Due to accelerating climate change, which entails more extreme storm conditions and exponential sea level rise, the protective function of our dikes and levees is under growing pressure. The presence of so-called forelands (e.g., beach, mudflat, and tidal marsh) are promising Nature-based Solutions to reduce the probability of dike failure and to mitigate the consequences after a dike breach. However, the positive effects of forelands depend, for instance, on sediment composition, geometry, and the presence of vegetation. These effects have not been studied in detail and have not been incorporated into reliable physical and validated mathematical models yet.

This report describes an experimental study on the stability of different foreland types. Particularly, the following aspects were considered: investigating whether surface and headcut erosion are indeed the dominant foreland erosion mechanisms, plus providing experimental data for validation purposes for models such as the extended BRES model.

Twelve experiments were performed in the sediment flume facility of the Hydraulic Laboratory of the faculty of Civil Engineering and Geosciences at the Delft University of Technology. A foreland during breaching conditions (i.e. high flow velocities) was simulated by an experimental setup, a so-called broad-crested weir. This weir was developed, constructed, and built within the facility to simulate and study erosion processes of two foreland sediment types, sand and clay. Geotechnical properties were obtained by performing sediment analyses in the Geoscience and Engineering Laboratory. In order to try to exclude the influence of cracks, a special adjustment of the facility was made by covering the sediment package with plywood panels; these experiments are referred to as ‘Tompouce’. For collecting data, the facility was fully instrumented with hydraulic sensors (discharge, flow velocities, and water levels) and synchronized cameras. The erosion process was analyzed and quantified from the visual data.

The experiments enabled us to answer the main research question, “Which erosion mechanisms are important to consider for forelands under dike breach conditions?”. For non-cohesive sediments, surface erosion should be considered the dominant erosion mechanism. In the case of cohesive sediments, however, headcut block erosion should be considered the dominant erosion mechanism. Here, when cracks are present, the erosion rate increases considerably. Especially the latter effect is a new finding; this mechanism should be included in foreland-dike breach development modeling. More research is needed into the quantitative effect of cracks on the stability of a vegetated foreland when it is subject to (sudden) high-water events.