Author(s): Norio Tanaka; Ichitsura Suzuki
Linked Author(s): Norio Tanaka
Keywords: Impulsive force; Floating debris; Tsunami inundation; Mitigation; Coastal forest
Abstract: The tsunami caused by the Great East Japan Earthquake passed through coastal forest with breaking trees and washing out trees. The coastal forest in Sendai Plains in Miyagi Prefecture could be assumed to reduce the tsunami force behind forest but it also produced much driftwood and could increase the impulsive force on houses. The advantage and disadvantage of coastal forest are usually pointed out but have never been compared quantitatively. Therefore, the objective of this study is to analyze quantitatively the advantage to reduce the tsunami force on houses behind coastal forest and the disadvantage to increase the additional impulsive force by driftwoods that acted on houses. In this research, the numerical simulation model including the judgment of tree breaking and washing out condition, motion of trees after the washout, and collision of houses has been developed and the influence of the driftwoods on houses was analyzed. For the tsunami-driftwood interaction model, two-dimensional non-linear long wave equations were solved by finite difference method with staggered grid in space and leap frog method in time, and the equation of motion for driftwoods have been analyzed. The collision between the driftwood and houses was supposed to be the completely inelastic collision and the collision force was analyzed by impulse-momentum approach. Field investigation on the measurement of tsunami trait water depth and the damage situation of trees and houses were conducted in Miyagino District in Sendai City. The simulation model of tsunami flow was validated by comparing the maximum simulated water depth with the observed tsunami trait. The simulation could reproduce well the final location of driftwood. The rate of the collision force of driftwood and the maximum drag force on a house was around 0. 01 - 0. 10. The driftwood has been found not to be a main factor for washing out houses.