The effect of geometrical transitions on the wave overtopping load

Erosion by overtopping waves can lead to failure of grass-covered earthen flood defences. Previous studies have indicated that cover failure starts at changes in material or geometry defined as transitions. In this study, the overtopping flow at geometrical transitions is modelled and their effect on the hydraulic load is quantified. A numerical model is developed in the OpenFOAM software and simulates the overtopping flow over the crest and the landward slope of a grass-covered flood defense. Two types of geometrical transitions were studied: slope changes and height differences. These transitions are representative for the landward crest line, the landward toe, erosion holes on the slope and a road on the crest. New relations are developed for the maximum shear stress and the maximum normal stress as result of the geometrical transitions using design parameters such as the geometry and the overtopping volume. These relations can be used in existing calculation methods to include the effects of geometrical transitions on the hydraulic load for the design and the safety assessment of flood defences.