Abstract
Edge waves, the infragravity waves trapped by near-shore topography, are important in morphodynamics and flooding on mildly sloping beaches. Edge waves are usually generated by swell via triad interactions. Here, we examine the possibility that edge waves might be also generated directly by wind. By processing data from the SandyDuck’97 near-shore experiment, we show that pronounced directional asymmetry of edge waves does occur in nature, apparently unrelated to the direction of swells and along-shore currents. These observations exhibit edge waves propagating in the downwind direction under moderate wind against the along-shore currents, while swell is incident nearly normally to the shoreline, which strongly suggests generation of edge waves by wind. We examine theoretically possible mechanisms of edge-wave excitation by wind. We show that the ‘maser’ mechanism suggested by Longuet-Higgins (Proc. R. Soc. Lond. A, vol. 311, issue 1506, 1969b, pp. 371–389) in the context of excitation of free water waves is effective under favourable conditions: nonlinearly interacting random short wind-forced waves create a viscous shear stress on the water surface with the variation of stress being phase linked to edge waves, which allows self-excitation of a coherent edge wave. The model we put forward is based upon the kinetic equation for short wind waves propagating on the inhomogeneous current due to an edge wave. The model needs a dedicated experiment for validation. Analysis of plausible alternative mechanisms of generation via Miles’ critical layer and via the viscous shear stresses induced by the edge wave in the air revealed no instability in the consideration confined to the main mode and constant slope bathymetry.
Highlights
By processing data from the SandyDuck’97 near-shore experiment, we show that pronounced directional asymmetry of edge waves does occur in nature, apparently unrelated to the direction of swells and along-shore currents
After the initial discovery of edge waves by Stokes in 1846, in the last decades of the 20th century research into edge waves has been primarily driven by the interest in understanding of their role in generating along-shore-periodic shoreline features, such as beach cusps (Guza & Inman 1975)
If wind did not play any role in the generation of the IG wave field, one would expect to see approximately directionally symmetric IG wavenumber–frequency spectra; i.e. low resolution maximum entropy estimates should show a spectral centre of mass located roughly at κ = 0
Summary
After the initial discovery of edge waves by Stokes in 1846 (see Ursell & Taylor 1952; Leblond & Mysak 1978; Stokes 2009), in the last decades of the 20th century research into edge waves has been primarily driven by the interest in understanding of their role in generating along-shore-periodic shoreline features, such as beach cusps (Guza & Inman 1975). We show that pronounced directional asymmetry does occur in nature, apparently unrelated to the direction of the swells and along-shore currents This observational evidence strongly suggests the existence of direct edge-wave generation by wind and motivates a theoretical examination of a possible mechanisms of direct wind forcing. In this mechanism the central role is played by wind-forced short free wind waves of gravity and gravity–capillary range which generate vorticity near the surface and viscous shear stresses.
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