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Impact of drought on the strength of grass and the underlying clay layer in dike systems

Stam, E.B. van (2025) Impact of drought on the strength of grass and the underlying clay layer in dike systems.

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Abstract:Dikes are vital for flood protection and water management in the Netherlands. With climate change increasing the frequency and severity of droughts, there is a growing need to understand how drought affects dike integrity. This thesis aims to quantify how drought conditions influence the erodibility of grass and clay layers in Dutch dikes (specifically Wadden Sea dikes), focusing on wave attack scenarios on the waterside slope (GEBU, a standardized failure mechanism in Dutch dike safety assessment). A key objective of this study is to assess how drought alters a newly developed erosion coefficient (from the ongoing “Onderzoek Erodeerbaarheid Klei” project) and to identify which clay material properties contribute most to increased erosion risk. The effect of drought on grass revetment was studied using theory from the BM Gras software. The analysis involved categorizing drought-affected grass into existing grass quality classes, which influence erosion resistance under wave loading. For the clay layer, insights from the recent OEK (Onderzoek Erodeerbaarheid Klei) project were used to examine how measurable clay properties (such as consistency index, water content, and dry density) influence erosion behavior through the erosion coefficient. Additionally, this research compares the Mourik formula (currently used to understand the erosion of clay) with the OEK formula to evaluate their assumptions and predictive capabilities. The results of the thesis show a degradation of strength in both the grass and clay layers of dike systems under drought conditions, making them more erodible and at greater risk during wave attack. The effect of drought on grass cover is strongly influenced by pre-drought maintenance and site conditions; for example, south-facing slopes are significantly more affected. The most vulnerable slopes experienced severe degradation, with large portions of the surface reduced to open patches due to grass die-off. This condition leaves the dike highly susceptible to wave attack and necessitates reseeding to restore erosion resistance. For the clay layer, drought was found to increase erodibility, particularly near the surface. In clay layers less than 0.5 m deep, drought conditions can more than double the erosion rate, while deeper layers are less affected due to limited moisture loss with depth. These findings highlight the urgency for further research, particularly in light of current data limitations and knowledge gaps related to modeling drought-induced degradation. A major constraint in this study was the limited availability of experimental data on drought-affected grass and clay. With more comprehensive datasets, it would be possible to derive drought-specific parameters that better represent grass behavior in the BM Gras Buitentalud model, thereby improving simulations of erosion under drought conditions. Additionally, further clay erosion experiments are essential to deepen our understanding of drought effects on clay layers and to strengthen the empirical basis for modeling their behavior.
Item Type:Essay (Bachelor)
Faculty:ET: Engineering Technology
Programme:Civil Engineering BSc (56952)
Link to this item:https://purl.utwente.nl/essays/107876
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