Experimental investigation of wave attenuation by mangrove forests with submerged canopies

Experimental investigation of wave attenuation by mangrove forests with submerged canopies

Zhang, R. (Hohai University; State Environmental Protection Key Laboratory of Marine Ecosystem Restoration)
Chen, Y. (Hohai University)
Lei, Jiaxin (Hohai University)
Zhou, Xin (Hohai University; Ministry of Land and Resources of China)
Yao, P. (Hohai University)
Stive, M.J.F. (TU Delft Coastal Engineering)

2023

Mangroves can function as a ‘bio-shield’ to protect coastal communities from harsh environments because of their strong ability to attenuate wave energy. However, as mangroves are usually oversimplified as rigid cylinders in antecedent studies, the effects of complex mangrove morphology on wave attenuation have not been well researched. Although increasing attention has been paid to the wave dissipation induced by varying mangrove morphologies, most of them focus on the bottom trunk and root components of mature mangrove trees. There are few investigations about the contributions of the canopies of young saplings and/or short species to wave attenuation. To bridge this knowledge gap, a series of laboratory experiments under regular waves were conducted to examine the hydrodynamic variations affected by varying mangrove morphology configurations. Three water depths were considered to explore the influences of the vertical-varying submerged volume of mangroves when the artificial mangrove models are submerged, nearly emergent, and fully emergent. The mangrove forest model is 2 m long at a 1:10 scale. Three mangrove configurations, i.e. with no canopy, sparse canopy, and dense canopy were applied and compared to isolate the wave attenuation contributed by mangrove canopies. The results highlight the wave energy attenuation attributed to the canopy density. A linear correlation is found between the wave damping factor and a new variable named hydraulic submerged volume index (HSVI). The bulk drag coefficient, including canopy effects, was calculated to characterize mangrove-induced wave attenuation when the mangrove canopy is submerged. The relationships between the bulk drag coefficient C_D and the characteristic hydraulic numbers (i.e., Reynolds number, Keulegan–Carpenter number, Ursell number) are discussed in detail. Consequently, new generic formulas of C_D were deduced considering the effects of the submerged canopy. The employment of new C_D formulas improves the reliability of the prediction of the wave attenuation ability by mangroves since the canopy effects are incorporated.

Drag coefficient
Experimental analysis
Submerged mangrove canopies
Wave attenuation
Wave damping factor

http://resolver.tudelft.nl/uuid:9e066479-d288-4a8a-b962-5a312e56f337

https://doi.org/10.1016/j.coastaleng.2023.104403

2024-03-23

0378-3839

Coastal Engineering, 186

Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.

Institutional Repository

journal article

© 2023 R. Zhang, Y. Chen, Jiaxin Lei, Xin Zhou, P. Yao, M.J.F. Stive