Wave Attenuation by Sparse Seagrass Meadows with Non-negligible In-Canopy Flow Under Combined Wave–Current Conditions

Seagrass meadows provide nature-based coastal protection by dissipating wave energy and thereby reducing flooding and coastal erosion. While wave attenuation by seagrass has been widely assessed under pure waves and waves with following currents, the role of opposing currents is less explored, despite its relevance for tidally forced nearshore environments. Here we report 160 flume experiments using an artificial Enhalus acoroides meadow, comprising 32 pure-wave cases, 64 wave with following current cases, and 64 wave with opposing current cases. Following currents generally weakened wave attenuation, whereas opposing currents systematically enhanced it. Relative to pure-wave conditions, wave attenuation under following currents could decrease to approximately 38%, while opposing currents increased attenuation by up to a factor of four. Velocity measurements further show that, for the sparse vegetation used in this study, in-canopy currents are non-negligible: the mean current velocity at the downstream meadow edge differs by less than 20% from that at the upstream edge, indicating efficient shear-layer penetration into the canopy. The current effect on wave attenuation can be explained by the competition between (i) current-induced blade reconfiguration that reduces effective plant height and frontal area (reducing drag), and (ii) current enhanced in-canopy velocities and current modified wave-energy advection that alter the mapping from time-domain dissipation to spatial decay, leading to reduced attenuation for following currents but increased attenuation for opposing currents. These results provide process-based constraints for parameterizing wave dissipation by flexible, sparse vegetation under wave–current coupling in coastal-scale models.