[A-CG40] Coastal Ecosystems 2. Coral reefs, seagrass and macroalgal beds, and mangroves

[ACG40-07] Numerical analysis of tidal flow in mangrove forests using a new drag parameterization accounting for three-dimensional root structures

*Yoshikai Masaya¹, Takashi Nakamura¹, Keita Furukawa², Kazuo Nadaoka¹ (1.School of Environment and Society, Tokyo Institute of Technology, 2.NPO Association Shore Environment Creation)

Keywords:coastal vegetation, coastal protection, sediment transport, blue carbon

Mangrove forests with complex above-ground root systems attenuate flow and waves and protect coasts. This drag effect of mangrove forests also enhances sedimentation and effectively accumulates organic matter transported from outer system, contributing to the mangroves’ carbon sequestration ability. Thus, accounting for the drag effect is critical when assessing the mangroves’ ecosystem services. However, unlike other coastal ecosystems such as seagrasses and salt marshes where vegetation morphology is relatively simple, parameterizing drag effect of mangroves is challenging specifically due to their complex three-dimensional morphological structures of the above-ground roots. Hence, mangrove drag effects on tidal flow and associated substance transport are largely unknown. In this study, we parameterized drag effect of mangrove trees of Rhizophora species based on results from previous field measurements and implemented it into a hydrodynamic model – Regional Ocean Modeling System (ROMS). Using the new drag parameterization in the hydrodynamic model, we analyzed the effects of mangrove drag on tidal flow in a schematized mangrove forest of Rhizophora trees. The model showed that compared to a case without imposing mangrove drag effect, mangrove forests tend to retain more water during periods from high tide to low tide, and also generate tidal asymmetry. We also examined effects of different drag parameterization of mangrove forests. Results showed that compared to a parameterization using bulk drag coefficient which does not account for the vertical variations in the above-ground root morphology, the newly implemented drag parameterization predicted significantly lower velocity at the bottom, hence lower bottom shear stress, due to the denser root density near the ground. These results suggested the significant impacts of mangrove forests on tidal flow which may affect sediment and other substance transport and long-term geomorphological evolution in mangrove forests, highlighting the importance of appropriate drag parameterization for evaluation of mangroves’ ecosystem services.