Wave damping by seagrass meadows in combined wave-current conditions

Abstract

Laboratory experiments using an artificial seagrass meadow modeled after Zostera marina measured the impact of following currents on meadow-induced wave decay as a function of the imposed current velocity (Formula presented.), wave velocity (Formula presented.), and wave Cauchy number (Formula presented.), which is the ratio of hydrodynamic drag force to restoring force due to blade stiffness. For small wave-induced reconfiguration of individual seagrass blades (Formula presented.) and (Formula presented.) the addition of current enhanced reconfiguration, which decreased wave decay relative to pure wave conditions. In contrast, when the wave-induced reconfiguration was large (Formula presented.), the addition of following current did not significantly enhance blade reconfiguration or impact wave damping. Due to canopy drag, the current velocity within the submerged meadow, (Formula presented.), was significantly smaller than the depth-averaged current (Formula presented.), and a better prediction of wave decay was achieved using (Formula presented.) as the relevant velocity. The measured wave decay validated predictions based on a modified Cauchy number, defined for combined waves and current and using the in-canopy velocity (Formula presented.). Practical assumptions to facilitate the prediction of wave decay in the field are discussed and validated.