Modeling Wave Energy Dissipation by Bottom Friction on Rocky Shores

Rocky shores are characterized by rough, multi-scale bathymetric variations that result in enhanced wave energy dissipation by bottom friction compared to sandy beaches. Realistic SWAN simulations of surface gravity waves across the rocky shores of Monterey (CA, USA) are conducted, and model results are compared to 20 inner-shelf observational sites spanning 34–5 m water depth. The wave field was highly variable during the study, including alternately low energy waves dominated by southern swell and higher energy local waves aligned with strong north-westerly winds. Including a modified bottom friction parameterization is required for the model to reproduce bulk wave statistics with high skill across the entire inner shelf. The SWAN simulation with the default bottom friction parameterization overestimates significant wave height relative to observations because the friction factor fe parameterization has a maximum value of 0.3. Additional simulations included two empirical formulations relating fe to the normalized wave excursion Ab/kN in the large roughness regime Ab/kN<1. Both simulations incorporate a higher fe that is required to model strong bottom friction dissipation over rocky seabeds. The higher friction factors, with 80% falling within the range 0.43 to 5.38, are associated with variability in the normalized orbital excursion within 0.1<Ab/kN<1. This range corresponds to a large bottom roughness length scale, kN=0.5 m, characteristic of rocky shore environments.