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Seagrass blade motion under waves and its impact on wave decay

Journal article
Authors M. Luhar
Eduardo Infantes
H. Nepf
Published in Journal of Geophysical Research: Oceans
ISSN 2169-9275
Publication year 2017
Published at Department of marine sciences
Language en
Links doi.org/10.1002/2017JC012731
onlinelibrary.wiley.com/doi/10.1002...
https://gup.ub.gu.se/file/206924
Keywords Flexible vegetation, Seagrass, Wave-energy dissipation
Subject categories Biological physics, Liquid physics, Freshwater ecology, Marine ecology, Oceanography, Botany

Abstract

© 2017. American Geophysical Union. All Rights Reserved.The hydrodynamic drag generated by seagrass meadows can dissipate wave-energy, causing wave decay. It is well known that this drag depends on the relative motion between the water and the seagrass blades, yet the impact of blade motion on drag and wave-energy dissipation remains to be fully characterized. In this experimental study, we examined the impact of blade motion on wave decay by concurrently recording blade posture during a wave cycle and measuring wave decay over a model seagrass meadow. We also identified a scaling law that predicts wave decay over the model meadow for a range of seagrass blade density, wave period, wave height, and water depth scaled from typical field conditions. Blade flexibility led to significantly lower drag and wave decay relative to theoretical predictions for rigid, upright blades. To quantify the impact of blade motion on wave decay, we employed an effective blade length, le, defined as the rigid blade length that leads to equivalent wave-energy dissipation. We estimated le directly from images of blade motion. Consistent with previous studies, these estimates showed that the effective blade length depends on the dimensionless Cauchy number, which describes the relative magnitude of the wave hydrodynamic drag and the restoring force due to blade rigidity. As the hydrodynamic forcing increases, the blades exhibit greater motion. Greater blade motion leads to smaller relative velocities, reducing drag, and wave-energy dissipation (i.e., smaller le).

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