Details

Autor(en) / Beteiligte

• Hu, Z.
• Lian, S.
• Zitman, T.
• Wang, H.
• He, Z.
• Wei, H.
• Ren, L.
• Uijttewaal, W.
• Suzuki, T.

Titel

Wave Breaking Induced by Opposing Currents in Submerged Vegetation Canopies

Ist Teil von

• Water resources research, 2022-04, Vol.58 (4), p.n/a

Ort / Verlag

Washington: John Wiley & Sons, Inc

Erscheinungsjahr

2022

Quelle

Wiley Online Library Core Title

Beschreibungen/Notizen

• 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. Plain Language Summary Nature‐based coastal protection has drawn much attention from coastal scientists, engineers, and managers. This measure conserves and (re)creates vegetated coastal wetlands, for example, saltmarshes, mangroves, and seagrasses as buffers to attenuate incident waves and reduce wave load on dikes. The mechanisms of wave dissipation in coastal wetlands have been extensively investigated. Existing studies generally attribute all the dissipation to the drag force exerted by vegetation, whereas indirect wave dissipation that may occur with accompanying tidal currents is generally neglected. By extensive flume experiments, we found that substantial indirect wave dissipation can be induced by triggered breaking. Such process takes place in submerged canopies with a suitable submergence ratio (e.g., 1< water depth/canopy height <4.5), where the currents above the submerged canopies is accelerated by the underneath vegetation drag. These insights highlight the importance of previously overlooked indirect wave dissipation in vegetation fields (not by vegetation drag but by breaking), which are likely to occur in shallowly submerged mangrove and saltmarsh canopies. Thus, the knowledge and the model developed in the present study provide a base for more precise predictions of coastal wetlands' protection values. Key Points A generic CD‐Re relation for various current‐wave combinations has been proposed Opposing currents can trigger wave breaking in submerged canopies, leading to greater wave dissipation than following currents An analytical model is developed to assess the contribution of wave breaking as indirect wave dissipation by vegetation