Calibration et validation d'un modèle hydraulique en milieu de mangrove en Guinée-Conakry

Abstract

Mangrove rice is a typical coastal crop in West Africa. They have the particularity of admitting sea water into their midst thanks to the forces of the equinox tides during the dry season. For two weeks, the water is accumulated in the plain because it fertilizes the soil thanks to the marine silt and, at the same time, it eliminates weeds thanks to the salt and asphyxiation of the soil. These crops therefore require a structure capable of accumulating sea water but also of regulating rainwater during the growing season. The dimensioning of these regulating structures can be carried out thanks to hydraulic modeling which makes it possible to predict the filling of the plain according to the incoming tide. A one-dimensional model was first created on the HEC-RAS software and was calibrated for Manning's parameters (n = 0.0216), the inlet (kin = 0.5 for rectangular pipes and 0.85 for circular pipes) and outlet (kex = 1) coefficients of the control structures and the lateral structure overflow coefficient (Cd = 0.4). The model was then validated on the same plain with another filling phase. The calibrated model was then use in two dimensions. This don’t bring any additional precision compared to the one-dimensional modeling. Finally, two rice field development scenarios were tested: one consisting in changing the dimensions of the regulating structure and the second in rearranging the water management on the plot. This work showed the functioning of HEC-RAS and the impact of the different structures and parameters, and finally presented the possibilities for use in the framework of a hydro-agricultural development in mangrove rice cultivation.