Numerical simulation estimating effects of tree density distribution in coastal forest on tsunami mitigation

After the 2004 Indian Ocean tsunami, several studies quantitatively investigated the effects of coastal vegetation on mitigating tsunami, but the effects of the tree density distribution in a forest on the tsunami-force reduction have not been elucidated yet. This study was conducted to investigate the tsunami mitigation effects of vegetation by changing the density distribution in a forest model. A one-dimensional numerical model using Boussinesq-type equations including porosity of vegetation and its drag and inertia forces was developed. Laboratory experiments were carried out in a wave channel, and vegetation was modeled simply by wooden cylinders. The applicability of the numerical model was confirmed by good agreement with experimental results. In uniform arrangement, even with the same dn (where d=diameter of a cylinder, n=number of cylinders in the streamwise direction per unit of cross-stream width), the velocity behind the vegetation was reduced by about 17% as the tree density increased about 25 times. In combined arrangements of cylinders with different densities, the velocity behind the vegetation was reduced by about 8% as the tree density increased about 8 times. Denser vegetation increased the reflection, and the resistance by vegetation increased because the water surface slope in vegetated region increased.

► We investigated tsunami mitigation by changing the density distribution in forest.
► Boussinesq models with porosity of vegetation and resistance forces were developed.
► Applicability of simulation was confirmed by good agreement with experimental result.
► The mitigation effect increased with increasing tree density, even under same dn.
► Denser vegetation increased the reflection, and resistance by vegetation increased.