Sedimentation and its effects on selected small dams in Lusaka province, Zambia

In Zambia, the need to conserve water resources through technologies that can easily be managed by the local communities has resulted in the construction of small dams. However, small dams are adversely impacted by sedimentation due to soil erosion in the catchment area. The aim of this study was to assess the water storage capacity loss for the selected small dams due to sedimentation in Lusaka Province. The studied small dams were; Lwiimba, Silverest, Morester and Katondwe dam. Data was collected by bathymetric survey for each small dam using hydrographic boat with echo sounding. The shape and area of each reservoir were determined by mapping reservoir perimeter with a boat equipped with DGPS, by taking several position points along the shore line. The initial storage capacity data were collected from the dam owners and through interviews with three key informants on the effects of sedimentation on dam uses and on the existence of sediment control measures. Analysis of data involved determination of reservoir surface area and storage capacities using the Area and Volume tool under ArcGIS 3-D Spatial Analyst. The deposited sediment volume was estimated by subtracting the measured storage capacities from the initial storage capacities of reservoirs. Thematic analysis was applied in analysing the effects of sedimentation on dam uses and the existence of sediment control measures. Results of the study revealed that the measured reservoir storage capacities for Lwiimba, Silverest, Morester and Katondwe dams were 101,051.43 m3, 368,331.5 m3, 14,724.32 m3 and 10,714.88 m3, respectively. The estimated sediment volumes equivalent to storage capacity loss for each dam were; Lwiimba (99,044.57 m3), Silverest (379,480.5 m3), Morester (13,805.68 m3) and Katondwe dam with 9,937.12 m3.The accumulation of these sediment volumes have led to capacity losses, drying of reservoirs especially in the dry season and reduced life spans of the dams. The estimated rates of sedimentation for Silverest dam was found to be 14,595.40 m3yr-1. At this rate, the reservoir lifespan was 26 years; Lwiimba (2,200.99 m3yr-1), with the lifespan of 46 years; Katondwe (283.92 m3yr-1), 38 years, and for Morester dam the rate was 251.01 m3yr-1 giving a lifespan of about 58 years. Natural vegetation cover was found to be the main sediment control measure used in the catchments to reduce sediment deposition by runoff. It is concluded that the studied small dams are seriously affected by high rates of sedimentation from the time of their construction. This calls for periodic dredging of deposited sediment in small dams in order to increase reservoir storage capacity for sustainable use of the water resources. More sedimentation studies need to be conducted in Zambia.