Impact of flow on the deposition and sediment trapping capacity in a West African reservoir (case of the Lobo River reservoir, west-central of Côte d’Ivoire)

Abstract

Built to improve the water supply of the populations of the city of Daloa, the water reservoir of the Lobo River is no longer functioning properly due to sedimentation phenomena. This study aims to understand the hydrosedimentary functioning of the reservoir for better management. Based on the river flows, bathymetric surveys, suspended solids concentrations, settling velocity, a two-dimensional modelling of the flows and sedimentation of the Lobo River reservoir was carried out using the MIKE 21FM/MT software. The simulation results indicate that the hydrodynamic model could reproduce the variation of water levels at the free surface of the reservoir. The results are very sensitive to the boundary conditions of the model but also to the initial conditions. A strong influence of the initial conditions on the long-term stability of the models was observed. It could be seen that in all cases the deposition rate tends towards a final value which only depends on the river flow. The trapping efficiency of the reservoir is between 26.7% and 80.4%. The different simulation scenarios indicate that the deposition thicknesses are between 0.02 and 48 mm and that the deposition occurs when the flow is loaded, during the rise and fall phase of the hydrograph. The old intake, the new intake and the weir are the preferred areas for sediment deposition in the Lobo reservoir.