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D4) Overtopping flow and cover erosion

Start: 10/2017
End: 10/2021
Status: Active

Contact details

Vera van Bergeijk

University of Twente

Expected outcome

Better models for the wave overtopping flow and dike cover erosion that include the effect of several transitions to improve the design of flood defences.

Top: Road on a grass-covered sea-dike (photo by Vera van Bergeijk). Bottom-left: Wave overtopping on a grass-covered dike with a road on top (source: Tipner Lake Coastal Defences, 2019). Bottom-right: Erosion of the grass cover at the inner toe during field tests (source: Hoffmans, G. 2014).

Motivation and practical challenge

In the Netherlands, the introduction of multi-functional flood defences have led to an increase in the number of transitions on flood defences. Examples of these transitions are roads on grass-covered dikes (top photo) and geometrical changes such as horizontal berms and objects including stairs and trees. During storms, high waves can overtop the dikes and run down on the landward slope. The large forces of these overtopping waves lead to erosion of the grass cover (bottom-left photo). Once the cover is eroded, the core material of the dike starts to erode, weakening the dike and resulting in the end, in a dike breach. Wave overtopping was indeed one of the main failure mechanisms that led to dike failure during the flood of 1953. Recent experiments and numerical studies have shown that transitions are weak spots along the dike profile (bottom-right photo). At these locations, the erosion by overtopping waves starts. However, we do not know how these transitions affect the overtopping flow and dike cover erosion, thus it is hard to include transitions in current calculation methods for dike failure.

Research challenge

Fascinated by the force of waves and their strength to erode, I develop analytical and numerical for the wave overtopping flow and dike cover erosion. Using these models, I investigate how the hydraulic forces and the cover strength are affected by transitions.

Innovative components of the research approach.

Innovative components

To address the above challenge, I develop two type of models. The first model is simple and fast, while the second numerical model calculates the forces pulling on the dike cover in more detail (top-left figure). In the models, locations where the hydraulic forces are high result in erosion of the grass cover and failure of the dike. See top-right figure for a zoom into the slope at a location with a high load that pulls hard on the grass cover leading to erosion.

Lessons learned from the detailed model are simplified and implemented in the fast model. Moreover, existing calculation methods can only be applied to one location of the dike profile. These new models calculate the forces along the entire dike profile and therefore calculate the upstream and downstream effect of transitions on the flow. Thereby, I study three types of transitions (bottom-left figure):

  • cover type: an asphalt road on a grass-covered dike.
  • geometry: slope changes such as a horizontal berm
  • height differences: existing erosion holes or irregularities in the profile.

To determine the effect of transitions, we use field tests on a grass-covered dike with a road on top near Millingen a/d Rijn. We further use the new green design of the Afsluitdijk to find vulnerable locations for grass-cover erosion (on the map below).

Relevant for whom and where?

Professionals or organizations involved in the design, assessment and maintenance of transitions on flood defences. The modelling approach developed in this study can be used to determine the failure probability of wave overtopping for complex flood defences with several transitions.

The models use dataset of experiments on a grass-covered dike near Millingen and are futher applied at Afsluitdijk.

Progress and practical application

In this study, two models are developed that are freely available and widely applicable. These models are more accurate than existing calculation methods and can be applied to flood defences with several transitions. To develop the models, I used a dataset of experiments on a grass-covered dike near Millingen aan de Rijn. A model study of the new design of the Afsluitdijk showed that transitions result in a lower critical overtopping discharge that is 10 times as small as flood defences without transitions. Furthermore, the inner toe was the weakest cross-dike location because of high flow velocities at this location. Numerical simulations have shown that the forces related to turbulence are high on the lower slope and the pressure increases at the inner toe. This means that the erosion at the inner toe is not only caused by high flow velocities, but also by an increase in the pressure and turbulence.

Status for day-to-day practice

The effect of transitions on the overtopping flow and resulting cover erosion need to be taking into account during the design of flood defenses to find the optimal location and design of the transitions.

Next steps

The next steps include modelling the hydraulic forces near transitions using the detailed model to understand how the flow and cover erosion changes due to transitions. The insights obtained from the detailed model will be simplified in the fast analytical model to study the effect of transitions on the failure probability.

Last modified: 06/07/2020

Contributing researchers

Vera van Bergeijk

University of Twente

Dr. Jord Warmink

prof. dr. Suzanne Hulscher

Contributing partners

Project outputs

Modelling the wave overtopping flow over the crest and the landward slope of grass-covered flood defenses

02/07/2020 by Vera van Bergeijk et al.

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Bevat: Publication open access journal

An analytical model of wave overtopping flow velocities on dike crests and landward slopes


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Bevat: Publication upon journal access

Failure probability by wave overtopping over grass-covered and damaged dikes

Failure probability by wave overtopping over grass-covered and damaged dikes and show that damages to the dike cover, such as an animal burrowing or an erosion hole can increase the failure probability significantly.

03/03/2021 by Vera van Bergeijk et al.

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Bevat: Conference proceedings Publication open access journal

  • Warmink, J. J., van Bergeijk, V. M., Chen, W., Aguilar Lopez, J. P., Bomers, A., & Hulscher, S. J. M. H. (2018). Modelling wave overtopping for flood defense reliability. 1-5. Paper presented at 3rd International Conference on Protection against Overtopping 2018, Grange-over-Sands, United Kingdom.