Wave Breaking Induced by Opposing Currents in Submerged Vegetation Canopies

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Wave height attenuation in vegetation canopies is often all attributed to the drag force exerted by vegetation, whereas other potential dissipation process is often neglected. Previous studies without vegetation have found that opposing currents can induce wave breaking and greatly increase dissipation. It is not clear if similar process may also occur in vegetation canopies. We conducted systematic flume experiments to show that wave breaking in opposing currents can occur in vegetated flows, but only in submerged canopies with shear currents above vegetation top. Subsequently, we developed a new analytical model to understand and assess the contribution of both drag-induced dissipation in the lower vegetation layer and current-induced breaking in the upper free layer. A new generic drag coefficient relation was applied in the model to quantify drag-induced dissipation with various current-wave combinations. It shows that breaking induced by opposing currents constitutes an essential part (up to 87%) of the total dissipation, which leads to considerably higher dissipation than the cases with following currents. Breaking can occur with various submergence ratios and with small opposing currents in the submerged vegetation field. It indicates that similar breaking process is likely to occur in real vegetation fields. The present study reveals and quantifies the current-induced wave breaking process that has not been reported before, which can improve our understanding of vegetation wave dissipation capacity in field conditions.