Tsunami mitigation by coastal vegetation considering the effect of tree breaking |
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Authors: | Nguyen Ba Thuy Norio Tanaka Katsutoshi Tanimoto |
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Institution: | (1) National Centre for Hydrometeorological Forecasting — NCHMF, No 4 Dang Thai Than Street, Hanoi, Hoan Kiem District, Vietnam;(2) Institute for Environmental Science and Technology, Graduate School of Science and Engineering, Saitama University, 255 Shimo-okubo, Sakura-ku, Saitama 338-8570, Japan;(3) Professor Emeritus of Saitama University, 255 Shimo-okubo, Sakura-ku, Saitama 338-8570, Japan; |
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Abstract: | Damage to vegetation by tsunami moment and reduction of potential tsunami force are discussed based on a numerical simulation.
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 to estimate tree breaking. The numerical model was then applied to a
coastal forest where two dominant tropical vegetation species, Pandanus odoratissimus and Casuarina equisetifolia, were considered. The threshold water depth for tree breaking increased with increasing forest width, and the analysis was
consistent with the field investigation results that the critical tsunami water depth for breaking is around 80% of the tree
height for P. odoratissimus. C. equisetifolia is stronger than P. odoratissimus against tsunami action, but P. odoratissimus can reduce a greater tsunami force than C. equisetifolia due to its complex of aerial root structures. Even if breakage occurs, P. odoratissimus still has high potential to reduce the tsunami force due to its dense aerial root structures. Previous numerical models that
do not include the breaking phenomena may overestimate the vegetation effect for reducing tsunami force. The combination of
P. odoratissimus and C. equisetifolia is recommended as a vegetation bioshield to protect coastal areas from tsunami hazards. |
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