Wave attenuation properties of seagrass canopies in combined wave-current flows

In the framework of SHIEELD, a European Research Project with the aim of assessing the protective value of seagrass in nature-based coastal defence, a series of laboratory experiments using artificial vegetation have been performed in an open-channel flume, measuring meadow-induced wave decay in combined wave-current flow conditions. Artificial vegetation reproduces four species (Posidonia oceanica, Zostera marina, Zostera noltii, Cymodocea nodosa) and four different plant densities; hydraulic conditions imposed allow to replicate a series of current velocities, submergence ratios and wave characteristics observed in the field. Wave decay was calculated in terms of the dissipation coefficient Kd,wc and compared with dissipation coefficient Kd,pw for the corresponding experiment in pure wave conditions, showing general lower dissipation caused by the following current. Two physically-based models present in literature were compared with measured wave dissipation with a decent agreement. The main differences are attributed to uncertainty in the estimation of experimental parameters (drag coefficient Cd and vegetation deflected height hd), to the wide range of Keulegan-Carpenter numbers KC and vegetation densities involved in experiments and to the different mimic vegetation properties. Further, the dependence of Kd,wc on some adimensional parameters has been investigated, showing that Kd,wc increases with Uc ⁄ Uw, the ratio between imposed current velocity and orbital wave velocity at canopy top, and with plant density and decreases with the Cauchy number Ca and the Keulegan-Carpenter number KC.