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DIKE FAILURE CAUSED BY FLOW OVERTOPPING: A COMPARISON OF TWO MODELLING METHODS

In order to model the process of flooding induced by fluvial dike breaching, 2-D shallow water equations are often used. They can be coupled with a sediment transport model simulating the dike erosion and the erosion of the neighbouring bed areas. However, such a model is not dedicated to the specific process of embankment erosion. One may prefer using a specific breach model, simulating the breach expansion and outflow hydrograph, associated to a 2D shallow water model simulating the flood propagation in the areas protected. In the present work, the two approaches are compared on a field case and on laboratory experiments. For the field case, the short overtopping duration involves that the results are very sensitive to the erosion parameters associated to the dike breaching, particularly the critical shear stress necessary for the initiation of surface erosion. The breach development depends on the modelling approach that can favour either the breach deepening or widening. With a fast breach deepening (breach model), the breach peak outflow is high whereas a high breach widening (sediment transport model) yields a peak flow reduced by more than 90%. The laboratory experiments represent a long flood event leading to a total erosion of the dike over a long distance with a large part of the upstream flow being deviated into the floodplain through the breach. The two approaches (breach model and sediment transport model) provide rather similar results and close to the laboratory measurements although the breach model provides faster initial erosion and is less sensitive to the type of erosion rate equations.