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)

Built to improve the water supply for the population 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 hydro-sedimentary functioning of the reservoir for better management. Based on the river flows, bathymetric surveys, suspended solids concentrations, and settling velocity, 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 water level fluctuations in the Lobo river reservoir. The results are very sensitive not only 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. In all cases, the deposition rate tends towards a final value that 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/year and that the deposition occurs when the flow is loaded during the rise and fall phase of the hydrograph. The old and new intake and the weir are the preferred areas for sediment deposition in the Lobo reservoir.