The wave overtopping load on landward slopes of grass-covered flood defences: Deriving practical formulations using a numerical model

Vera M Van Bergeijk,Jord J Warmink,Suzanne J.M.H Hulscher

doi:10.1016/j.coastaleng.2021.104047

Abstract

Overtopping waves exert a high hydraulic load on the landward slopes of flood defences leading to erosion of the grass cover and finally to a dike breach. The hydraulic load is an important variable in erosion models and a detailed description of the load is necessary to determine where and when the grass cover erodes. We use a numerical model to simulate the flow of a single overtopping event over a flood defence with a grass-covered crest and landward slope. The model results show that the flow velocity, the shear stress and the pressure are maximal at the landward toe and can be used to describe grass erosion by shear forces. For steep slopes, the flow separates at the crest line and impacts on the upper slope. The normal stress is maximal at the location of impact and describes the grass erosion by normal forces. Practical formulations are developed for the maximum flow velocity, the maximum pressure, the maximum shear stress, the maximum normal stress and the impact location using three main design parameters for the landward slope: the overtopping volume, the slope steepness and the slope length. The formulations are able to accurately predict the overtopping load with Nash–Sutcliffe model efficiency factors between 0.41 and 0.90. The model output and these new formulations are used to calculate the erosive power of the overtopping waves predicted by eight erosion indices to show how the simulated load can be used in erosion models.

Highlights

Grass-covered earthen flood defences are widely used to protect coastal and fluvial areas from flooding
We use the numerical OpenFOAM® (OpenCFD Ltd., 2019) model setup used by Van Bergeijk et al (2020b) to investigate the hydraulic load on the landward slope of grass-covered dikes This numerical model calculates the four hydraulic variables that are used to describe the load in existing erosion models – flow velocity, shear stress, normal stress and pressure – as function of time and location including the effect of turbulence
The model simulation for the Wijmeers case with V = 1.662 m3/m shows that the overtopping flow separates from the dike profile at the crest line and reattaches on the landward slope (Fig. 6, see the supplementary material for a video)

Summary

Introduction

Grass-covered earthen flood defences are widely used to protect coastal and fluvial areas from flooding. One of the main failure mechanisms of these types of flood defences is grass cover erosion by wave overtopping. Waves flow over the dike crest and accelerate along the landward slope resulting in high turbulent flow velocities with large erosive power. Erosion at the landward toe is the result of high flow velocities at the end of the slope in combination with the slope change resulting in an additional impact (Warmink et al, 2020; Van Bergeijk et al, 2020a) (Fig. 1b). In case of steep slopes, the overtopping flow can separate from the dike profile at the crest line and reattaches on the slope where this impact leads to a high load on the upper slope (Ponsioen et al, 2019) (Fig. 1a)

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