A study of the effect of the slope angle of a green dike on the failure of the grass revetment due to wave impact
Peters, M. (2020). A study of the effect of the slope angle of a green dike on the failure of the grass revetment due to wave impact. MSc Thesis. University of Twente: Twente. 77 pp.
Over the past years, nature, biodiversity and climate change have played an increasingly large role in ood protection projects in the Netherlands. One of these ood protection projects is the \Wide Green Dike" at the Dollard. A wide green dike is a wide dike with a grass cover on the entire waterside slope. The dike, in comparison to most sea dikes, does not contain a hard revetment to deal with the incoming waves. Unfortunately, it is uncertain under which exact storm circumstances the seaside grass revetment fails. The assessment of the strength of the grass revetment against wave impact is captured in the \Wettelijk Beoordelingsinstrumentarium" (WBI), but the slope angle is not included although it has an important effect on the revetment strength. Therefore, the objective of this research was to determine the effect of the slope angle on the duration until failure of the grass revetment due to wave impact, also termed resistance-duration. The \Wide Green Dike" at the Dollard is a demonstration project and has an estimated slope of 1:7. For this project, it is necessary to obtain knowledge about resistance-duration of the revetment on a gentle slope. Next to computation of the relation between the slope angle and the resistance-duration, the return period of the storm when the revetment fails for the case was studied. Results of executed experiments were gathered with a literature study and were used to establish the relation between the slope angle and the resistance-duration. The results of the experiments with different slope angles were compared with the predicted resistance-duration curves of the WBI and the Wave Impact Pressure Erosion (WIPE) model. A linear negative correlation between slope angle and resistance-duration described this relation the most accurately. This means that a grass revetment on a slope of 1:6 has twice the resistance-duration compared to a revetment on a slope of 1:3 with similar wave conditions. With the found relation, the resistance-duration curves for slopes between 1:3 and 1:8 were generated. The resistance-duration curves were applied on the case of the project \The Wide Green Dike" at the Dollard. For different storm conditions with a specific return period, the frequency of the storm to occur, the moment of failure was calculated. This resulted in a return period of 90 years for a slope of 1:7, while the WBI, that does not take the slope angle into account, predicts a return period of less than 10 years. The slope angle thus substantially reduces the probability of failure for the grass revetment in case of a gentle slope. Finally, a sensitivity analysis was conducted. Different grass parameters were changed to determine the effect of the uncertainty of the quality of the grass on the resistance-duration. The resistance-duration was very sensitive for the root tensile strength, which describes the force that breaks the roots. The tensile strength depends on the type of grass and the maintenance of the grass revetment. However, this aspects is only included in the WIPE model and not in the WBI for the safety assessment. It is advised to further study the tensile strength and to implement this parameter in the WBI model. From this research, the primary recommendation is to implement the found relation between the slope angle and the resistance-duration in the safety assessment, because it does have a large impact on the assessment of the grass revetment. Additionally, this study found that waves below 0.5 meter do not cause damage to the dike and will not result in failure. The WBI suggests a threshold value of 0.25 meter, but the results from this study indicate that an increase of this threshold value to 0.5 meter is reliable.
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