Modelling of the effect of transitions on wave overtopping flow and erosion for flood defence reliability

Book of Abstracts

NCK Days 2018

Geo-logic in coastal and shelf research: a matter of multi-disciplinarity

March 21-23

Teylers Museum – Haarlem

Sponsored by:

Organized by:

85

M

V.M. van Bergeijk*, J.J. Warmink, S.J.M.H. Hulscher University of Twente, Marine and Fluvial Systems

* v.m.vanbergeijk@utwente.nl Introduction

The major part of the Dutch flood defence consists of dikes. One of the main failure mechanisms of dikes is wave overtopping. Waves that overtop the dike crest run down on the landward slope and cause erosion on landward side of the dike, resulting in weakening of the dike and, in the end, in a dike breach.

For this reason, it is important to learn more about the wave-overtopping process and the erosional effects as well as the about the effect of transitions in dike cover and geometry on wave overtopping.

Research plan

The goal of the project is increased understanding of the effects of transitions on the wave overtopping flow and of the erosion due to wave overtopping. In this project, an idealised wave model and a detailed hydrodynamic model will be developed to simulate the overtopping flow over the dike crest and down the landward slope. These models will be coupled to an erosion model for the dike cover to investigate the erosional effects due to wave overtopping. This abstract reports the first results of the idealised wave model.

Idealised wave model

The idealised wave model is built using the theoretically derived formulas of Van Gent (2002) for the maximum flow velocity umax and the maximum layer thickness hmax of the overtopping tongue exceeded

by 2% of the overtopping waves. Figures 1a and 1b show the decrease in flow velocity and layer thickness over the crest. When the overtopping tongue arrives at the transition towards the landward slope, the flow velocity increases to a constant value while the layer thickness decreases to a constant value. Figures 1c and 1d show the influence of the steepness of the landward slope. The velocity increases with increased steepness, while the opposite holds for the layer thickness, owing to continuity. During this project, the idealised wave model will be extended with other transitions.

Figure 1. (a) The simulated maximum 2% exceedance velocity umax, (b) the simulated maximum 2% exceedance layer thickness hmax and the dike profile with a slope of 1:3, (c) the velocity and (d) the layer thickness for

different slope angles.

Van Gent, M.R.A. (2002). Low-exceedance wave overtopping events: Measurements of velocities and the thickness of water-layers on the crest and inner slope of dikes. Delft Cluster Report DC030202/H3803. Delft Hydraulics, Delft.