Impacts of Fringing Oysters Reefs on Wave Attenuation and Marsh Erosion Rates

Mainland marshes in the Virginia Coast Reserve have been eroding at rates that vary both spatially and temporally. Data from these study sites showed statistically significant upward trends in the rate of shoreline retreat over the course of the last 52 years. This increase in erosion rates corresponds to an increase in the frequency of hurricanes and tropical storms passing within 100 km of the study sites. The correlation between hurricane frequency and erosion rates suggest that these high-energy storms could have been the main cause of rapid marsh-edge erosion. Average erosion rates from this study were similar to those measured by McLoughlin et al. (2011) on other mainland marshes in the Virginia Coast Reserve, as was the spatial variability. Erosion at the study sites was less than the regional erosion rate calculated in Hog Island Bay to be 1.2 m·yr -1 . Oyster reefs were shown to have successfully dissipated wind-wave energy, the primary driver of erosion in the Virginia Coast Reserve, and have the potential to work as an erosion control method. For significant waves, the mean dissipation of wave power was 49%. Attenuation of wave energy was determined by multiple factors, primarily water depth above the reef and significant wave height. An ideal range of depths at which reefs were most effective was identified, above which, additional increases in water depth diminished the interaction. This occurred because although waves continued to grow with greater water depth, the decay in orbital motion with depth was sufficient that the waves were no longer strongly modified by the underlying reef surface. Strategic installment of reefs based on prominent wind direction and fetch in relationship to marsh shorelines is likely key to greater erosion mitigation.

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