Numerical Study of the Variation of the Velocity of a Tsunami-Drifting Object in Front of a Building Just Before Impact at Different Angles of Tsunami Incidence

When tsunami run-up on land, containers, large vehicles, ships and other objects become drifting debris and collide with a building. However, it is known that the backwater created by the run-up flow in front of a relatively large building makes it difficult for drifting debris to collide with a wall of the building. The purpose of this study is to clarify how the velocity of drifting debris immediately before impact changes because of the ratio between the width of the building and the width of the drifting debris with each tsunami incidence angle. Besides, for that, regime characteristics of the flow field around the building is investigated. A uniform flow field was reproduced in a numerical tank by using the MPS method, which was a particle method. When the flow field was stable, a drifting object was dropped onto free surface. Then sufficient acceleration time and distance were kept. In the case where the tsunami incidence angle was small and the width of the building was greater than the width of the drifting object, the velocity of the drifting object just before impact was reduced due to the backwater in front of the building. On the other hand, in the case where the tsunami incident angle was 45°, the velocity of the drifting object just before impact was almost unaffected by the backwater in front of the building. These results suggest that the impact force of drifting objects is greater at the corner where the building columns are located than at the building wall. From the point of view of protecting buildings against tsunami-drifting debris, it is thought that damage can be reduced by placing trees and other structures at the corners of buildings in the direction of tsunami impact.