Standing infragravity waves over an alongshore irregular rocky bathymetry

Abstract

Rocky reef coastlines typically feature highly variable and often abrupt cross‐shore and alongshore changes in bathymetry. The effects of this irregular rocky bathymetry on the dynamics of infragravity waves are largely unknown. Most models of infragravity wave dynamics have been developed and validated on smooth alongshore‐uniform bathymetries, which may break down over these highly variable bathymetries. A 2 week field experiment was conducted on a rocky reef‐fringed beach to investigate how the variable bathymetry affects the spatial and temporal variability of infragravity waves. The height of short (sea‐swell) waves decreased over the shallow reef due to breaking, whereas the height of infragravity waves increased toward the shoreline. Both during a storm event (Hm0 = 2.3 m) and under moderate wave conditions (Hm0 = 1.0–1.8 m), the infragravity waves formed a persistent cross‐shore standing wave pattern along the entire shoreline, despite the irregular bathymetry. In addition, the alongshore components of infragravity waves refracted by the presence of the nearshore reef were observed to propagate in opposite directions up and down the coast resulting in a local alongshore standing wave pattern. Thus, the presence of highly variable nearshore bathymetry, which commonly occurs along rocky reef coastlines, may produce both cross‐shore and alongshore standing wave patterns.

Geologically controlled sandy beaches: Their geomorphology, morphodynamics and classification

“…These low-frequency oscillations can occur due to nonlinearities in the short wave field, and include bound and free long waves (Karunarathna and Tanimoto, 1995;Payo and Muñoz-Perez, 2013). Moreover, measurements indicate that the energy spectrum on coral reef flats is dominated by infragravity frequencies (Young, 1989;Brander et al, 2004;Winter et al, 2017), and reef topography can lead to excitation of resonant modes (Péquignet et al, 2009), such as by wave groups (Gallop et al, 2012). In addition, on beaches resting on platforms, the frequency of waves is altered as they propagate across the platforms, with wave breaking filtering out gravity waves and increasing infragravity wave height (Beetham and Kench, 2011;Ogawa et al, 2012).…”

“…Thus, while submerged rock substrates supporting beaches can dissipate waves, significant amounts of wave energy can still impact the shoreline during particular topographic and forcing conditions. It was demonstrated by Winter et al (2017) that cross-shore standing water elevation patterns can be generated by infragravity waves, even in environments with highly irregular alongshore bathymetry such as coral reefs; and refraction of infragravity waves by nearshore reefs can also propagate in opposite alongshore direction causing a local standing wave pattern.…”

Reconstruction of Directional Spectra of Infragravity Waves

“…Most of the reflected FIG waves are expected to be trapped due to wave refraction and residuals leak to offshore as leaky waves (Herbers et al, 1995; Okihiro et al., 1992). The trapped waves can generate an alongshore standing wave structure, which intensifies the local magnitude of IG waves (Thomson et al., 2007; Winter et al, 2017). The leaky waves are known to be able to cross ocean basins and reach distant coastlines (Ardhuin et al., 2014; Rawat et al., 2014).…”

Coherence between infragravity waves and ambient water motions over cape-associated shoals

“…Cross-shelf exchanges of material off capes are therefore governed by flow separation of wind-driven currents (Kumar et al, 2013;Lamas et al, 2017) and the well-known Ekman transport (Pedlosky, 1987, Section 4.3). Over cape-related shoals, cross-shelf exchange may also be influenced by short-wave-related motions (Lentz et al, 2008), and infragravity waves (Thomson et al, 2005;Pomeroy et al, 2012;Winter et al, 2017).…”

“…Long gravity waves (LGWs, also known as infragravity waves) have been found to exert a first order control on nearshore morphodynamics (Bertin et al, 2018, and references therein). Under non-breaking conditions, irrotational LGWs (typically from 5 to 50 mHz, or 20 to 200 s) propagate either as oscillations bound to short-wave groups (Longuet-Higgins and Stewart, 1962;Herbers et al, 1994), nearshore-trapped edge waves (Lippmann et al, 1999;Sheremet et al, 2005;Winter et al, 2017), or free waves (Herbers et al, 1995). Both forced and free infragravity motions have been found to contribute to nearshore sand fluxes, especially during storms (Ruessink et al, 1998;Aagaard and Greenwood, 2008;de Bakker et al, 2016a).…”