Field Measurement and Prediction of Drag in a Planted Rhizophora Mangrove Forest

Drag effects of mangrove forests with complex root systems modulate flow and substance transport and promote sedimentation, all of which are linked to mangroves' ecosystem services (e.g., coastal protection, trapping of suspended organic carbon) and resilience (e.g., vertical accretion relative to sea‐level rise). Previous flume studies proposed a predictive model of drag by Rhizophora mangrove forests based on the quadratic drag law using two parameters‐drag coefficient (CD) and vegetation projected area density (a). However, its general applicability to mangrove forests in the field has not been tested. To fill this knowledge gap, this study quantified drag in a 17‐year‐old planted Rhizophora mangrove forest using comprehensive measurements of hydrodynamics and vegetation morphology. The site showed a much higher value for a than the conditions assumed in previous flume studies, mainly due to root branching and high tree density. We found that despite the much higher vegetation complexity, the derived CD was 1.0 ± 0.2 for tide‐driven currents, consistent with previous flume studies. By using the mean value of derived CD (1.0), we confirmed that the quadratic drag model expresses well the field‐measured drag. We also showed that the value of parameter a can be predicted by only two vegetation parameters (stem diameter and tree density) using an empirical Rhizophora root model, and confirmed the successful prediction of drag from these vegetation parameters and the derived CD. This provides insights into effectively implementing the drag model in hydrodynamic models for better representation of flow, transport, and geomorphic dynamics in mangrove forests. Plain Language Summary Mangrove forests with Rhizophora trees that have complex above‐ground root systems exert drag against water flow. This modulates sediment transport and promotes the trapping of sediments, which contributes to the sequestration of suspended carbon and sediment vertical accretion that mitigates the impacts of sea‐level rise. Previous flume experiments proposed a predictive model of drag by Rhizophora mangrove forests. However, model root systems examined by previous studies are rather simple to adequately represent actual conditions. This necessitates the investigation of the applicability of insights from flume experiments to mangrove forests in the field. We measured drag in an actual Rhizophora mangrove forest and showed that the predictive model can express well the measured drag,