Wave Dissipation by Bottom Friction on the Inner Shelf of a Rocky Shore
Casey J. Gon, Jamie MacMahan, Edward B. Thornton, Mark W. Denny
Abstract
Abstract Approximately 32% of the measured wave energy flux by sea and swell waves was dissipated over distances less than 130 m, outside of wave breaking on the inner shelf, over a rocky shore in southern Monterey Bay, CA. The bottom roughness of the rocky shore is defined by the standard deviation of bottom vertical variability, σ b , that is 0.9 m, which is of similar magnitude to previously measured σ b for rough coral reefs. Spectral wave energy flux balanced by bottom friction is modeled and compared with observations. Measured average wave reflection was 0.08 and is neglected in the model. The average energy dissipation owing to bottom friction over the rocky shore results in energy friction factors, f e , ranging 4 to 34. The observed f e are larger than previously measured f e on coral reefs. An empirical power law relationship is developed for f e as a function of the ratio of wave orbital excursion amplitude, A b , and σ b , based on combined data from coral reefs, rocky platforms, and this rocky shore. As σ b increases, f e increases. Numerical simulation by Yu et al. (2018, https://doi.org/10.9753/icce.v36.waves.57 ) of waves over large bottom variations, similar to observed on coral reefs, suggests that drag forces do not account for the large observed f e . Therefore, it is hypothesized that bottom friction on rocky shores is a function of multiscale physical and biological roughness.