[A-CG51] Coastal Ecosystems-2. Coral reefs, seagrass and macroalgal beds, and mangroves

[ACG51-07] Hydrodynamics and sediment transport in mangrove forests: field measurement and modelling

★Invited Papers

*Yoshikai Masaya¹, Takashi Nakamura², Kazuo Nadaoka³, Julia Mullarney¹ (1.University of Waikato, 2.Institute of Science Tokyo, 3.Kajima Technical Research Institute)

Keywords:mangrove, coastal vegetation, blue carbon, nature-based solutions

Mangrove forests with complex above-ground root systems can attenuate flow and waves and protect coasts. This additional drag from mangrove forests also enhances sedimentation by effectively trapping suspended sediment and particulate organic carbon transported from external systems, contributing to mangroves’ high carbon sequestration efficiency. However, due to the fundamental difficulty in quantifying vegetation morphological structures and hydrodynamics in mangrove forests, quantifying the extent to which mangroves alter hydrodynamics and associated sediment transport remains challenging. To address this knowledge gap, we conducted field measurements of morphological structures of above-ground root systems and hydrodynamics in a mangrove forest dominated by Rhizophora species. Using the results of these measurements, we parameterized and implemented the drag effect of Rhizohora mangroves in the Coupled-Ocean-Atmosphere-Wave-Sediment Transport Modeling System (COAWST). Through this model, we demonstrate the vital role of Rhizophora mangroves in dissipating flow and wave energy, enhancing sedimentation and preventing resuspension of sediments. We also present results from manipulative field experiments to examine flow and sediment transport within and around an artificial patch of vegetation mimicking pneumatophores of Avicennia mangroves. The results showed, in some cases, there was a distinct reduction of turbulent kinetic energy near the bed owing to the presence of pneumatophores, thus contributing to sediment retention within and behind the vegetation patch. However, under some conditions, vegetation was found to increase turbulent kinetic energy and cause sediment erosion, with differences attributed to the vegetation patch geometry (patch height and density). These results underscore the need to correctly account for such contrasting impacts of vegetation geometries on flow and sediment transport when evaluating sedimentation and organic carbon sequestration in mangrove forests.