Kyoto, Japan -- Mangrove forests are natural wonders that protect coastal areas, particularly in tropical and subtropical regions. They are able to dissipate wave energy and limit flooding, which can even mitigate tsunamis and coastal inundations during tropical cyclones. For this reason, mangroves are attracting attention as Nature-based Solutions, or NbS: natural infrastructure with the potential to enhance coastal resilience in an environmentally friendly way.
As climate change is altering ocean conditions and intensifying storms, many coastal communities face growing risks from flooding and extreme wave events; hence mangroves can serve to both mitigate disasters and help communities adapt to climate change. However, these forests remain underutilized in engineering applications due to a limited understanding of how they interact with hydrodynamic forces. Accurately modeling their complex root structures, known as prop-roots, while quantifying their wave attenuation effects has posed a particular challenge.
A collaborative team of researchers from Kyoto University's Disaster Prevention Research Institute resolved to address this knowledge gap. "Japan has a long history of using pine trees for coastal defense, and we want to apply this knowledge to mangroves to develop smart, cost-effective disaster risk reduction," says first author Yu-Lin Tsai.
Drawing on their previous tree morphology surveys in the field and wave flume experiments, the team set out to develop a numerical model capable of evaluating mangrove wave attenuation. Focusing on the species Rhizophora apiculata, found throughout Southeast Asia and the western Pacific, the team gathered detailed measurements of 3D root shapes, creating a vegetation model accounting for wave attenuation as a function of water depth and wave height. They then evaluated this process using a numerical Boussinesq wave model, incorporating drag and inertia forces to estimate the attenuation of water momentum by mangroves.
The results revealed that wave attenuation varies significantly with vertical root morphology and water depth, and that estimates of wave attenuation levels can differ by 20–50%. This shows that the level of root submersion must be accounted for in assessing the effectiveness of coastal protection.
The team's numerical model and resulting formulas are expected to be valuable tools for integrating mangroves into future coastal disaster risk reduction planning. This study also highlights the critical importance of moving beyond previous knowledge based on simplified mangrove shapes to considering realistic root morphology and submergence conditions.
"We enjoy the full spectrum of our research, but the best part of all is definitely getting to work amidst the beautiful scenery of mangrove forests," says team leader Nobuhito Mori.
In the future, the team plans to develop manuals based on these findings to support mangrove-based disaster mitigation strategies in Southeast Asia, the Pacific Islands, and other regions. They also hope these findings can be applied to additional efforts including mangrove reforestation.
