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

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.