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.

Embankments are one of the key aspects of flood protection programs. As the sea levels are expected to rise the coming decades coastal protection is becoming increasingly important. To better prepare for the future, increased understanding of embankment breaching is desired. An area of embankment breaching that has been relatively left untouched is the impact of a foreland during a breaching event. This study examines the different effects caused by the presence of a foreland, namely reduced flow rate through the breach, limited breach growth and an elliptical erosion pattern in the foreland.
Embankments are generally more erodible than forelands, and erosion of the foreland occurs after the adjacent embankment has been breached to its base level. As water continuous to flow over the foreland, a plunging jet will form, falling from the high foreland into the breach bottom. This causes undermining in the foreland and subsequent headcut erosion. A distinction is made between cohesive and non-cohesive forelands. Cohesive soil layers experience rotational failure and non-cohesive layers experience sliding failure.
A model is presented that calculates headcut erosion in a foreland, flow characteristics over the foreland and erosion shape of the foreland. The erosion shape of the foreland affects the discharge through the breach and is captured in the non-dimensional constant m. The presented model is implemented as a module into the existing breaching model BRES. The added functionality will increase the accuracy BRES model results.
Two experiments were performed to verify the model. The first experiment examined soil failure of a foreland in dry conditions by pushing an extracted foreland soil layer from a table to observe the failure type and the effects of vegetation on the strength of the soil. The second experiment was performed in collaboration with the University of Antwerp. A soil layer was extracted from a foreland and placed in a large flume. The foreland was subjected to a plunging jet to simulate a breaching event. The outcomes of the model were in line with observations made during both experiments.
The presented model serves as a significant first step towards implementing forelands in flood protection programs, but further research is suggested. The uncertainties stemming from assumptions in the model and the underlying sediment transport relations are significant. Most importantly uniformity of the soil is assumed, whereas in reality root systems in fertile forelands can lead to significant local strengthening. The assumptions in this model are conservative, but additional studies can further increase the accuracy of the foreland- and BRES model.

Due to the advancements made in the GPS technology, a new smartphone RTK-GPS system can be used to do measurements with high accuracy, while the equipment is much cheaper than before. In addition, the equipment is relatively small. This makes the system useful for measuring the velocity of the water currents with the use of a drifter. Currently there are no cheap drifters available in which the RTK-GPS smartphone system is integrated. Therefore, such a drifter must be designed. The design needed to be compact, lightweight, watertight and must be designed for use in rivers and small streams. Concept drifters have been made to test the shape, dimensions, material and watertightness of different designs. Using the obtained knowledge, a final drifter design for the bachelor thesis was made, which met most of the requirements for a working design. All the steps taken in the design process of the drifter are described in the report. Further steps in the process of designing a drifter with an integrated smartphone RTK-GPS system have been described in the conclusion of the report.