Propagation of swell across a wide continental shelf

The transformation of ocean swell across a wide, shallow (nominal depths 25 -50 m) continental shelf is examined with data from a 100 km long transect of bottom, pressure recorders extending from the shelf break to the beach a t Duck, North Carolina. The analysis is restricted to periods with light wi nds when surface boundary layer processes (e.g., wave generation by wind an d wave breaking in the form of whitecaps) are expected to be relatively uni mportant. The majority of the observations with low-energy incident swell c onditions (significant wave heights <1 m) shows weak variations in swell en ergy across the shelf, in qualitative agreement with predictions of a spect ral refraction model. Although the predicted ray trajectories of waves prop agating over the irregular shelf bathymetry are quite sensitive to the deep water incident wave directions, the predicted spatial energy variations fo r broadbanded wave fields are small and relatively insensitive to incident wave conditions, consistent with the observations, Whereas swell dissipatio n on the shelf appears to be insignificant in low-energy conditions, strong attenuation of swell energy levels (a factor 4 between the shelf break and nearshore sites) was observed in high-energy conditions (significant wave height 2.5 m), This decay, not predicted by the energy-conserving: refracti on model, indicates that dissipative bottom boundary layer processes can pl ay an important role in the transformation of swell across wide continental shelves.