Swell Transformation across the Continental Shelf. Part I: Attenuation and Directional Broadening

Abstract

Extensive wave measurements were collected on the North Carolina–Virginia continental shelf in the autumn of 1999. Comparisons of observations and spectral refraction computations reveal strong cross-shelf decay of energetic remotely generated swell with, for one particular event, a maximum reduction in wave energy of 93% near the Virginia coastline, where the shelf is widest. These dramatic energy losses were observed in light-wind conditions when dissipation in the surface boundary layer caused by wave breaking (whitecaps) was weak and wave propagation directions were onshore with little directional spreading. These observations suggest that strong dissipation of wave energy takes place in the bottom boundary layer. The inferred dissipation is weaker for smaller-amplitude swells. For the three swell events described here, observations are reproduced well by numerical model hindcasts using a parameterization of wave friction over a movable sandy bed. Directional spectra that are narrow off the shelf are observed to broaden significantly as waves propagate over the inner shelf, although refraction theory predicts a narrowing. This broadening generally agrees with predictions of Bragg scattering of random waves by the irregular seafloor topography.