Évaluation d'un modèle à base physique pour l'estimation des flux de sédiments lors des crues soudaines - Evaluation of a physically-based hydrological model for estimating sediment fluxes during flash floods

Abstract: This study evaluates the performance of a physically-based, distributed hydrological model, called MARINE, for simulating suspended sediment transport during flash floods at catchment scale. Intense flash floods can induce significant soil erosion and sediment transport, leading to long-lasting soil loss. MARINE takes into account the spatial variability of catchment characteristics and precipitation. The study was carried out on two small catchments in southern France, La Claduègne in Ardèche (42.3 km²) and Auradé in Gers (3.28 km²). MARINE uses a mean-flow advection equation to simulate the spatio-temporal evolution of non-cohesive suspended sediment concentration, taking into account source terms representing raindrop erosion and shear stress erosion.

The study examines the impact of various parameters on the simulation of suspended sediment concentration during flash floods in these catchments, including: (i) the median diameter of sediment particles, (ii) the location of the interface between bed load and suspended load, (iii) the soil sensitivity coefficient to shear stress erosion, (iv) the soil sensitivity coefficient to raindrop erosion. The results of the sensitivity analyses applied to selected flash flood events highlight the model’s sensitivity to two key parameters for both catchments: the soil sensitivity coefficient to shear stress erosion and the median sediment particle diameter. Thanks to the sensitivity analysis of the model parameters, the simulations led to classify floods into two categories: those dominated by raindrop erosion and those dominated by shear stress erosion. Analysis of the results also highlights the need to take into account the spatial variability of soil sensitivity to erosion, in particular by identifying the location of potential sources of sediment.

To assess the model’s performance in estimating erosion, the results for the Claduègne catchment are compared with two widely used empirical models derived from the Universal Soil Loss Equation (USLE), namely RUSLE and MUSLE. To compare the three models with MARINE as reference, a sensitivity analysis is carried out on the land-use factor involved in MUSLE and RUSLE. Although the erosion simulated by the three methods is generally comparable, variations appear for events dominated by raindrop erosion, suggesting the need for further research to improve raindrop erosion modelling within the MARINE model.

MARINE model simulations also offer the possibility of generating erosion/deposition hazard maps which, combined with a vulnerability map, can be useful to environmental decision- makers and planners in identifying areas at risk from erosion and deposition.

Jury

• Benoit Camenen, Directeur de recherche INRAE Lyon – Rapporteur Luis Cea, Senior lecturer, Universidade da Coruña – Rapporteur
• Denis Dartus, Professeur émérite Toulouse INP
• Roger Moussa, Directeur de recherche INRAE Montpellier – Rapporteur Anne Probst, Directrice de recherche émérite CNRS
• Pierre Sagnes, Ingénieur, OFB
• Directeur/trice(s) de thèse Ludovic Cassan, Hélène Roux