Observations of nearshore infragravity waves: Seaward and shoreward propagating components

Abstract

[1] The variation of seaward and shoreward infragravity energy fluxes across the shoaling and surf zones of a gently sloping sandy beach is estimated from field observations and related to forcing by groups of sea and swell, dissipation, and shoreline reflection. Data from collocated pressure and velocity sensors deployed between 1 and 6 m water depth are combined, using the assumption of cross-shore propagation, to decompose the infragravity wave field into shoreward and seaward propagating components. Seaward of the surf zone, shoreward propagating infragravity waves are amplified by nonlinear interactions with groups of sea and swell, and the shoreward infragravity energy flux increases in the onshore direction. In the surf zone, nonlinear phase coupling between infragravity waves and groups of sea and swell decreases, as does the shoreward infragravity energy flux, consistent with the cessation of nonlinear forcing and the increased importance of infragravity wave dissipation. Seaward propagating infragravity waves are not phase coupled to incident wave groups, and their energy levels suggest strong infragravity wave reflection near the shoreline. The cross-shore variation of the seaward energy flux is weaker than that of the shoreward flux, resulting in cross-shore variation of the squared infragravity reflection coefficient (ratio of seaward to shoreward energy flux) between about 0.4 and 1.5.

Refraction and reflection of infragravity waves near submarine canyons

“…Contributions from forced, or group bound, waves that shoal proportional to a theoretical maximum h À5 ratio [Longuet-Higgins and Stewart, 1962] are small everywhere except the surfzone. Also similar to previous results, the relative contribution of seaward propagating leaky waves (with depth dependence h À1/2 ) is estimated to be a significant portion of the total infragravity energy [Sheremet et al, 2002].…”

“…Assuming nearly shore-normal linear wave propagation close to shore, the total seaward energy flux F( f ) is estimated at each infragravity frequency f using the observations of pressure (P) and cross-shore (U) and alongshore (V) velocities as [Sheremet et al, 2002] …”

“…In the surfzone, infragravity waves transfer some of their energy back to motions with higher frequencies [Thomson et al, 2006;Henderson et al, 2006], before reflecting from the shoreline [Suhayda, 1974;Guza and Thornton, 1985;Nelson and Gonsalves, 1990;Elgar et al, 1994;Sheremet et al, 2002]. Reflected infragravity waves radiate seaward as freely propagating waves, some of which leak out to deep water and some of which return to the shore as refractively trapped edge waves [e.g., Eckhart, 1951;Huntley et al, 1981;Oltman-Shay and Guza, 1987].…”

Wave-mud interaction over the muddy Atchafalaya subaqueous clinoform, Louisiana, United States: Wave processes

“…The numerical results show that important features of the time frequency shape of wave dissipation (Figure 3) are due to nonlinear interactions; the peak of the dissipation distribution follows the spectral peak because of transfer of energy toward both the infragravity band, and high frequencies; narrower distributions of dissipation rate occur when nonlinear interaction is stronger, e.g., for larger waves. Growth rates in the infragravity band are exclusively due to transfer of energy from the spectral peak, an essentially nonlinear process [Sheremet et al, 2002].…”

Transport of larvae and detritus across the surf zone of a steep reflective pocket beach

“…Owing to the steep beach profile, shoreward (+) and seaward (−) energy, E, from the co-located pressure and ADCP velocities (2nd bin from the bottom) was computed as: (4) where Co represents the co-spectrum, f is frequency, x is cross-shore location, k is radian wave number, z is instrument height, h is water depth, ω is radian wave frequency, and subscripts u and p represent cross-shore velocity and pressure (Tatavarti et al 1988, Sheremet et al 2002. The energy flux, F, is defined as:…”