Dynamique et modélisation des crues dans le bassin du Mono à Nangbéto (Togo/BEN)

Abstract

The devastating floods in recent years in West Africa call for improvement in hydrological forecasts to reduce the vulnerability of local communities. We studied flood dynamics in the Mono basin, in Togo and Benin, upstream of the Nangbeto Dam, using statistical methods. The increase in precipitation in the 23-year period from 1988 (following completion of the Nangbeto Dam) modified the regine of the Mono River, with an increase in stage discharge. There is little difference between the river discharge at the entrance and the exit of the Nangbeto Reservoir, at least at peak flow, which emphasizes that the dam, due to the small capacity of the reservoir, has a limited effect on flow regulation. The annual peak flows entering the reservoir can be modelled using a Frechet-type generalized extreme value (GEV) distribution, while those at the exit follow a Weibull-type GEV distribution. The main mechanism for peak flow generation in the basin is a steady rise of water as a result of increasing soil moisture saturation, rather than isolated rain spells. Peak flows between 1988 and 2010 do not show any change in magnitude and frequency. The GR4J hydrological model is shown to be effective in reproducing peak flows, but its skill decreases as we move away from the calibration period. Therefore, it may be risky to use this model for long-term projections of future flood risks and other approaches should be considered.