Over the dike top: modelling the hydraulic load of overtopping waves including transitions for dike cover erosion

Abstract

Grass-covered flood defences are globally used to protect low-lying areas against flooding. Grass cover erosion by overtopping waves is one of the main failure mechanisms of flood defences leading to a dike breach and flooding. Accurate descriptions of the wave overtopping load are essential to predict the grass cover erosion by overtopping waves. Additionally, observations have shown that most failures occur near transitions - defined as any change in cover material or geometry - that are especially vulnerable locations for cover erosion by overtopping waves. The aim of this thesis is to quantify the hydraulic load of overtopping waves on the crest and the landward slope of grass-covered flood defences with transitions by modelling. This thesis shows that the overtopping load is affected by transitions and anomalies along the dike profile. An analytical model, a numerical model and practical load formulations are developed to calculate the hydraulic load of overtopping waves along the dike profile. The effect of transitions on the hydraulic load was quantified and can now be included in existing erosion models using the tools developed in this research. The tools lead to more accurate calculations of the hydraulic load, the cover erosion and the failure probability that are necessary to improve the design and theassessment of grass-covered dikes with and without transitions. Moreover, the results show that overall geometric transitions have a larger impact on the hydraulic load compared to transitions in cover type. An example of a geometric transition is a damage to the grass cover. This study shows that damages can increase the failure probability with a factor 100 underlying the importance of a good grass cover to protect the flood defence against failure.