Author(s): Vera Van Bergeijk; Jord Warmink; Suzanne Hulscher

Linked Author(s): Vera van Bergeijk, Jord Warmink

Keywords: Erosion; Flood defence; Cover; OpenFOAM; Grass

Abstract: Wave overtopping is one of main failure mechanism of flood protective structures. More extreme weather conditions and sea level rise due to climate change will result in more frequent overtopping. Especially structures with a grass coverare vulnerable for climate change because droughts decrease the strength of the grass cover. This calls for dike cover designs that are more erosion resistant for wave overtopping, for example innovative covers such as geogrid. The hydraulic forces on the cover are required to determine the stability and the erosion resistance of these covers. Detailled modelling of the overtopping flow provides insights in the hydraulic load on the cover of flood defences. Moreover, these models are able to compute the effect of transitions on the downstream flow and cover erosion contrary to analytical and empirical models. In this study, we focus on geometrical transitions including slope changes, such as the transition from the crest to the landward slope, and cliffs that can form at erosion holes. The flow can separate from the dike surface at geometrical transitions resulting in hight impact forces at the location of reattachment. This process significantly influences the pressure, flow velocity, shear stress and normal stress of the overtopping flow and thereby affect the cover erosion. We have developed a detailed hydrodynamic model in the open source software OpenFOAM that calculates the hydraulic load on grass-covered flood defences. We show that transitions in cover type result in acceleration of the flow for smoother covers and deceleration for rougher covers, that is accurately calculated using an analytical model. Height transitions result in flow separation and high hydraulic loads at the location of impact. The model results show that transitions in cover type have a limited effect on the hydraulic load and height transitions have a major impact on the hydraulic load leading to more than doubling of the hydraulic load. Therefore, the effects of transitions in height on the hydraulic load dominate over transitions in cover type. The findings in this research are the first step towards fundamental understanding of the behaviour of dike covers under hydrodynamic wave loads, and their soil strength. We explored the hydrodynamic wave loads during storm events, but identified that knowledge about the geotechnical strength of vegetated dike covers is limited. In future studies are working on the transition towards Future-Proof dikes as a nature-based solution, which are safe, are constructed from environmentally friendly local soil with a biodiverse vegetation and are resilient against climate change.

DOI: https://doi.org/10.3850/IAHR-39WC252171192022428

Year: 2022