Observations of surface-gravity-wave scattering and dissipation by an isolated shoal related to a cuspate foreland

Abstract

Inner-shelf and shoreline morphology exert control on the variability of gravity waves. Although gravity waves can influence nearshore morphodynamics, their behavior is not well understood at inner-shelves close to cuspate forelands with complex bathymetry. In order to study the effects of cape shoals on gravity wave variability, water level and velocity data were collected during Fall 2013, Spring 2014, and Fall 2014 within the swales on either side of Shoal E, near Cape Canaveral, Florida. Short wave energy fluxes (periods from 3 to 20 s, or frequencies from 50 to ~300 mHz) were calculated from time series of spectral densities, whereas cross-shoal infragravity energy fluxes (periods from 20 to 500 s, or frequencies from 2 to 50 mHz) were calculated from cross-spectra of pressure and cross-shoal velocities. Ratios of short-gravity-wave energy fluxes between inner and outer swales of Shoal E ranged from 0.5 to 3. Values < 1 were typically associated to large (wave heights > 1 m) and short waves (periods < 5 s) likely related to dissipation. Ratios > 1 were exhibited by large and long (periods > 10 s) waves because of refraction. Values close to 1 were related to small waves regardless of the wavelength. In addition, infragravity reflection coefficients were quantified as the ratio of seaward to landward infragravity energy fluxes. Reflection coefficients and spatial variations in infragravity fluxes indicate that instruments were located in the shoaling zone during Fall 2013 and Fall 2014. Infragravity energy flux differences between inner and outer swales further suggest Shoal E may have acted as a source of infragravity energy. Differences in reflection between swales on either side of Shoal E could be attributed to asymmetric short-wave shoaling and breaking, partial infragravity reflection, and infragravity energy trapping near the shoal. However, instrument configuration did not allow for along-shoal calculations, which hindered resolution of edge waves that might develop over the shoal. Our results provide evidence, albeit incomplete, of surface-gravity-wave energy scattering and dissipation over cape-related shoals. Such wave transformations, including infragravity energy forcing, may provide a positive feedback for the long-term stability of cuspate forelands by preventing the full complement of deep water wave energy from reaching the shoreline.