Subsurface Dams as Sustainable Water Harvesting Option

Water availability is declining at rapid pace for humans, livestock and agriculture in most parts of the Sudan. Erratic rainfall, and cyclic drought is leading to desertification in it. Rapid increase in human & livestock populations, and corresponding decline in grazing rangelands and decrease in available water resources have caused tribal conflicts and wars. Wadi Kaya, Darfur starts flowing intermittently with the first heavy rains of May and runs continuously by mid-June up to September. The annual rainfall is 496mm with total average annual flow volume of more than 43MCM. But due to absence of water storage sites, 85% of it is lost by flash floods. If this water is not harnessed timely, then rest of the year there is huge water scarcity situations. So this study was done to identify suitable water harvesting techniques to capture/store the water of flash flood and to recharge groundwater aquifers. Topographic surveys, Geology and geotechnical investigations, (including field exploration & laboratory testing) and hydrological studies were carried. Water availability is calculated for June to September as rest of the months contributes minimum rainfall, which cannot generate runoff. Rainfall data of Nyala meteorological station from 1923 to 2000 is used for rainfall flood frequency analysis. The computer model HEC-HMS developed by the US Army Corps of Engineers is used to model the Wadi catchment. The results of HEC - HMS in terms of inflow hydrographs developed for different years` return periods and the peak discharges and storages against 5, 10, 20, 50 and 100 year return period were calculated. Consultants’ team observed that the proposed sites are not suitable for construction of surface storage dams due to their high cost, high water evaporation rate, low lands on both sides of Wadi, social problem due to Inundation of large arable lands in the Wadi flood plain and local people unacceptability. It was found that it would be feasible to obstruct the flow of Wadi Kaya’s alluvial aquifer of study area by construction of subsurface dams at suitable locations and reduce the variation of the level of the groundwater table. These will solve all described problems of surface water dams and sub-surface dams are more stable & safe and does not require maintenance. Three alternative sites were also proposed for further investigations and assessment of their suitability for construction of subsurface dams based on meteorological and topographical analysis.