STUDY ON THE CAUSES OF DAMAGE IN IWANE OHASHI BRIDGE AT THE 2018 FLOOD OF YUBETSU RIVER IN HOKKAIDO

Abstract

 In July of 2018, a flood with return period of 1/15 years attacked the Yubetsu River basin in Hokkaido. The Iwane Ohashi Bridge over the Yubetsu River suffered great damage. This flood resulted in a subsidence of a bridge pier, causing a great deformation of the bridge girder. The detailed understanding of the reason of the damage will provide a useful insight into future management of the river and bridge. In this study, we investigate the main cause of this disaster by using numerical model of flow and bed evolution, iRIC-Nays2DH. As the results of numerical calculation of two-dimensional bed variation, bars are not able to form during the high discharge condition. The presence of the bridge pier greatly affects to the flow and bed evolution around the bridge pier, resulting in a deep scouring around the pier. This scouring might be the main factor causing the damage of the bridge.