FIELD-STUDY OF WAVE ATTENUATION ON AN OFFSHORE CORAL-REEF

A major field experiment was conducted which obtained measurements of the attenuation and transformation of short gravity waves as they cros s the windward edge of an offshore coral reef. Water level data were c ollected for over 3000 individual time series during a wide range of e nvironmental conditions. At the innermost measurement site, which is l ocated on the horizontal reef flat after the completion of wave breaki ng, the upper bound of significant and maximum wave heights is limited to less than 40% and 60% of the reef flat water depth, respectively. As with significant wave height, both the shape and energy level of th e reef flat spectra are strongly affected by changes in reef nat water depth. For higher tide levels the spectra on the reef flat closely mi mic the corresponding incident spectra, However, the attenuation is gr eater for both lower frequencies and higher-energy portions of the spe ctra, This causes the reef flat spectra to be broader than those measu red windward of the reef. At lower water levels, considerable energy l osses due to wave breaking and bottom friction occur. Most of the ener gy loss comes from the vicinity of the spectral peak, and energy shift s to harmonics of the peak of the spectrum can be seen.