QUANTIFYING THE INFLUENCE OF TRANSITIONS ON GRASS COVER EROSION BY OVERTOPPING WAVES

Jord Warmink,Paul Van Steeg,Marc Frankena,Suzanne Hulscher,Vera Van Bergeijk

doi:10.9753/icce.v36v.papers.39

Abstract

Transitions in vegetated dike covers, such as geometry changes or roughness differences, are identified as weak spots in dikes for grass cover erosion by wave overtopping. Although several erosion models exist to model grass cover erosion on dikes, it is unclear how the effect of transitions on grass cover erosion must be included in these models. Therefore, we have developed a model approach to analyze the effects of transitions on grass cover erosion using field experimental data and to derive representative influence factors for one transition type. The model approach has been applied to the transition at the landward toe where the slope changes to a horizontal plane. The model approach is general applicable and can be transferred easily to other transitions. The derived factors can be used to improve predictions of dike cover erosion near transitions.Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/YVH6PN4-Er0

Highlights

A strong grass cover is essential for reliable flood defences that are subject to wave action
The derived critical flow velocity is 0.5 m/s higher using the constant flow acceleration factor compared to the critical flow velocity that results from the analytical model
The calibrated threshold flow velocities (Ut) for the Grass-Erosion Model (GEM) method are much larger than the calibrated critical velocity (Uc) for the Cumulative Overload Method (COM)

Summary

Introduction

A strong grass cover is essential for reliable flood defences that are subject to wave action. When waves overtop the dike, the overtopping water rushes over the crest and down the inner slope, leading to erosion of the grass cover layer (Schüttrumpf 2001, Van der Meer et al 2010, Warmink et al 2018). Geometry or roughness is called a transition. These transitions affect the strength of grass covers (because the continuous grass cover is interrupted) and they affect the wave overtopping flow by increasing turbulence downstream of the transition. Transitions in vegetated dike covers are identified as weak spots in dikes for grass cover erosion by wave overtopping (Fig.1)

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Conclusion