Guidelines for the design and assessment of coastal dikes with shallow foreshores, taking into account long (infragravity) waves.
Top: Dike-foreshore system in the Wadden Sea, near Eemshaven in the North of The Netherlands (Photo by Jaap van Duin). Bottom: Recent field campaign to measure waves and currents during the yearly winter storms in the same location (Photo by Pieter van der Gaag).
Motivation and practical challenge
To incorporate nature-based solutions, such as the effect of salt marshes and mudflats, in the design and assessment of sea dikes, we must fully understand their impact on waves and the likelihood of flooding during extreme storms. While the influence of such shallow environments on short-period wind waves (periods less than 25 seconds) is well understood and accounted for, what happens to longer-period infragravity waves (periods of minutes) is still not fully understood. During extreme storms, these waves typically propagate reaching up to levees or coastal dikes. Despite their importance for flood safety and coastal dynamics, the current tools neglect or only indirectly consider them into the analysis. In the Netherlands, this challenge presents itself in the Wadden sea, which is quite shallow for kilometres and experiences waves generated both locally and in the North Sea. The improved understanding of these waves propagating over the foreshores is also useful for building with nature in other coastal areas such as the Caribbean Islands where I am from.
My research seeks to answer under what conditions are these infragravity waves significant; and given their significance at the dike, what is the impact on the flood safety?
Components of the research to estimate the influence of nearshore waves according to the offshore and dike charactheristics for more accurate dike designs and flood risk assessments.
To answer the above questions, I use the following methods validated as much as possible with field measurement campaigns:
- Numerical Modelling: while field measurements and physical model tests are often difficult and expensive to implement, numerical models may be used to better understand the interaction between waves and the foreshore, in a timely and cost-effective manner. In my research, state-of-the-art numerical modelling tools such as SWAN, SWASH, XBeach and OpenFOAM are applied to estimate the nearshore wave heights and the volumes of waves overtopping the dike.
- Empirical Modelling: Using existing physical model tests and new numerical data, the relationship between the foreshore, nearshore waves and the volume of water that may overtop the dike can be captured in simple empirical relations. These relations may then guide coastal advisors towards more accurate dike designs and flood risk assessments. Thereby, they can estimate the influence of infragravity waves that are often enhanced due to shallow waters according to: (1) the magnitude of the offshore waves; (2) the foreshore characteristics such as the slope and vegetation coverage; and (3) the slope of the dike.
Relevant for whom and where?
The improved understanding of wave propagation is useful for coastal engineers, researchers and flood risk advisors. Moreover, once the research is completed, a practitioners’ guide will summarize the main findings.
The research components are applied into a case study located in the North of The Netherlands.
Progress and practical application
Findings from the numerical modelling so far indicate that infragravity waves become significant for offshore swell waves that are relatively steep and directionally narrow-banded. The influence of infragravity waves in the nearshore is further significant for shallower water depths; milder foreshore slopes; reduced vegetated cover; and milder dike slopes. Moreover, with empirical adjustments, phase-averaged models like SWAN —which on their own do not model infragravity waves— can be used to estimate infragravity waves.
Status for day-to-day practice
Once published, the empirical model that I am developing would give practitioners a method to quickly assess the significance of infragravity waves at their coast—and whether different modelling tools or approaches are required.
So far, the research has focused on numerical and physical model data. In order to ensure that the proposed methods are valid, the next steps are focused on application to field sites (in the Netherlands and elsewhere). Interesting datasets are always welcomed.
Last modified: 09/07/2020
Christopher H. Lashley
Delft University of Technology
- Lashley, C. H., Bertin, X., Roelvink, D., and Arnaud, G. (2019a). “Contribution of Infragravity Waves to Run-up and Overwash in the Pertuis Breton Embayment (France).” Journal of Marine Science and Engineering, 7(7), 205. https://www.mdpi.com/2077-1312/7/7/205
- Lashley, C. H., Bricker, J. D., van der Meer, J., Altomare, C., and Suzuki, T. (2019b). “Infragravity-Wave Dominance at Sea-Dikes Fronted by Very and Extremely Shallow Foreshores.” The 29th International Ocean and Polar Engineering Conference, International Society of Offshore and Polar Engineers, Honolulu, Hawaii, USA, 7.
- Lashley, C.H., Bricker, J.D., van der Meer, J., Altomare, C., Suzuki, T., 2020. Relative Magnitude of Infragravity Waves at Coastal Dikes with Shallow Foreshores: A Prediction Tool. Journal of Waterway, Port, Coastal, and Ocean Engineering 146, 04020034. https://doi.org/10.1061/(ASCE)WW.1943-5460.0000576
- Lashley, C.H., Zanuttigh, B., Bricker, J.D., van der Meer, J., Altomare, C., Suzuki, T., Roeber, V., Oosterlo, P., 2020. Benchmarking of numerical models for wave overtopping at dikes with shallow mildly sloping foreshores: Accuracy versus speed. Environmental Modelling & Software 130, 104740. https://doi.org/10.1016/j.envsoft.2020.104740