The effects of transitions in cover type and height on the wave overtopping load on grass-covered flood defences

Abstract

Grass cover erosion by overtopping waves is one of the main failure mechanisms of grass-covered flood defences. Observations have shown that grass cover erosion often starts at transitions such as changes in cover type and geometric changes. However, it is unclear how the effect of transitions on the overtopping load can be included in existing erosion models. In this study, the increase in load as the result of transitions in cover type and height are quantified using a numerical model in OpenFOAM®. The hydraulic load by overtopping waves is simulated for various transitions in cover type on the crest and a landward berm as well as height transitions on the crest and the slope. The model results show that transitions in cover type have a limited effect on the hydraulic load contrary to height transitions that lead to more than doubling of the hydraulic load. Formulations for the maximum shear stress and the maximum normal stress as result of height transitions are determined based on a fit through the model results. These formulations can be used to include the effects of height transitions in existing calculation methods for the erosion by overtopping waves and the safety assessment of flood defences.