Observed directional spectra of shoaling and breaking waves

Abstract

The evolution of the frequency-directional wave spectrum, E(f,theta), across the inner continental shelf and beach was examined with measurements collected at the U.S. Army Corps of Engineer's Field Research Facility during the recent SandyDuck experiment. Arrays of bottom pressure sensors were deployed on the shelf in 20 m depth and on the beach in depths ranging from 2 - 5 m. These arrays were complemented by a directional wave buoy in 20 m depth and an array of pressure sensors in 8 m depth maintained by the U.S. Army Corps of Engineers. A preliminary analysis of these data is presented here focused on four case studies that illustrate the observed wave shoaling evolution in both non-breaking and breaking conditions. Estimates of E(f,theta) extracted from array cross-spectra at six cross-shore locations are compared to predictions of linear refraction theory. The present observations support conclusions from previous studies that the cross-shore evolution of dominant wave propagation direction is well described by linear refraction theory. Observations of harmonic peak development at directions aligned with the dominant waves are consistent with theoretical wave-wave interaction rules and previous observations. In both non-breaking and breaking conditions, the observed E(f, theta) are directionally broader than predicted. In contrast to previous observations on a barred beach, the present observations on a planar beach do not show a dramatic broadening of directional wave spectra in the surf zone