Quantifying wave attenuation to inform coastal habitat conservation

Coastal vegetation can protect people and property from erosion and flooding, potentially providing a win-win solution for conservation and development. However, the conditions under which natural habitats provide protection have been controversial, partly because the geomorphic, ecological, and hydrodynamic factors that determine wave attenuation vary greatly among locations, times, and studies. We re-analyzed existing wave attenuation studies in kelp, mangrove, marsh and seagrass habitats and found that much of the variation in wave attenuation can be explained by differences in vegetation characteristics and by the change in bulk drag with flow conditions. We found that vegetation can exert substantial drag on passing waves, but that the bulk drag coefficient declines in flow conditions characterized by high Reynolds numbers. This decline is important because storm conditions are highly turbulent (typical Reynolds numbers are greater than 10 4), and we lack empirical measurements of bulk drag coefficients from such conditions. Failing to account for the decline can over-estimate wave attenuation in storms by 19% to 1600%. These results suggest that larger habitat areas will need to be set aside for coastal protection than previously thought. Our approach provides predictions for designing practical habitat conservation and restoration plans that also protect humans and property from flooding and erosion.