Wave Dissipation by Bottom Friction on the Inner Shelf of a Rocky Shore

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, sigma (b), that is 0.9 m, which is of similar magnitude to previously measured sigma (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 sigma (b), based on combined data from coral reefs, rocky platforms, and this rocky shore. As sigma (b) increases, f(e) increases. Numerical simulation by Yu et al. (2018, ) 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. Plain Language Summary During the spring and the fall of 2018, two experiments were conducted to examine how waves change as they progress from offshore to onshore over a rough rocky reef on the Central California coast. Wave statistics from a buoy offshore, which marked the edge of the rocky reef, were compared to wave measurements acquired farther onshore located seaward of wave breaking. Wave heights decreased between the edge of the reef and the corresponding onshore wave measurement stations. Since the observations are outside of wave breaking, the decrease in wave height is attributed to energy dissipation by bottom friction. Bottom friction is a function of bottom roughness, which was found to be as much 7 times larger than the roughest coral reef. Wave energy dissipation is hypothesized to be a result of the large and small rocks and biological growth on the rocky reef. Understanding wave transformation due to friction is important as this region is the home to a diverse ecosystem. Key Points Very large bottom roughness and energy dissipation factors are observed on a rocky shore Energy dissipation by bottom friction on rocky shore is hypothesized to be a function of multiscale physical and biological roughness Energy transformation on a rocky shore outside of wave breaking is dominated by bottom friction