Abstract
In August of 2016, a huge flood attacked the Ishikari River upstream basin. The Kougen Ohashi Bridge in Kamikawa town over the upstream of Ishikari River suffered great damage. The collapse of the abutment and the sinking of the piers occurred during the flood. We conducted a numerical simulation of the river channel processes during the flood using a numerical simulation software iRIC Nays2DH. It was clarified that the causes of the damage were the concentration of flow and the riverbed fluctuation accompanying the plane shape of the river course. And it was clarified that the extension of the bridge length for the purpose of changing the shape of the river course is a very effective measure.
