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Reflection: Towards a realistic approach of resistance against wave overtopping

Posted at 10/06/2021 by Dr. Jord Warmink

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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Dr. Jord Warmink

*This reflection emerged from the discussion between the webinar team and the 28 to 46 participants to the All-Risk webinar organised in June 10, 2021.

Would you like to view the presentations of the webinar team? You can view them in the above video.

The amount of wave overtopping – and thereby the load on the cover – can be reduced using a berm on the outer slope in combination with roughness elements. Weiqiu Chen performed physical experiments and numerical simulations to improve the existing empirical formulations for wave overtopping including the effect of these two components. Additionally, the effects of transitions on the wave overtopping flow and resulting cover erosion on the inner slope are studied in the All-Risk program. Vera van Bergeijk developed multiple models to gain more insights into the load and erosion of overtopping waves near transitions. These two aspects are combined in a case of Hillblock where a numerical model is developed to calculate the load on the dike cover along with the entire dike profile. The gained insights into the load are used to develop block revetments to reduce the load and to increase the strength of the cover on both the outer and the inner slope.

The discussion

The maximum allowable amount of overtopping is expressed as the critical overtopping discharge, a measure for the volume of water that is allowed to flow over the dike per second without causing failure of the dike cover. Failure of the dike cover is defined as the exceedance of 20 cm erosion depth related to the depth of the topsoil where the roots of the grass cover lead to additional cover strength. At the moment, 10 l/s/m is the maximum allowable overtopping discharge used in dike designs although overtopping tests have shown that the grass cover is able to resist larger amounts of wave overtopping. The participants respond positively to allowing more wave overtopping provided that the grass cover is able to resist these amounts. It is not possible to perform inspections and emergency measures during these large amounts of wave overtopping. This should not be a problem since inspections are never performed during design conditions and it will probably be too late for emergency measures during these extreme overtopping conditions.

There are multiple methods to strengthen a dike for wave overtopping such as improving the grass quality, reducing the wave load, strengthening of the weak spots such as transitions or heightening of the dike. The best method depends on the situation, for example, the effect of a rougher outer slope only has a limited effect for river dikes where the waves are small. Strengthening weak spots and improving the grass quality are interesting options for dikes with limited available space. The effect of these options remains uncertain while the heightening of the dike will always work, however, against which costs?

Increasing knowledge and inspection/monitoring are mentioned as options to reduce risks and existing knowledge gaps. Little observations of overtopping on the inner slope are available next to the experiments with the wave overtopping simulations, but insights into the grass cover strength can also be gained from studying wave run-up. The quality and damages to the grass cover are easier to inspect during conditions without wave overtopping. The wave overtopping simulator is a useful tool to gain knowledge during design conditions.

The main uncertainties are related to transitions and water levels and therefore these two components have a major impact on the future risk. Draught seems to have a smaller impact: observations during last summer (2020) showed that the grass cover coloured brown but the root structure that provides the main resistance was not damaged. Other challenges are related to knowledge gaps in the erosion process such as residual strength, head-cut erosion and the maintenance of transitions and the grass cover. Additionally, social challenges exist related to the multi-functionality of dikes such as the use of space and working with opposing interests.

Towards a realistic approach of resistance against wave overtopping

The dike cover has failed according to the current failure definition when the upper 20 cm are eroded, while the dike is still able to fulfil its water-retaining function. The failure definition would become more realistic and less conservative when the strength of the underlying clay layers is included. However, the follow-up erosion processes remain uncertain and the resistance of the remaining soil layer for overtopping is unknown. More research into these two topics is required before the failure definition can be extended. This also became clear during the webinar where the participants indicated that it is essential to increase the knowledge on the entire dike cover (grass cover and underlying clay layer) to make the dikes future proof. Additionally, information on the strength of the dike core is desired since it is currently unknown what will happen with the dike core during overtopping and if the dike core can contribute to the residual dike strength. This holds specifically for river dikes where the strength of the dike core will be essential to reduce the number of reinforcements when the strength of the core can be incorporated in the assessment.

About experiments and numerical models

Wave overtopping experiments provide many insights into the load that the dike cover can resist. However, these experiments are expensive and it is often not possible to adapt the dike geometry. This means that the results are only applicable for the conditions that were tested. Moreover, the overtopping experiments showed that the dike cover often fails near anomalies such as animal burrowings or transitions. In these cases, it is difficult to determine the dike cover strength. Numerical models can help to extend the results, such as formulas and calculation methods, to other conditions and dike geometries. Additionally, models can be used to increase our understanding of the failure process of the dike cover. For example, the formulas for the overtopping discharge are extended so the effects of a berm and roughness elements can be calculated more accurately. Numerical models are also useful to study the processes and gain insights into the load on the dike cover which is difficult to measure during experiments. Each process can be studied separately in a model to determine the dominant process at transitions. For example, it is challenging to measure the effect of turbulence during wave overtopping conditions. Simulation of wave overtopping experiments with a numerical model makes it possible to determine the amount of turbulence and thereby increase our knowledge of the hydraulic load and strength of the dike cover.

It became clear during this webinar that we are open to allowing more wave overtopping and wish to incorporate the residual strength in the current assessment. However, we need to increase our knowledge on the erosion processes before this could be implemented, for example, the strength of the soil layers under the grass cover and the processes near transitions and damages in the dike profile.

All-Risk recommendations

  • Numerical models can be used to extend calculations methods outside of the tested range and to gain insights into the important processes.
  • A combination of a berm and roughness elements on the outer slope can reduce the amount of overtopping. This reduction is calculated more accurately using the new formulas.
  • It is important to calculate the load and erosion along the crest and landward slope because it is not known upfront what the weakest point along the dike profile is and transitions also have an effect on the flow downstream.
  • More research into the strength of the grass cover, underlying clay layers and dike core can help to adapt the failure definition and thereby make the assessment of grass erosion by overtopping waves more realistic and less conservative.
  • The erosion process by overtopping waves remains uncertain for the erosion near transitions and the erosion mechanisms at larger erosion depths.

Last modified: 20/10/2021