Abstract

Green infrastructures, known as ecosystem-based disaster risk reduction units of nature-based solutions (NbS), have been employed by several countries to mitigate coastal flooding and waves. Mangroves are one of the well-known green infrastructures, which have been found to efficiently reduce tsunami waves and contribute to mitigating the effect of climate change such as carbon sequestration. However, the capacity of mangroves to dissipate coastal waves has not been fully understood, particularly with their diverse structural geometries.

This lecture will focus on wave attenuation by Rhizophora mangroves, which are distinguished by their unique prop root systems. Wave flume experiments and numerical model simulations were conducted to investigate their performance. Mangrove forests, consisting of scaled Rhizophora apiculata mangrove tree models with prop roots, were subjected to variant regular wave conditions in a wave flume. Drag and inertia coefficients were evaluated from direct force measurements, and their relationships with Reynolds and Keulegan–Carpenter numbers were formulated. The vegetation module was developed in a fully nonlinear Boussinesq model incorporating vertical variations in mangrove geometry. Numerical simulations, using drag and inertia coefficients obtained from experiments, showed good agreement with experimental data in predicting wave height attenuation. However, more significant uncertainties in wave attenuation simulations using drag and inertia coefficients related to Reynolds and Keulegan–Carpenter numbers were observed in the regions closer to the end of mangrove forests, particularly under partial submergence conditions of the mangrove prop roots. These findings provide more quantitative evidence for the effectiveness of Rhizophora mangroves in reducing wave energy. More results and insights will be presented in the Kamide Lecture of Ocean Science at the AOGS 2025 conference.