The Influence of Infragravity Waves on the Safety of Coastal Defences: A Case Study of the Dutch Wadden Sea

Abstract

Abstract. Many coastlines around the world are protected by coastal dikes fronted by shallow foreshores (e.g. saltmarshes and mudflats) that attenuate storm waves and are expected to reduce the likelihood of waves overtopping the dikes behind them. However, most of the studies to-date that assessed their effectiveness have excluded the influence of infragravity (IG) waves, which often dominate in shallow water. Here, we propose a modular and adaptable framework to estimate the probability of coastal dike failure by overtopping waves (Pf). The influence of IG waves on wave overtopping is included using an empirical approach, which is first validated against observations made during two recent storms (2015 and 2017). The framework is then applied to compare the Pf of the dikes along the Dutch Wadden Sea coast, with and without the influence of IG waves. Findings show that including IG waves results in 1.1 to 1.6 times higher Pf values, suggesting that safety may be overestimated when they are neglected. This increase is attributed to the influence of the IG waves on the design wave period, and to a lesser extent the wave height, at the dike toe. The spatial variation in this effect, observed for the case considered, highlights its dependence on local conditions – with IG waves showing greater influence at locations with larger offshore waves and shallower water depths. Finally, the change in Pf due to the IG waves varied significantly depending on the empirical wave overtopping model selected, emphasizing the importance of tools developed specifically shallow foreshore environments.

Using salt marshes for coastal protection: Effective but hard to get where needed most

“…This could be attributed to the higher storm surge levels found in the east compared to the west (LMM: X 2 = 192.03, p < 0.001) (Table 1; Figure 5a). It should be noted that Lashley, Jonkman, et al (2021) investigated the influence of non-linear wave interactions in the form of infragravity waves, also referred to as dynamic wave setup across the Dutch Wadden sea, and while there is some influence, it was found to be more prominent in the west than in the east.…”

“…Elevated foreshores fronting a dike will reduce the water depth so that large incident waves will be depth limited and forced to break (Bouma et al, 2014). The wave shoaling and breaking induced by shallow foreshores leads to several nearshore processes, including a rise in mean water level, known as wave setup and the growth of long‐period infragravity waves, also known as ‘surfbeat’ (Keimer et al, 2021; Lashley, Jonkman, et al, 2021; Lashley, van der Meer, et al, 2021). Lashley, Jonkman, et al (2021) and Lashley, van der Meer, et al (2021) examined these effects in detail and concluded that shallow foreshores have a net positive effect on coastal safety by reducing the extent to which waves run‐up and overtop the structure behind them.…”

“…Dike overtopping, where wave run‐up reaches and passes over the dike crest, forms a major flood‐safety risk, as it may induce sea‐wall breaching by eroding the back of the dike (Schüttrumpf & Oumeraci, 2005; Zhu et al, 2020). Hence, the seawall or dike height is typically designed to resist high water levels during a storm surge and the corresponding wave run‐up height or overtopping volume (EurOtop, 2018; Lashley, Jonkman, et al, 2021). Elevated foreshores fronting a dike will reduce the water depth so that large incident waves will be depth limited and forced to break (Bouma et al, 2014).…”

“…The wave shoaling and breaking induced by shallow foreshores leads to several nearshore processes, including a rise in mean water level, known as wave setup and the growth of long‐period infragravity waves, also known as ‘surfbeat’ (Keimer et al, 2021; Lashley, Jonkman, et al, 2021; Lashley, van der Meer, et al, 2021). Lashley, Jonkman, et al (2021) and Lashley, van der Meer, et al (2021) examined these effects in detail and concluded that shallow foreshores have a net positive effect on coastal safety by reducing the extent to which waves run‐up and overtop the structure behind them. Salt marshes will be more effective in wave attenuation compared to bare tidal flats as a result of having a higher and more stable bathymetric elevation, combined with imposing vegetation friction to the orbital water motion (e.g.…”