varying slopes. The results indicate that settling basins are not an optimal alternative and that designing canals with non-uniform sections and slope leads to better performance. It is concluded that the former are not applicable in spate systems. Copyright (C) 2011 John Wiley & Sons, Ltd.