Numerical simulation about impulsive force on a house by tsunami-driven woody debris from coastal forest

Abstract

One of the advantages of coastal forest against tsunami inundation is to reduce fluid force behind forest. However, the coastal forest also has a disadvantage to increase the damage to people and buildings because it produces floating debris when the trees are broken and washed out. The advantage and disadvantage are usually pointed out but have never been compared quantitatively. The tsunami caused by the Great East Japan Earthquake on 11 March 2011 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 impact force on houses. Therefore, 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 and the equation of motion for driftwoods have been combined. Driftwood‟s behavior was analyzed by the Lagrangean approach. The collision between the driftwood and houses was supposed to be the completely inelastic collision and the collision force was analyzed by the equation of motion. Numerical simulation was conducted in Miyagino District in Sendai City with the area of 4000m×100m. In the simulation, a collision force on houses by driftwood was analyzed and the magnitude of the collision force was compared with that of the drag force. 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 0.05 - 0.15. In the case of the collision force on the house which was judged as washout, the largest rate was 0.08. Therefore, the driftwood has been found not to be a main factor for washing out houses. However, it has a possibility to slightly influence the washing out situation of the house where the fluid force was close to the critical value for washing out houses.