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-to-SS-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.
Highlights
BackgroundInfragravity (IG) waves, often referred to as “long,” “surfbeat,” or “tsunami-like” waves, are widely recognized as the driving force behind several critical nearshore processes: beach and dune erosion (Roelvink et al 2009), the development of seiches in harbors (Okihiro et al 1993), and wave-driven coastal inundation (Stockdon et al 2006)
9Visiting Researcher, Dept. of Hydraulic Engineering, Delft Univ. of Technology, Stevinweg 1, 2628 CN Delft, Netherlands. These long-period, low-amplitude waves are formed through nonlinear interactions of sea and swell wave components (Longuet-Higgins and Stewart 1962), such as those locally generated by wind, and those generated by distant storms
Before using XB-NH to generate the synthetic dataset, we first verify that it accurately simulates the hydrodynamics of shallow foreshore environments by comparing it to the observations of the physical experiment
Summary
BackgroundInfragravity (IG) waves, often referred to as “long,” “surfbeat,” or “tsunami-like” waves, are widely recognized as the driving force behind several critical nearshore processes: beach and dune erosion (Roelvink et al 2009), the development of seiches in harbors (Okihiro et al 1993), and wave-driven coastal inundation (Stockdon et al 2006). Recent observations of the impact of IG waves include: unexpectedly high runup levels observed at the rocky coast of Banneg Island on the island of Simeulue off the coast of Sumatra (Sheremet et al 2014); extensive damage and casualties that occurred along a coral reef-lined coast in the Philippines during Typhoon Haiyan (Roeber and Bricker 2015; Shimozono et al 2015); and on the west coast of France, where several dunes were eroded and “over-washed” (Baumann et al 2017; Lashley et al 2019a) In each of these cases, the observed extreme water levels and resulting damage have been attributed to the presence or dominance of nearshore IG waves. This enhancement and subsequent freeing of the bound IG wave is considered to be the main generation mechanism of nearshore IG waves on mild slopes (βb ≤ 0.3)], where the normalized bed-slope parameter (βb) is defined as (Battjes et al 2004)
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