Effects of tree crown height on the flow hydrodynamics and tsunami energy reduction around the coastal forest

Coastal forest acts as a natural solution against floods and tsunami events; however, the difference in the management techniques can significantly affect the forest effectiveness. This study clarified the role of tree crown height and forest thickness by elucidating the flow hydrodynamics and tsunami energy reduction around coastal forests. Through physical laboratory tests in an experimental flume with quasi-steady subcritical flow conditions, this paper elucidated the flow hydrodynamics while investigating the mitigation function of coastal forest and the reduction potential of approaching flow velocity and fluid force around it. The outcomes demonstrated that a forest model with a low tree crown height i.e. crown height to tree height ratio of 0.3 (lowest considered among 0.7, 0.5, and 0.3), caused a strong reflection (back movement of water wave due to backwater rise) of flow, followed by large flow resistance capability in case of large forest thickness (W) than that in half forest thickness (W/2) configuration. The highest reduction potential of approaching flow velocity and fluid force i.e., around 50%, was observed in the case of the lowest tree crown height and largest forest thickness, and such forest configuration had shown to dissipate the maximum energy i.e., around 40%, of the inundating tsunami current. The study found a strong relation of flow energy loss to that of low tree crown height and large forest thickness, highlighting the improved reduction potential efficiency of such coastal forest designs against tsunami disasters.