Session: 12-03-01 Coastal Processes I

Paper Number: 123915

123915 - Evaluating Wave Attenuation by Mangroves: An Integrated Laboratory and Numerical Investigation 

In the face of escalating threats posed by intensifying storms and rising sea levels, the use of Natural and Nature-based Features (NNBF) has gained substantial recognition for enhancing coastal resilience. Mangrove forests, among the natural features, have emerged as an exceptional exemplar against coastal hazards. With their complex root systems, mangroves possess the capability to dissipate wave energy, decelerate flow velocity, and mitigate the impact of extreme waves and surges.

This study aims to understand the effects of mangroves on water waves and the consequent wave attenuation. Our approach integrates laboratory experiments and numerical modeling. In the laboratory experiments, we employed three-dimensional tree models that replicated the structure of typical Rhizophora mangroves at a reduced scale. By measuring both the wave forces acting on the mangrove models and the fluid velocity in their vicinity, we established empirical formulas for force coefficients that account for drag and inertia. These coefficients serve as representations of the resistance induced by mangroves to water waves. Additionally, by closely examining fluid velocity patterns and turbulence within the proximity of mangroves, we identified the blockage effects and the heightened turbulent kinetic energy induced by the presence of mangrove roots. This underscored the significant influence of mangrove roots on hydrodynamics. To futher investigate the behavior of mangroves under extreme wave conditions, we conducted prototype-scale experiments on a real mangrove. The observed swaying and bending of the mangrove highlighted its flexibility, a crucial aspect that cannot be adequately captured in model-scale tests. These observations emphasize the importance of considering the flexibility of mangroves when quantifying their effects on wave attenuation.

Leveraging the empirical formulas for force coefficients derived from the laboratory results, we developed a vegetation module in a phase-resolving Boussinesq-type wave model. This vegetation module was to parameterize the effects of mangroves in simulating the propagation of water waves through mangrove forests. Our model results demonstrated that accounting for both drag and inertia effects improves the model simulation of experimental measurements without the need for calibration. Overall, the simulated wave attenuation agrees well with experimental data. We also conducted sensitivity analyses to evaluate the impact of uncertainties in the estimated drag and inertia coefficients. In addition, we compared two different formulations for representing vegetation effects in the numerical model, one incorporating depth-varying velocity and the other using reference velocity. Further detailed findings will be presented and discussed at the conference.

Presenting Author: Che-Wei Chang University of Rhode Island

Presenting Author Biography: Dr. Che-Wei Chang is an Assistant Professor at the Department of Ocean Engineering, University of Rhode Island. He earned his PhD in Civil and Environmental Engineering from Cornell University in 2017. Before joining the faculty at URI, he worked as an Assistant Professor at the Disaster Prevention Research Institute of Kyoto University in Japan.

His research interests include coastal engineering, nearshore hydrodynamics, coastal resilience, and coastal processes. His current research passion primarily centers on exploring the potential of natural and nature-based solutions to mitigate coastal hazards and enhance resilience, driven by a profound sense of duty to address pressing coastal challenges posed by rising sea levels and extreme weather events.

Authors:

Che-Wei Chang University of Rhode Island
Yu-Lin Tsai Kyoto University
Nobuhito Mori Kyoto University