Wave dissipation by flexible vegetation

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Abstract

[1] Dissipation of waves propagating through natural salt marsh vegetation was about half the dissipation expected for rigid vegetation. This low dissipation was predicted by a theoretical model that accounts for bending of vegetation motions. A transect of 3 pulse-coherent Acoustic Doppler Profilers recorded water velocity and pressure (at 8 Hz) within the dense (650 stems/m2) canopy of semi-flexible single-stem vegetation (Schoenoplectus americanus). Most wave energy (56–81%) was dissipated within 19 m of the marsh edge. Two dissipation models, the first assuming rigid vegetation, and the second simulating wave-forced vegetation motion using the theory for bending of linearly-elastic beams, were tested. After choosing optimal drag coefficients, both models yielded a good fit to the observed dissipation (skill score = 0.96–0.99). However, fitted drag coefficients for the rigid model (0.58–0.78) were below the range (0.98–2.2) expected for the observed Reynolds numbers (13–450) and canopy densities (accounting for interactions between stem wakes), whereas drag coefficients for the flexible model (0.97–1.6) were nearer the expected range, indicating that prediction of wave dissipation was improved by simulating vegetation motion.

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