Laboratory study on wave attenuation by submerged mangrove canopies

With global climate change and sea level rise, the frequency and strength of coastal hazards are increasing. Nature-based solutions have been recognized as a kind of sustainable approach to coastal protection. Among them, mangroves play a crucial role in attenuating coastal waves, tides, and storm surges. From top to bottom, mangrove trees have a typical three-layered structure, consisting of leafy canopies, thick trunks, and intertwined roots. Most previous studies investigating mangrove-wave interaction oversimplified mangroves as rigid cylinders. In this way, the effects of the vertical morphology structure of mangroves on wave decay are ignored and have not been fully understood. To bridge this knowledge gap, we carried out a series of flume tests to compare the differences in the wave attenuation ability of artificial near-natural mangrove models and rigid cylinder models. The wave damping factor was calculated based on fitting the measured wave height evolution through the mangrove zone to the wave decay formula proposed by Dalrymple et al. (1984). It has been observed that the submerged mangrove canopies significantly enhanced the wave attenuation rate. To unify the correlations between the wave decay parameter and the varying submerged mangrove volumes under different waves and water depths, we proposed a new parameter, the hydraulic submerged volume index (HSVI), to quantify the wave damping contributions by mangrove canopies, stems, and roots respectively. A fitted linear correlation between the HSVI and wave damping factor was induced. Then the bulk drag coefficient versus nondimensional hydraulic parameters, i.e. Reynolds number Re, Keulegan-Carpenter number KC, and Ursel number Ur, were discussed in detail, and modified correlations considering the effects of varying characteristic length scales of nondimensional hydraulic parameters were proposed.