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D5) Berms and roughness elements

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

Contact details

Weiqiu Chen

University of Twente

Download here the PhD thesis related to this project.


In this project, we developed new empirical equations and numerical models for predicting the effects of berms, roughness and oblique waves on wave overtopping at dikes. Better estimates of these effects can lead to more accurate predictions of wave overtopping, which are important for dike design and reinforcement. Physical model results demonstrated that roughness elements applied on the upper part of the waterside slope are more effective in reducing the overtopping discharges. The newly developed numerical models can predict the overtopping discharges and overtopping flow parameters with a good accuracy, which shows a potential to become a complementary tool with empirical equations for predicting wave overtopping at dikes.

Figure 1 - 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 worldwide, 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 and riverine dikes 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 (Figure 1, top photo). Thereby, permeable and impermeable blocks over the slope transitions of dikes can dissipate the energy of the overtopping discharge (Figure 1, bottom photo). The presence, absence, or combination of these elements naturally leads to the question: To what extent these reinforcement measures can reduce 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 (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 the 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 waterside with a berm. The equations were further validated against the numerical model to estimate the average overtopping discharge at dikes.
  2. Numerical model of overtopping discharge. We used OpenFOAM software to model the overtopping at dikes to accurately predict the average overtopping at dikes with berm and roughness elements. This model is also extended to 3D to include the effects of the oblique waves.
  3. Numerical model of overtopping flow. We extended 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 an 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 with a berm. We found that the roughness elements located on the upper slope contribute the most to reducing overtopping discharge. The results show that the new equations significantly improved the predictive accuracy of overtopping discharge compared to existing prediction methods from available technical guidances (TAW, 2002; EurOtop, 2018). We also developed an OpenFOAM numerical model within which it is easy to change the dike configurations and estimate the average overtopping discharge and flow parameters.

Recommendations for practice

  • The roughness factor is not constant but varies with wave properties and crest freeboard, which should be taken into account for estimates of overtopping discharges.
  • It is recommended to consider applying roughness elements at the higher part of the waterside slopes.
  • The dependency of oblique wave influence on the berm width should be included for a more accurate prediction of overtopping discharges when oblique waves and a berm are present at the same time.

Last modified: 27/12/2021

Contributing researchers

Weiqiu Chen

University of Twente

Supervisory team

dr. Marcel van Gent


Dr. Jord Warmink

Prof. dr. S.J.M.H. Hulscher

Contributing partners

Project outputs

Hydrodynamic modelling of wave overtopping over a block-covered flood defence

This new model developed in OpenFOAM simulates the wave forces, such as the pressure and the shear stress, on the entire dike profile from the waterside slope to the landward toe. The model can be used to study different designs and locations of the blocks leading to the safest design of the flood defence.

10/01/2022 by Vera van Bergeijk et al.

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Bevat: Publication open access journal

Numerical investigation of the effects of roughness, a berm and oblique waves on wave overtopping processes at dikes

Introducing a roughness factor to existing empirical formulas leads to better estimates of the flow charac-teristics. We found that the flow characteristics are more sensitive to the variation of the berm width than to the berm level. In order to take oblique waves into account, the 2D numerical model is extended into the 3D model domain. The numerical model computa-tions confirm that the reductive influence factor of oblique waves is dependant on the berm width.


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Bevat: Publication open access journal

Numerical modelling of wave overtopping at dikes using OpenFOAM

A 2D numerical model based on OpenFOAM® is set up in this study for predicting wave overtopping at dikes that have complex configurations with berms and roughness elements.


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Bevat: Publication open access journal

Experimental study on the influence of berms and roughness on wave overtopping over dikes

Physical model tests results show that there is a large difference between measured influence factors and calculated ones by using the existing equations there-fore new questions are necessary to better estimate wave overtopping.


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Bevat: Conference proceedings

Experimental Study on the Influence of Berms and Roughness on Wave Overtopping at Rock-Armoured Dikes

Empirical equations are derived based on the analysis of ex-perimental data from new physical model tests. The influence of roughness of the rock armour applied on parts of water-side slopes is estimated by introducing the location weighting coefficients.


Bevat: Publication open access journal

The influence of a berm and roughness on the wave overtopping at dikes

New empirical formulae are derived from the analysis of the experimental data from physical model tests. The new formula is derived for the roughness influence factor of combined multiple roughness elements along the bermed slope.


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Bevat: Publication upon journal access



Reflection: Towards a realistic approach of resistance against wave overtopping

Wave overtopping results in a high hydraulic load on the dike cover and can lead to erosion of the grass cover. The overtopping research in the All-Risk program was focused on two aspects: methods for the outer slope and methods for the inner slope.

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Modelling the waves forces on innovative covers for dikes


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