Wave overtopping: Impact of water jets on grassed inner slope transitions

Abstract

The safety of a large part of the Netherlands depends on sea dikes. Due to climate change, a sea level rise is predicted. Together with stronger storms and more wave attack, wave overtopping on the current dikes will increase. As a response, crest levels of the dikes need to be raised in order to meet the present regulations for wave overtopping. Alternatively, these regulations could be lowered if the dikes can be proven strong enough to cope with the increased loads. The SBW ('Sterkte & Belastingen Waterkeringen' or 'Strength and Loads on Water Defences') project with respect to grassed inner slopes focuses on the improvement of reliable overtopping criteria for the present dike structures. Experiments are carried out using a wave overtopping simulator. This device is developed to create full scale overtopping conditions on a inner slope. Experiments show initiation of erosion of the grass layer downstream of the transition of the slope and a horizontal part. The focus of the present research is modelling of the erosion process at a transition. The erosion process is described by two different models. The Transition Model (TM) is derived to describe the development of the scour hole due to the impinging forces of the overtopping wave tongue, known as surface erosion; The SSEA (Sites Spillway Erosion Analysis) model is taken to describe the headcut erosion process which occurs after the scour hole has reached a certain depth. The TM is based on a oblique impinging jet and a depth variable strength profile, in where near the surface the root system dominates, whereas with increasing depths cohesion and internal friction of the clay dominates. Since irregularities, found at the real dikes, are not taken over in the model discrepancies between model and experimental results occurred. According to the defined erosion model, the present guidelines are stern and the actual strength of a grassed inner slope is underrated. The work is done in cooperation with Deltares.