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D5) Berms and roughness elements

Start: 09/2017
End: 09/2021
Status: Active

Contact details

Weiqiu Chen

University of Twente

Expected outcome

Better methods for quantifying the berm and roughness influence for the dike design and reinforcement.

Top: Various types of roughness elements and a berm applied at a dike (source: EurOtop, 2018). Bottom: Wave run-up and overtopping at dikes in The Netherlands and embankments in China (source: EurOtop, 2018 and

Motivation and practical challenge

Due to climate change, sea-level rise and land subsidence, there is an increasing risk of coastal flood disasters all over the world, especially in low-lying countries like The Netherlands and densely populated countries such as my home-land China. Against this background, some existing flood defences such as coastal dikes, levees and embankments may not satisfy the safety standard and therefore require reinforcement. To reduce the average overtopping discharge at dikes, reinforcement measures are typically built over the slope of the waterside. These measures include transitions with almost horizontal slopes also called berms and roughness elements such as block revetment (top photo). Thereby, permeable and impermeable blocks over the slope transitions of dikes and embankments can dissipate the energy of the overtopping discharge (bottom photo). The presence, absence or combination of these elements, naturally lead to the question: To what extent these reinforcement measures can reduce the overtopping rates? Improved prediction methods are necessary for more efficient dike design and reinforcement.

Research challenge

As shown in the above pictures, almost horizontal berms and various types of roughness elements are combined along the waterside slope, but what are the effects of these elements on the average overtopping discharge at dikes?

Main components of the research, including component 3) that is future work. (Photos provided by Weiqiu Chen and scheme adapted from EurOtop, 2018)

Innovative components

This project gives better insights on the influence of berms and roughness elements on wave overtopping to define more accurate guidelines for design and safety assessment of dikes. The main components of my research are:

  1. New empirical equations for estimating the berm and roughness influence. These new equations are derived based on experiments for a combination of permeable, impermeable and smooth revetments over slopes of the water side with a berm. The equations are further validated against the numerical model to estimate the average overtopping discharge at dikes.
  2. Numerical model of overtopping discharge. We use OpenFOAM software to model the overtopping at dikes to accurately predict the average overtopping at dikes that have a berm and roughness elements. This model will probably be extended to 3D to include the effects of the irregular waves.
  3. Numerical model of overtopping flow (future work). We will probably extend the 2D OpenFOAM model to study the influence of berms and roughness on the overtopping flow velocity and layer thickness at the waterside edge of the dike crest. These flow parameters can be used as the inputs for erosion models.

Relevant for whom and where?

Designers and advisors concerned with reducing flooding risks by applying berms and/or roughness elements at dikes.

Findings from this project are developed in a experimental lab but are applicable to dike locations where the berms and combined roughness elements are applied.

Progress and practical application

We conducted physical model tests with four configurations of permeable, impermeable and smooth surfaces in the experimental facilities of Deltares in the Netherlands. We derived new empirical equations for estimating the influence of berms and roughness on average overtopping discharges based on the analysis of the experimental data. The new roughness equation can deal with varying roughness along the slopes that have a berm. We found that the roughness elements located on the upper slope contribute the most to the reduction of overtopping discharge. The results show that the new equations significantly improved the predictive accuracy of overtopping discharge compared to existing prediction methods form available technical guidances (TAW, 2002; EurOtop, 2018). We also developed an OpenFOAM numerical model within which it is easy to change the configurations of the dike and to estimate the average overtopping discharge.

Status for day-to-day practice

Empirical equations have applicable ranges. The wave conditions and dike configurations should be checked if they are within the applicable ranges before using the new empirical equations. As for the numerical model, it is important to ensure the accuracy of incident wave generation.

Next steps

We will extend the 2D OpenFOAM model to 3D to simulate the influence of oblique waves on wave overtopping discharge. Additionally, the overtopping flow velocity and layer thickness at the waterside edge of the crest may also be investigated. It would be nice if the users can provide some experimental data to validate the numerical model.

Last modified: 21/06/2020

Contributing researchers

Weiqiu Chen

University of Twente

Supervisory team

dr. Marcel van Gent


Dr. Jord Warmink

prof. dr. Suzanne Hulscher

Contributing partners