Wave attenuation characteristics of Kandelia obovata mangroves and their synergistic coastal protection with a dike

The wave attenuation and disaster mitigation capabilities of mangroves have received increasing attention in recent years. However, studies specifically examining the wave dissipation capacity of mangrove canopies remain scarce, and the synergistic wave-mitigation mechanism between mangroves and seawalls under storm surge conditions is still not fully understood. In this study, field observations and mechanical property tests of Kandelia obovata were conducted around Maoyan Island in Yuhuan City, southern Zhejiang Province. Based on these results, a similarity theory for flexible mangrove canopy wave attenuation was proposed. To ensure dynamic similarity between the model and the prototype, the elastic modulus scaling ratio \lambda _E should be set equal to the geometric scale ratio \lambda _l . Additionally, geometric similarity in total canopy leaf area should be maintained to achieve force similarity in the canopy structure. Based on field observations of the typical morphology of Kandelia obovata, two types of mangrove models—leafless and leafy—were designed using a geometric scale of 1∶10 and fabricated via 3D printing. A series of physical experiments were conducted under both fully and partially submerged conditions to evaluate the wave attenuation performance of the mangrove branches and canopies. Furthermore, overtopping experiments and numerical simulations were performed for leafy mangrove–dike combinations under elevated storm surge conditions. The experiments were conducted in a wave flume measuring 60 m in length, 0.8 m in width, and 1.4 m in height at the Nanjing Hydraulic Research Institute Laboratory. Results indicate that both water depth and canopy leaves have substantial influence on wave attenuation. At low water depth (prototype 2 m), wave height was reduced by approximately 45% for leafless mangroves and around 70% for leafy mangroves. In contrast, at high water depth (prototype 4 m), leafy mangroves still achieved a reduction of about 40%, whereas leafless mangroves produced less than 20% attenuation. These findings highlight the crucial role of canopy leaves in wave energy dissipation under storm surge conditions. At low water depth, both branches and leaves contribute to attenuation, whereas at high water depth—when mangroves are fully submerged—the contribution from leaves becomes dominant. Due to the exponential decay of orbital velocity with depth, the drag force exerted by vegetation rapidly diminishes with increasing submergence, leading to a marked reduction in wave attenuation. Under such conditions, the canopy leaves become the primary structural component responsible for wave energy dissipation. Experimental results also demonstrate that mangrove belts on intertidal flats can substantially reduce wave heights in front of the dike and lower overtopping discharge at the dike crest. At a prototype design water depth of 3.6 m, a 64 m-wide mangrove belt reduced overtopping by more than 75%. Although the wave attenuation capacity of mangroves is reduced under storm surge conditions with elevated water levels, their effectiveness in reducing wave overtopping remains substantial. The dimensionless wave overtopping rates obtained from physical experiments were validated through numerical simulations, and the effects of varying mangrove belt lengths and additional wave conditions were further investigated. These numerical results further confirm that mangroves can effectively reduce both the frequency and intensity of overtopping events. When the relative length of the mangrove belt B/h exceeds 13.6 and the relative freeboard R_c/H_\mathrms ranges from 0.7 to 1.33, the non-dimensional overtopping discharge q^\mathrm* can be decreased by 60%. The overtopping reduction rate increases with wave period and relative wave height, but decreases with increasing wave steepness. This study also proposes new formulations and recommended values for the bulk drag coefficient C_D and the branch/leaf drag coefficients C_D,b\left(c\right) . In addition, an empirical formula for the non-dimensional overtopping discharge q^\mathrm* , incorporating the effects of mangroves, is developed. These results provide valuable references for numerical simulations of wave–mangrove interactions and for evaluating overtopping behavior in the presence of mangroves. This study also demonstrates that even under storm surge conditions with high water levels and complete submergence, mangroves can effectively collaborate with dikes to attenuate wave energy, serving as a green barrier for disaster prevention and mitigation.