What is the research about?
Wave overtopping is one of the main failure mechanisms of dikes. During a storm, high waves break on the waterside slope, flow over the dike crest, and flow down on the landward slope. These waves pull on the dike cover resulting in damages that can result in a dike breach.
Innovative blocks are developed to protect dikes against the forces of waves. Blocks on the waterside slope will slow down the waves and therefore reduce the amount of water flowing over the dike. The current reinforcement of the Afsluitdijk along the lake IJssel shows the use of different types of blocks to protect the dike against the wave forces of the Wadden Sea in the north of the Netherlands. The company Hillblock develops blocks that can be placed on the waterside slope, the crest and the landward slope of a dike. As part of Luuk Barendse’s MSc study, All-Risk researchers investigated the usage of their Grassblocks, which are placed underneath the grass cover to protect the dike against the wave forces once the grass is eroded.
Figure 1. Impressions of both the a) experimental tests (Steeg et al. 2017) and b) the numerical model (Barendse et al. 2021, p20)
Investigating the wave forces on the dike cover
Back in 2017, Paul van Steeg et al. tested a concrete dike with Grassblocks on the crest and landward slope in the Delta flume at Deltares. The performance of the blocks under extreme conditions can be investigated using these types of tests (see Figure 1a). However, it is hard to measure the wave forces on the blocks during these tests and the tests are expensive so only a limited series of tests have been performed.
Therefore, we developed a numerical model using OpenFOAM where we can calculate the wave forces on the dike at every location (see Figure 1b). This numerical model was based on the outputs of All-Risk projects on the effects of berm and roughness elements on the amount of wave overtopping (Weiqiu’s project) and the effect of transitions on the overtopping flow and dike cover erosion (Vera’s project). Luuk Barendse further developed the existing numerical model in his master research project to calculate the forces on the entire dike profile where blocks can be included at multiple locations.
Figure 2. Comparison between experimental and model results on a point at the crest of the dike (Barendse et al. 2021, p35)
A numerical model to help bridging the gaps of experimental tests
In this 2021’s study, the flume tests with the Grassblocks were simulated. The comparison between the experimental and model results at multiple locations of the dike allowed us to calibrate the numerical model (see Figure 2). Once calibrated, we used the numerical model to determine the forces of the waves on the blocks, which could not be measured during the experimental tests. The water levels and wave heights can be changed so the performance of the blocks under different storm conditions can be studied. Additionally, we can change the characteristics of the blocks in our model to develop the best design of the blocks.
For example, the size of the blocks and the open space within the blocks (see photo) can be varied in the numerical model. Overall, the model has proven to be a useful tool for the development of innovative covers to protect the dikes against waves. The wave forces on the dike cover can be assessed at every location on the dike making it possible to study covers on the waterside slope as well as the landward slope.
- Read the MSc thesis of Luuk Barendse: Hydrodynamic modelling of wave overtopping over a block-covered dike
- Visit the website of Hillblock to know more about the Grassblock itself.
Last modified: 15/09/2021