Wave dissipation by flexible vegetation

Abstract

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 by wave motions. A transect of 3 pulse-coherent Acoustic Doppler Current 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 = 0.95 - 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.