AbstractNearshore wave dissipation by bottom friction can significantly attenuate surface waves when seabed roughness is large. Wave dissipation is parameterized with a friction factor fe, depending upon the wave orbital excursion at the seabed Ab, and the seabed roughness kN. Parameterizations have been developed assuming small roughness kN relative to Ab, but whether they yield accurate fe for rough seabeds, such as rocky shores, is unclear. Observations from a month-long experiment measured wave transformation on a rough rocky shore, with a large standard deviation of bottom depth σh of 0.5–1.5 m. The explicit fe dependence on variable rocky seabed σh has yet to be demonstrated. Sea-swell energy flux consistently decays shoreward of 8-m water depth, which is well offshore of the surfzone given the time-mean incident significant wave height of 1 m. The observed cross-shore flux convergence yields fe estimates across the instrument array. Quality control criteria are implemented to reduce noise in estimated fe. Hourly fe vary from 1 to 10 and increase with smaller Ab/σh, and binned means indicate a power-law scaling. When using a spatially averaged standard deviation , the scatter around binned means increases, demonstrating that fe is related to σh. Intercomparison with previous experiments is challenging due to different methodologies and definitions of fe. Nevertheless, observations from multiple experiments are broadly consistent with a power law in terms of Ab/σh. Given high-resolution bathymetry, our empirical fe scaling can be used to parameterize wave dissipation over rough seabeds of coral reefs and rocky shores.Significance StatementIn contrast with sandy beaches, the large seabed roughness of coral reefs and rocky shores can induce significantly larger wave dissipation by bottom friction. We present observations over a rough rocky shore, where incoming sea-swell waves are largely dissipated by bottom friction offshore of the surfzone. While theoretical expressions can estimate the wave friction factor fe for small seabed roughness, our results provide an empirical power law for fe, which can be used to parameterize dissipation in wave transformation models over rough seabeds.
