Wave attenuation by juvenile and mature mangrove Kandelia Obovata with flexible canopies

Abstract

Mangroves have notable wave attenuation capabilities, crucial for protecting coastal ecosystems. Most studies have focused on Rhizophora, particularly its complex aerial root systems, with limited research on Kandelia obovata, a widespread species characterized by short roots, radiating branches, and large canopies. To address this gap, wave attenuation by juvenile and mature Kandelia obovata, both with and without canopy, was investigated using wave flume experiments. The wave attenuation equation was modified to account for the complex mangrove morphology. The effective bulk drag coefficient of the entire vegetation $C_D$, the elastic branch $C{}_{D,b}^{\prime }$ and flexible canopy $C{}_{D,c}^{\prime }$ were calculated. The results highlight the wave energy attenuation capabilities of flexible canopy in both mature and juvenile cases.Even sparse canopy of juvenile mangroves can produce wave attenuation comparable to that of mature tree branches. Juvenile mangroves exhibit acceptable energy dissipation primarily due to their canopies, but only at low water levels. Both branches and canopies of mature mangroves significantly attenuate waves, but as water level increases, the canopy gradually dominates. A new parameter, the hydraulic length scale $HL$, was proposed to predict the wave damping factor β. A new characteristic length scale hydrodynamic diameter $D_e$ was used to calculate the vegetation Reynolds number $Re$ and the Keulegan-Carpenter number $KC$. Principal component analysis (PCA) indicated that combining $Re$ and $KC$ best predicts $C_D$, with Re alone being the second-best. While $C{a}^{-1/3}$ correlates with $C{}_{D,c}^{\prime }$, the underlying mechanism of this relationship may be complex and requires further research. The adaptability of the theoretical model for emergent vegetation is also explored. This study may contribute to the design of eco-coastal defenses using mangroves for protection.