Field Observations of Infragravity, Sea and Swell Directional Spectra
Abstract
Infragravity, sea and swell wavenumber-frequency (k-f) spectra from an alongshore-aligned array of bottom-mounted pressure sensors in 8 m depth (800 m offshore) are analyzed for their insight into edge wave generation mechanisms. Eight months of nearly continuous data are analyzed. Earlier analysis of infragravity k-f spectra from this site identified the presence of trapped edge waves, distinct from leaky waves at this offshore location (Oltman-Shay et al., 1992). Here, edge wave variance is separated from other infragravity motions (leaky and bound waves) by their k-f location in the spectra. It is observed that on average edge waves account for -50% of the total infragravity variance at this offshore location, and occasionally dominate with -80% of the variance. Significant wave heights greater than 20 cm are observed. Correlation analysis of up- and down coast progressive edge wave variance with sea and swell directional distributions suggest that swell waves propagating from the same directional quadrant as the edge wave are the dominant generation source; sea waves had significantly lower correlations. An order of magnitude difference in of up- and down coast progressive edge wave variance is occasionally observed and is shown to be expected from the theory of resonant excitation of edge waves through nonlinear difference interaction of swell waves incident to the beach at oblique angles to the normal. The correlation between swell and edge waves from the same directional quadrant is approximately linear and accounts for more than 90% of the up- and down coast edge wave variance over the 8 months.
Document Details
- Document Type
- Technical Report
- Publication Date
- Jan 01, 1992
- Accession Number
- ADA266439
Entities
People
- Joan Oltman-shay