An Experimental Study of Mangrove‐Induced Resistance on Water Waves Considering the Impacts of Typical Rhizophora Roots

Abstract

AbstractThis paper studied the interactions between mangroves and water waves through laboratory experiments. Using 1:7 scale 3D‐printed trees based on the scanned image of a typical Rhizophora species, we replicated the root structure of natural mangroves and constructed a model forest. Three arrangements of mangrove models, two model forests of different stem densities and a single tree in isolation, were adopted. To investigate the relationships between mangrove resistance and waves, we applied the approach of direct force measurement, measuring wave forces on mangrove models, fluid velocity, and free surface elevation along the wave flume. Testing regular and solitary waves of various conditions, we observed non‐negligible inertia effects in the measured forces. Based on the direct measurements, we estimated drag and inertia coefficients in the Morison‐type equation (Morison et al., 1950, https://doi.org/10.2118/950149-g) and established empirical relationships between the two coefficients and Reynolds and Keulegan‐Carpenter numbers. These relationships of drag and inertia coefficients can be used in mathematical/numerical simulations to parameterize mangrove effects under similar scale conditions. Compared to previous studies, the drag coefficients for mangroves were more scattered than rigid cylinders, especially in shallower water depths. By testing different water depths, we observed different patterns of drag and inertia coefficients when the root system was partially or fully submerged. In addition, the fluctuating fluid velocity and enhanced turbulence kinetic energy at the vertical positions near prop roots indicated the blockage effects and the shear‐induced turbulence due to the complex root structure. These findings suggested the influence of mangrove roots on water waves.