Abstract

Abstract. Many coastlines around the world are protected by dikes with shallow foreshores (e.g. salt marshes and mudflats) that attenuate storm waves and are expected to reduce the likelihood and volume 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 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 values 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 is 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, such as those behind tidal inlets, 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 for shallow foreshore environments.

Highlights

  • 1.1 BackgroundCoastal defences, fronted by wide, shallow foreshores, protect many coastlines around the world

  • This study considered the influence of IG waves on the wave period at the toe using the Hofland et al (2017) empirical model but neglected their influence on the wave height

  • It should be noted that the increase in the spectral wave period due to IG waves is sensitive to the estimated foreshore slope (Eq 18)

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Summary

Introduction

1.1 BackgroundCoastal defences (e.g. dikes or seawalls), fronted by wide, shallow foreshores, protect many coastlines around the world. Examples include the sandy foreshores along the Belgian coast (Altomare et al, 2016); the wide shelves of the Mekong Delta, Vietnam (Nguyen et al, 2020); and the intertidal flats of the Wadden Sea along the northern coast of the Netherlands (Vuik et al, 2016). These bodies of sediment reduce the water depth in front of the structure such that large incident waves are forced to break.

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