Presenter: Masaya Yoshikai (Tokyo Institute of Technology)

Description:
Mangroves grow in intertidal zones in tropical and subtropical regions and are known to form complex structures in their above-ground roots. These structures attenuate flow energy and enable them to provide coastal protection as one of their ecosystem services. While laboratory-based studies have proposed values of drag coefficient (C_D) and the use of a quadratic drag law as a model for predicting drag in mangrove forests, its applicability has not been verified in actual mangrove forests. This study aimed to quantify drag and verify the applicability of the proposed drag model in an actual mangrove forest, which may contribute to a better representation of the coastal protection function of mangrove forests in hydrodynamic models. Field measurements were conducted in a 17-year old planted Rhizophora mangrove forest in Panay Island, the Philippines. Precise water surface slope measurement, velocity profiling, and bottom shear stress measurement were done to quantify the mangrove drag force from the momentum balance. Morphological structures of the above-ground roots and stems were also measured extensively to estimate the mangrove projected area density (a). The results showed a quadratic dependence of drag on velocity. The C_D derived from the quadratic drag law with the measured value of a showed values of 1.0 ± 0.2, which are within the variations of the values derived from laboratory studies; this justifies the use of the drag model proposed by laboratory studies for predicting drag in an actual mangrove forest. We also tested the use of a previously developed model for the above-ground root morphological structure as a predictor of the parameter a in the drag model. The result showed reasonable accuracy, suggesting that the drag exerted in mangrove forests can be predicted once the input parameters of the model for a are given. The parameters are stem diameter and tree density, which can be easily measured in the field.