Tsunami force mitigation by tropical coastal trees, Pandanus odoratissimus and casuarina equisetifolia, Considering the effect of tree breaking

Abstract

A numerical model based on two-dimensional nonlinear long-wave equations that include drag forces and turbulence induced shear force due to the presence of vegetation was developed for estimating tree breaking. The numerical model was then applied to a coastal forest, where two dominant tropical vegetation tree species were considered, Pandanus odoratissimus and Casuarina equisetifolia. Quantitative effects of the coastal forest destruction by tsunami on the decrement of tsunami force behind the forest were evaluated with or without including the destruction mode into the model. The analysis satisfies the previous field investigation knowledge that the critical breaking tsunami water depth is around 80% of the tree height when tree height is larger than 2m for P. odoratissimus. P. odoratissimus can reduce tsunami force higher than C. equisetifolia due to the complex of aerial root structures. Even when the trees are destructed at just above the aerial roots, tsunami force reduction rate are decreased by 20%, 10% for 2-4m trees, 6-8 m trees, respectively, because of the existence of dense aerial roots in case of a P. odoratissimus. The previous numerical models that do not include the breaking phenomena have a possibility to overestimate the vegetation effect for reducing tsunami force. The reduction of tsunami mitigation effect by breaking is larger for C. equisetifolia, however their growth rate is larger than P. odoratissimus and is hardly broken. The combination of P. odoratissimus and C. equisetifolia was recommended as a vegetation bioshield to protect coastal area from tsunami hazards.