Wave Dissipation by Bottom Friction on a Rough Rocky Reef

An estimated 75% of the world’s coastlines can be described as rocky. However, only a few wave transformation measurements have been made, limited to rocky shore platforms that constitute about 20% of rocky shorelines, leaving the majority of shorelines unstudied. Two, month-long experiments were conducted to examine wave transformation over a rough, rocky reef shoreline on the Central California coast. Wave measurements from a buoy offshore that marked the edge of the rocky reef in 17m depth compared to measurements acquired onshore in 7 m depth, found that approximately a third of the energy flux was dissipated over 130m between stations. Measured wave reflection was negligible. Owing to the measurements being outside of wave breaking, the only mechanism responsible for wave energy dissipation is bottom friction. The bottom roughness of the rocky reef defined by the standard deviation of bottom vertical variability (σ) is 0.9 m, which is 8-10 times larger than previously measured for coral reef and rocky platforms. The energy dissipation, ε, attributed to bottom friction, results in energy friction factors (f_e) ranging 0.03 to 43.8. The observed f_e are 4-5 times larger than on rough 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 for combined coral reef, rocky platform and the rough rocky reef studies. f_e increases as A_b/σ decreases. Owing to the large σ, the rocky reef measurements fall in the domain A_b/σ≤1, a domain where inertial forces, which do not dissipate wave energy, dominate drag forces contrary to the observed results. It is hypothesized that bottom friction on the rocky reef is caused by multiple roughness scales including small, unresolved undulations. ε by bottom friction on a rough rocky reef is an important physical process that controls the evolution of coastal landforms and coastal ecosystem habitats.