Comparison of reduction of tsunami fluid force and additional force due to impact and accumulation after collision of tsunami-produced driftwood from a coastal forest with houses during the Great East Japan tsunami

Coastal forests on the Sendai Plain reduced the force of the 2011 Great East Japan tsunami, but the tsunami also produced driftwood that increased the impact force or additional drag force due to accumulation in front of houses (damming). The advantage of reducing the tsunami force and disadvantage of increasing the force on houses after collisions of driftwood were repeatedly pointed out after the tsunami, but have never been compared quantitatively. Therefore, a driftwood model was developed to evaluate tree washout, motion of the drifting trees, and collisions with houses. Two-dimensional non-linear long wave equations were coupled with the equation for motion of driftwood, and the impact force was solved assuming the collision to be inelastic. Experiments were conducted to clarify the driftwood movement and determine the impact duration in the simulation. The ratio of the impact force of driftwood to the maximum drag force on a house was comparatively small, around 0.01–0.10. On the other hand, when driftwood piled up in front of a house, the increasing water depth caused additional fluid force. The rate of accumulation of driftwood was not large in experiments, but the magnitude of the drag force increase after accumulation was far larger than that of the impact force of collision. However, in comparison with the reduction of fluid force by the coastal forest, the fluid force due to the damming in front of a house was found to be smaller and not a main factor in the washout of houses.