Wave Energy Reflection from a Rocky Shoreline

Energy reflection, R², of surface gravity waves from a rough rocky shoreline was investigated by two 15-day deployments off the Hopkins Marine Station in Monterey Bay, California. R² is defined as the ratio of the offshore to onshore energy flux. The first deployment consisted of an Acoustic Doppler Current Profiler (ADCP) deployed in 9 m water depth, 100 m offshore of the rocky shoreline. The second deployment consisted of the same ADCP deployed in 2.5 m water depth at the shoreline. The mean R² for both deployment locations was ~0.1, suggesting that the primary reflector is the shoreline and not scattering by the rough subaqueous bottom. R² statistically decreased (at 95% significance) by 0.05 during the transition from low to high tide. This difference is considered negligible. R² was found to be independent of onshore wave energy. The experimental mean direction of the incoming and outgoing waves was relatively equal and opposite, though the outgoing waves displayed more directional variability. This increase is believed to occur from the alongshore variability of the shoreline. The outgoing directional variability was weakly correlated with tidal elevation, suggesting the effective shoreline variability is modified between high and low tide. These results are similar to other observations of wave energy reflection observed at rough coral reefs and breakwaters. The results suggest that wave dissipation plays a larger role than wave reflection at rocky shorelines, which influence biological community structures and nutrient transport.