Spectral wave dissipation over a barrier reef

TLDR

Fore reef wave dissipation results from combined frictional dissipation and wave breaking. A two‑week field study measured wave heights and velocities across the fore reef and reef flat, estimating dissipation from breaking and bottom friction, and surveyed bottom roughness to link hydraulic roughness to physical roughness. Reef flat dissipation is dominated by bottom friction at rates higher than sandy beaches, driven by organism‑generated roughness, and a spectral friction model shows a single hydraulic roughness length captures frequency‑dependent dissipation; on the fore reef, bottom friction is comparable to wave breaking, and overall most incident wave energy in Kaneohe Bay is lost to bottom friction rather than breaking.

Abstract

A 2 week field experiment was conducted to measure surface wave dissipation on a barrier reef at Kaneohe Bay, Oahu, Hawaii. Wave heights and velocities were measured at several locations on the fore reef and the reef flat, which were used to estimate rates of dissipation by wave breaking and bottom friction. Dissipation on the reef flat was found to be dominated by friction at rates that are significantly larger than those typically observed at sandy beach sites. This is attributed to the rough surface generated by the reef organisms, which makes the reef highly efficient at dissipating energy by bottom friction. Results were compared to a spectral wave friction model, which showed that the variation in frictional dissipation among the different frequency components could be described using a single hydraulic roughness length scale. Surveys of the bottom roughness conducted on the reef flat showed that this hydraulic roughness length was comparable to the physical roughness measured at this site. On the fore reef, dissipation was due to the combined effect of frictional dissipation and wave breaking. However, in this region the magnitude of dissipation by bottom friction was comparable to wave breaking, despite the existence of a well‐defined surf zone there. Under typical wave conditions the bulk of the total wave energy incident on Kaneohe Bay is dissipated by bottom friction, not wave breaking, as is often assumed for sandy beach sites and other coral reefs.