We developed several tools for the hydraulic load on the dike cover by overtopping waves so the load can now be calculated along the dike crest and the landward slope, including the effects of transitions. Transitions in geometry, such as slope changes and height differences, have a major impact on the overtopping load, contrary to transitions in cover type that have a limited effect on the load. The developed tools can be used to account for the effects of transitions in existing calculation methods for the design and the safety assessments of dikes to determine the best location and design of transitions.
Figure 1 - 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 has led to an increase in the number of transitions on flood defences. Examples of these transitions are roads on grass-covered dikes (Figure 1, top photo) and geometrical changes such as berms and objects, including stairs and trees. During storms, high waves can overtop the dikes and run down the landward slope. The large forces of these overtopping waves lead to erosion of the grass cover (Figure 1, 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 in the dike profile (Figure 1, 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.
Therefore, the challenge is to quantify the hydraulic load of overtopping waves on the crest and the landward slope of grass-covered flood defences, including transitions. We use models to study the important processes at transitions and quantify the load so the effects of transitions can be included in existing calculations methods to improve the designs of transitions on dikes.
To address the above challenge, I develop two types 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 (Figure 2, top-left). In the models, locations are identified where the hydraulic forces are high, resulting in erosion of the grass cover and failure of the dike. See the 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.
Moreover, existing calculation methods can only be applied to one location of the dike profile. These new models calculate the forces along in the entire dike profile and therefore calculate the upstream and downstream effect of transitions on the flow. Lessons learned from the detailed model are simplified and implemented in the fast model. Thereby, I study three types of transitions (Figure 2, bottom-left):
- 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 the data of 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 organisations 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 further applied at Afsluitdijk.
Progress and practical application
In this project, different modelling tools 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. A model study of the new design of the Afsluitdijk showed that transitions result in a lower critical overtopping discharge that is ten times as small as flood defences without transitions. Furthermore, the inner toe was the weakest cross-dike location because of the high flow velocities.
Another transition in geometry is the inner crest line. The wave can separate at the crest line for steep inner slopes resulting in high impact forces at the reattachment location. A similar process was observed at transitions in height that occur at damages leading to an additional load. Additionally, simulations with the fast model show that the failure probability increases significantly for damaged dikes due to cover erosion or slope instability. For a detailed description of the findings, check the related outputs. The models use a dataset of experiments on a grass-covered dike near Millingen and are further applied at Afsluitdijk.
Recommendations for practice
- Calculate the hydraulic load of overtopping waves and the erosion along the dike profile to find vulnerable locations for cover failure.
- Account for transitions and other anomalies in the calculation methods for the design and the assessment of grass-covered dikes.
- Height transitions have a major impact on the hydraulic load and have the most erosive power. Therefore, reducing the number of height transitions can make flood defences more resistant to overtopping.
- Determine the strength of the cover layer and the core materials of flood defences to develop more realistic failure definitions.
Last modified: 21/12/2021
Vera van Bergeijk
University of Twente
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.
Bevat: Publication open access journal
The wave overtopping load on landward slopes of grass-covered flood defences: Deriving practical formulations using a numerical model
Flow separation at the landward crest line occurs for steep slopes resulting in high impact forces by the overtopping wave at the reattachment location and the landward toe. We were able to capture both process accurately and used the model outputs to calculate erosion indices that show when and where erosion of the grass cover is expected.
11/11/2021 by Vera van Bergeijk et al.
Bevat: Publication open access journal
Modelling the wave overtopping flow over the crest and the landward slope of grass-covered flood defenses
A new detailed model that is able to calculate the load of the overtopping waves on the grass cover and provides insights in how the load changes along the profile.
02/07/2020 by Vera van Bergeijk et al.
Bevat: Publication open access journal
An analytical model of wave overtopping flow velocities on dike crests and landward slopes
New formulas for the flow velocity over the dike crest and landward slope that can be applied to a wide range of geometries and dike covers.
Bevat: Publication upon journal access
Failure probability by wave overtopping over grass-covered and damaged dikes
We calculate the failure probability by wave overtopping of 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.
Bevat: Conference proceedings Publication open access journal
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.