On the use of the diffusive wave for modelling extreme flood events with overbank flow in the floodplain

Catastrophic flood events are frequently chosen as reference events, for modelling the impact of scenarios of river bank remediation to prevent inundations. However, these rare events are poorly understood due to the poor hydrological knowledge available, and due to the lack of data. A model for routing flood events adapted to the few available data during historical extreme events was developed. The model subdivides the floodplain into cells linked to the main channel, and the diffusive wave model with overbank flow was used to route floods in the main channel and through the inundation floodplain. The linearized equations are resolved by an implicit numerical scheme to ensure stability and convergence. The flood event of October 1958 on the Hérault river, which caused one of the largest inundations Southern France during the XXth century, was studied as the application case. As input data, the model requires a Digital Elevation Model, geographical map and cross sections in order to subdivide the channel network into cells. The model parameters are the Manning roughness coefficients in the main channel and in the floodplain. The model was applied to simulate the flood event of 1958, and was validated on other flood events under similar hydro-meteorological conditions. Then, the model was applied to quantify the impact of scenarios of river bank remediation. Fourteen scenarios of construction of dams, embankments, or coupling both dams and embankments, were modelled. The model simulates hydrographs in the main channel, the water depth, the area and the volume on both right and left banks of the floodplain cells. Results show that the model developed herein, with relatively little hydraulic modelling, is well adapted to simulate extreme flood events, and is useful for environmental managers to study the impact of river bank remediation on inundations.