Relative Magnitude of Infragravity Waves at Coastal Dikes with Shallow Foreshores: A Prediction Tool

Abstract

Despite the widely recognized role of infragravity (IG) waves in many often-hazardous nearshore processes, spectral wave models, which exclude IG-wave dynamics, are often used in the design and assessment of coastal dikes. Consequently, the safety of these structures in environments where IG waves dominate remains uncertain. Here, we combine physical and numerical modeling to: (1) assess the influence of various offshore, foreshore, and dike slope conditions on the dominance of IG waves over those at sea and swell (SS) frequencies; and (2) develop a predictive model for the relative magnitude of IG waves, defined as the ratio of the IG-toSS wave height at the dike toe. Findings show that higher, directionally narrow-banded incident waves; shallower water depths; milder foreshore slopes; reduced vegetated cover; and milder dike slopes promote IG-wave dominance. In addition, the empirical model derived, which captures the combined effect of the varied environmental parameters, allows practitioners to quickly estimate the significance of IG waves at the coast, and may also be combined with spectral wave models to extend their applicability to areas where IG waves contribute significantly.

Characterization of Overtopping Waves on Sea Dikes with Gentle and Shallow Foreshores

“…As Altomare et al [6] indicated, the combination of u and h is linked to the hazard rather than the single maximum values of one of these parameters. According to Suzuki et al [16], gentle and very shallow foreshore will result in flatter spectrum at the toe of the dike and thus spectral wave period T m-1,0 is much longer than ones in deep water conditions due to infragravity waves contribution [17]. In such a situation, the waves have been transformed into bores and therefore overtopping characteristics, namely, flow pattern on dikes /promenades, might be also different from one which toe is at deep water.…”

Development of damage curves for buildings near La Rochelle during Storm Xynthia based on insurance claims and hydrodynamic simulations

“…In addition to the general sensitivity of damage curves to topographic data quality, the damage curves displayed in Figure 7 do not consider certain physical wave-driven phenomena such as wave overtopping of structures (Lashley et al, 2020a) or infragravity waves generated by waves breaking in shallow water (Roeber and Bricker, 2015). For instance Lashley et al (2019) discussed the importance of dike overtopping due to infragravity waves on nearshore developments that can induce wave-driven coastal inundation.…”

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

“…Influence on Significant Wave Height at Toe Lashley et al (2020a) investigated the influence of various dike-foreshore parameters on wave conditions at the dike toe using the XBeach non-hydrostatic numerical model (Kingsday release) in both 1D and 2D mode. In each simulation, the offshore wave height ( 0, ), offshore spectral wave period ( −1,0, ), wave directional spreading ( ), water depth at the toe (ℎ ), foreshore slope ( ), width of vegetated cover ( ) and structure slope ( ) were systematically varied, following a one-[factor]-at-a-time (OAT approach).…”

“…where ̅ , ̅ ℎ , ̅ , ̅ , ̅ are influence factors for wave directional spreading, water depth at the toe, foreshore slope, vegetated cover and structure slope, respectively; see Lashley et al (2020a) for further details on how each influence factor is determined.…”

“…From a design perspective, the presence of IG waves typically results in higher characteristic values of the two main parameters used to estimate : namely, the significant wave height and spectral wave period, both assessed at the structure toe (Lashley et al, 2020a). Vuik et al (2018b) assessed the overtopping failure probability of an idealised the dike-foreshore system, representative of the Dutch Wadden Sea coast, considering the effects of vegetation.…”