Effects of sedimentation on reservoirs in the Mushibemba catchment, Mkushi Farm Block, Central Zambia

Thumbnail Image
Mphande, Goodfellow
Journal Title
Journal ISSN
Volume Title
University of Zambia
Sedimentation is one of the problems that affects the storage capacity of most small reservoirs, and if not addressed on time, may lead to the dams being filled up with sediment and failing to meet the intended objective of providing agricultural water for food and economic security. The aim of this study was to assess the effects of sedimentation on the storage capacity losses of Moffat dam and GRZ weir located in the Mushibemba catchment of the Mkushi Farm Block in Central Province, Zambia. The Mushibemba catchment has the highest number of reservoirs that were constructed to meet increased irrigation water demands. However, there is lack of application of the knowledge on appropriate techniques of assessing sedimentation rates on reservoirs in the catchment. Understanding the nature of sedimentation in the Mushibemba river catchment is useful for mitigating against the reservoirs being filled up with sediment. The specific objectives of this study were to (i) determine the storage capacities of two reservoirs; (ii) estimate suspended sediment transport into the streams draining reservoirs and; (iii) determine the rates of sedimentation on the Mushibemba reservoirs. Bathymetric survey using hydrographic boat mounted with a differential GPS was used to collect reservoir depths, water surface elevation and reservoir perimeter. For water quality assessment, water samples were collected at the intake of each reservoir and analysed for concentration of total suspended and total dissolved solids. The results of the study revealed that the measured volumes of Moffat dam and GRZ weir were 1,180,462 m3 and 197,218 m3, respectively. The measured reservoir capacity values were then compared with the originally calculated designed capacity values to determine changes in storage capacity over the years. The storage capacity losses for Moffat reservoir was found to be 223,789 m3 (16 percent) whereas that of the GRZ Weir was 53,312m3 (21.3 percent). The estimated rate of sedimentation loss for Moffat reservoir was found to be 13,986.81 m3 yr-1 with a lifespan of 84 years while that of GRZ weir was 1,480.89 m3 yr-1 with a lifespan of 133 years. The sediment concentration inflows inflow into the Moffat reservoir and GRZ weir were in the same low order of magnitude, 0.6 mg/l and 0.4 mg/l respectively. The source of the suspended sediment was mainly attributed to the cleared commercial agricultural land which predominantly consists of clayey to loamy soils in the vegetated catchments. This also accounted for the high turbidity of the reservoir water. It is concluded that sedimentation in the catchment is fairly low but serious given reservoir capacity losses observed due to agricultural activities despite having a good vegetation cover. Soil conservation measures are needed to avoid degradation of the catchment in future. Keywords: Sedimentation, Bathymetry survey, Reservoir storage capacity, Suspended sediment, Central Zambia
Reservoir Sedimentation--Central Zambia , Sedimentation