Observations of Persistent Sea-Swell Nodes and Antinodes Near a Steep Cliff

Standing waves are analyzed near a steep cliff (slope β ≈ 0.44) in the US Virgin Islands over a nearly 3-month deployment. The tide at the field site is weak, and waves varied with height ranging from 0.5 to 2.7 m, with varying incident angles and directional spread. The experiment-averaged pressure p and cross-shore velocity u spectra in the sea-swell frequency band indicate persistent strong nodes (minima) and antinodes (maxima), and the phase difference between p and u varies between ± 90°, consistent with standing wave theory.  We identify 13 persistent nodes across frequencies and at different cross-shore locations.  A shallow-water standing wave model with dispersive effects and oblique-incidence correction predicts the observed nodal locations. We define a nodal strength parameter μ - n , which is consistently strong for most nodal frequencies in the swell band, indicating strong wave reflection and persistent standing waves.  The μ - n decreases linearly with increasing non-dimensional reflection extent (∆L - y /λ off ) 1/2 , which incorporates the incoherence effect of incident and reflected directionally spread random waves. The (∆L - y /λ off ) 1/2 can be calculated from nodal location, incident wave angle, directional spread, and offshore wavelength. The observed μ - n and (∆L - y /λ off ) 1/2 relationship can be reproduced by simulating idealized incident and reflected waves with a prescribed incident directional spectrum and assuming alongshore uniform and perfect specular reflection. This indicates that the observed μ - n variation is consistent with incoherence due to the directionally spread wavefield.