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Modelling the grass cover erosion in the wave run up zone of sea dikes : applying an OpenFOAM model to delta flume experiments

Bijen, N.G. (2021) Modelling the grass cover erosion in the wave run up zone of sea dikes : applying an OpenFOAM model to delta flume experiments.

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Abstract:flood standards came into effect recently whereby the design water level reached above the hard revetment of outer slopes of many sea dikes. Consequently, the grass revetment on the dike slope is subject to wave impact and wave run-up erosion whereby it is not known which erosion process is dominant. Scale tests are performed in the delta flume at Deltares in order to test the erosion resistance of the sea dikes along the Wadden Sea coast with the new flood standards. The erosion resistance of different grass and clay qualities are tested to provide insights into the effect of the cover layer quality on erosion rate. The objective of this study is to determine how grass covers on the outer dike slope erode and which hydraulic variables can be used to predict the erosion. This study is divided into three parts: (1) analysis of erosion data from the delta flume experiment, (2) creating a hydrodynamic model in OpenFOAM to simulate a dike from the delta flume experiments with and without grass cover erosion, and (3) computing the erosion using the OpenFOAM model results. In the first part of this study, elevation data of a laser scanner measuring grass revetment profile changes after each delta flume test were analysed and the maximum erosion depth and erosion volume was determined. The results showed that the grass cover eroded three times faster when the grass is dried out compared to normal grass. Therefore, it is suggested that the effect of dry summers, especially in the wake of expected climate change, are considered in the design of dikes with grass covers. The analysis on clay erosion showed that the higher quality clay contributed to a roughly 35% increase in erosion resistance compared to low quality clay. However, the clay quality did not seem to have a significant effect on the erosion of the grass cover. In the second part of the study, an OpenFOAM model was created and validated to describe the effect of an erosion profile on simulated hydraulic load. The model results showed that the hydraulic load on the eroded grass revetment profile is generally lower than on the initial profile. The lower half of the eroded grass revetment profile is mostly sheltered by the, not erodible, hard revetment slope, experiencing little hydraulic loading. However, a cliff is present towards the end of the grass revetment surface, which endures high dynamic pressures, flow velocities and shear stresses. Although the dynamic pressures at the cliff in the eroded profile are not higher than on the dike without erosion, the velocities and shear stresses are significantly larger. For the third part, results of the OpenFOAM model were used to compute the erosion that occurred in the experiment. Several empirical relations describing wave impact and wave run-up erosion were used and calibrated. The results show that wave impact relations using dynamic pressures and wave run-up relations using flow velocities are both capable of describing the erosion depth of the grass revetment, which is mostly situated above the wave impact zone. However, dynamic pressures show to be the most accurate when replicating the erosion profile measured in the delta flume experiments. Additionally, a head cut erosion model was used to compute the cliff erosion using the flow velocities and the water layer thickness on the grass revetment. To conclude; grass and clay quality have a major influence on the erosion rate of a grass revetment cover layer on the outer dike slope. The distribution of hydraulic variables on the grass revetment slope changes significantly when the grass revetment cover layer has eroded. Dynamic pressures can best be used for determining the erosion volume and the erosion depth, combined with a head cut erosion model to compute the erosion of significantly eroded cover layers.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Programme:Civil Engineering and Management MSc (60026)
Link to this item:http://purl.utwente.nl/essays/86062
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