Dike Failure Caused by Flow Overtopping: A Comparison of Two Modelling Methods

Along rivers, fluvial dikes are built to protect humans and property from flooding up to a certain water elevation. Overtopping of dikes during an extreme flood may lead to the formation of a breach, eventually creating a very fast flooding. In order to model the process of flooding due to dike breaching, 2-D shallow water equations are often used. They can be coupled with a sediment transport model simulating the erosion of the dike and 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 development model, simulating the embankment erosion, associated to a shallow water model simulating the flood propagation. The two approaches are compared on a field case and on laboratory experiments. In both cases, parameters are set to be as similar as possible. However, while it is easy to select the same parameters for the propagation component, it is difficult to select equivalent parameters for the breach development model. The comparison between results includes the breach dimension, the breach flow hydrograph and the consequences in the downstream area in terms of flooding and eventual erosion.

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 erosion. The breach development depends on the modelling approach that can favour either the breach deepening or widening. With a fast breach deepening, the breach peak flow is high whereas a high breach widening leads to a peak flow reduced by more than 90%. Adding the erosion at the dike toe induces a higher peak flow but the water elevation in the flooded area is slightly influenced.

The laboratory experiments represent a long flood event that can lead to a total erosion of the dike over a long distance with a large part of the upstream flow deviated into the breach. The end of the dike erosion occurs mainly because the breach becomes very wide (i.e. low flow velocity and shear stress throughout the breach). Adding potential vertical erosion increases the breach depth and accelerates the erosion process in the first step of dike erosion; however, in a second step the breach widening is slowed down, leading to contrasted results in terms of flooding parameters.