Spectral Wave Energy Dissipation by a Seagrass Meadow

Abstract

Existing formulations for predicting wave dissipation by submerged canopies generally fall into three categories where (a) an empirical coefficient (energy dissipation factor) is attributed to the canopy ignoring its physical properties; (b) estimates of canopy drag forces based on a bulk drag coefficient and undisturbed velocities above the canopy are used to estimate dissipation; and (c) canopy flow theory is used to account for how modifications to in-canopy flows influence canopy forces and associated dissipation. We measured rates of spectral wave dissipation across a dense seagrass meadow comprised of Posidonia australis in southwestern Australia, which also included high-resolution flow measurements within and above the seagrass canopy. These observations were used to quantify the effectiveness of the three different approaches to predict observed rates of spectral wave dissipation. The results showed that conventional approaches that do not account for canopy flow modifications and/or seagrass flexibility tend to overestimate both bulk and frequency-dependent wave dissipation. Conversely, approaches that consider frequency-dependent flow attenuation in canopies were found to improve predictions of wave dissipation, particularly when also accounting for how the deflection of flexible seagrass blades induced by flow modifies the effective canopy height. The results show that the canopy flow velocities induced by short period wind waves were less attenuated than longer period swell, explaining the frequency dependency of rates of wave dissipation, with shorter period wave heights being more efficiently attenuated by the meadow.