Wave Attenuation Over Reefs
COASTAL ENGINEERING RESEARCH CENTER VICKSBURG MS
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The transformation of waves across coral reefs is a complex problem, including the processes of refraction, shoaling, breaking, energy dissipation by bottom friction, and reflection. As waves pass from deep water over a steep reef face onto the reef flat, the waves become highly nonlinear. Wave energy is dissipated due to breaking, but energy is also transferred to both higher and lower frequencies in the wave spectrum, and the spectral shape becomes flat Young 1989, Hardy and Young 1991. The peak wave period shoreward of the reef face may become shorter as higher harmonics are transmitted as free waves Lee and Black 1978, or the period may increase as surf-beat dominates the spectrum. Breaking waves induce a setup of the water surface over the reef, and differences in breaking characteristics along the reef can cause variations in wave setup, producing significant longshore currents. Although it may seem that wave reflection off a nearly vertical reef would be significant, field data Young 1989, Roberts et al. 1975 have shown reflected wave height to be on the order of only 10 percent of the incident height due to the porosity of the reef. Energy losses due to bottom friction are usually negligible in wave transformation across sandy beach profiles, but may be significant over shallow, rough reef flats where the bottom friction coefficient may be an order of magnitude larger than for a sandy bed Roberts et al. 1975, Gerritsen 1980.
- Physical and Dynamic Oceanography
- Fluid Mechanics
- Radiofrequency Wave Propagation