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Great tsunamis passing through structures in some cases cause structural damage by the hydrodynamic force. Although there are many empirical methods to estimate hydrodynamic force which are expressed with a function of flow depth, flow velocity or Froude number, the predicted hydrodynamic forces exhibit considerable scatter. In order to investigate the characteristics of the hydrodynamic force acting on the front face of a structure and propose the prediction method of the hydrodynamic force induced by a tsunami, we have carried out three dimensional numerical simulations of tsunami passing a structure.

The relationship between the collision direction angle and the collision force of woody debris and its behavior is investigated through large-scale bore flow and airborne experiments. In the case of the airborne experiment's oblique collision angle being larger than 20°, the oblique collision force is reduced by up to half of the longitudinal collision force. In the case of the bore flow experiment's initial arrangement along the transverse direction, the wood-log moved with a rotation along the channel floor. Collision forces in the bore flow experiment indicated the trend less than that in the airborne experiment. The effects of water cushioning between the wood-log and vertical wall, and the surrounding water caught by the rotation of debris were considered. Major collision force formulas for collisions in the long axis direction of woody debris are verified through comparison of this study and recent experiments. By considering the reduction in collision energy by the rotational motion of debris after oblique collisions, their collision force formulas are improved to estimate the oblique collision force. The improved formulas can reproduce the reduction of significantly oblique collision forces compared with longitudinal collision forces by the above experiments. The effect of the collision angle's uncertainty on the estimation of the *Corresponding author. 1640022-1 M. !keno et al. collision force was investigated as a case study on a probabilistic evaluation of the collision force.

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