Laboratory study on the drag coefficient for mangrove forests in regular waves

Coastal vegetation such as mangrove forests plays a crucial role in coastal protection. The parameterization of the drag coefficient is of great significance for characterizing the vegetation-induced wave attenuation. However, the drag coefficient characteristics of mangroves with root systems remain unclear. Therefore, we constructed a mangrove forest model (10-m-long at a 1:10 scale) considering the roots based on a root parametric model, and two sets of cylindrical models (i.e., without roots) were considered for comparison. A series of physical experiments were conducted on the hydrodynamics of the cylindrical and mangrove models under regular wave conditions. The results reveal the characteristic of the vegetation-induced wave force, including its evolution along the vegetation field and the relationship between the wave force and velocity. Using the measured force-velocity data, the local period-averaged drag coefficients for mangrove and cylindrical models were obtained based on the direct force measurement method. Then, the relationships between the local drag coefficients and the Reynolds number (Re) and Keulegan-Carpenter number (KC) were discussed in detail, in which Re and KC were redefined using various characteristic lengths and velocities. Finally, the characteristic lengths and velocities suitable for characterizing the local drag coefficient of the mangrove and cylindrical models are identified, and several promising empirical formulas for revealing the local drag coefficient characteristics were proposed, which are expected to help lead to improved understanding and modeling of wave-mangrove interaction.