M. Irías Mata1*, A. Zubova2, M. de Ridder1
1 Dept. Coastal Structures & Waves, Deltares, Delft, The Netherlands; 2 Dept. Hydraulics for Infrastructure and Industry, Deltares, Delft, The Netherlands
With the increasing threads against the coastal zones and the nature (stronger wave conditions and sea level rise), the world demands solutions that comply with safety and climate adaption as well as nature preservation. One pilot project developed in the Netherlands that satisfies both needs is the Wide Green Dike project. The Wide Green Dike consists of a gradually sloping dike that smoothly transitions into the salt marshes and is located along the Ems-Dollard estuary. The dike is constructed with clay made from locally dredged sediment that is ripened to clay near the dike. Due to the innovative layout and materials used in the construction of the dike, physical tests were carried out in the Delta Flume on a real scale to assess the safety of the dike against high wave loads (up to 2 m) and to study the erosion rate of the clay. To extend the study and gain a better understanding on the process of clay erosion, an analysis of the pressures along the profiles of erosion of clay is desired.
The numerical flume consists of a CFD model OpenFOAM coupled with a potential flow solver OceanWave3D. The offshore wave conditions from the wave paddle are the input values in OceanWave3D, which then passes the water elevations η(x,t) to the OpenFOAM model, which simulates the wave-structure interaction. In this way, the physical tests are replicated in the numerical flume. The dike geometry is based on the subsequent clay erosion profiles measured during the physical modelling campaign. The numerical model is used to determine the pressures during the physical model experiments, since the pressure due to wave impacts is the main driver of erosion. Next, a relationship between the modelled pressure and the observed erosion is developed for the wave conditions during the physical experiment. Finally, the dataset is extended using the numerical model to simulate the pressures for different wave conditions, such as variation in wave steepness, with which the erosion could be calculated with the erosion-pressure relation.
A relation between the observed erosion, the modelled pressures and the wave conditions is determined that can be used in the safety assessment of these innovative flood defences. Moreover, the numerical flume provides insights into the important hydrodynamic processes that occur during extreme events and will be developed further to help improve our understanding of the erosion process on flood defences.
Figure 1: Wide Green Dike under wave attack. Left: Physical flume. Right: Numerical flume.
This study has been performed in cooperation with the Waterboard Hunze en Aa's and Sweco. Especially the contributions of Henk van Norel, Marco Veendorp and Jos van Zuylen are acknowledged.