Evaluating the Influence of Vegetation Breakage on Tsunami-Induced Structural Forces: An Experimental Study

This study experimentally investigated the influence of vegetation integrity, vertical architecture and morphology, flexibility, and patch length on tsunami bore attenuation and structural force reduction, using Pandanus odoratissimus (screwpine) as a model species. A key aspect of the experimental design was the simulation of vegetation breakage, defined as occurring when the tsunami water depth exceeded 80% of tree height, a critical threshold for structural failure. Results showed that vegetation under non-breaking conditions significantly attenuated water levels and hydrodynamic forces, with maximum tsunami force reductions of up to 70% for rigid and 66.5% for flexible vegetation, particularly when the patch extended further inland (i.e., longer vegetation length). In contrast, vegetation breakage led to a notable decline in protective performance, with horizontal and uplift force reductions dropping between 10.1–45.2% and 10.7–16.7%, respectively, in short patches. Flexible vegetation exhibited the greatest loss of effectiveness due to structural collapse. However, longer vegetation patches played a compensatory role, maintaining higher force reduction even under breaking conditions. Notably, broken P. odoratissimus still contributes to energy dissipation through its intact dense aerial roots that resist flow near the bed. These findings highlight the importance of maintaining vegetation integrity for effective tsunami mitigation, while also recognizing that partially damaged vegetation retains some protective function, particularly when configured in extended patches.