UNRAVELLING THE BREACH MECHANISM OF TOKORO RIVER TRIBUTARIES DURING HEAVY RAIN IN AUGUST 2016

Abstract

In recent years, bank breaching occurred high frequency. In August 2016, the east part of Hokkaido in Japan was attacked a heavy rainfall and the tributaries of Tokoro River (Shibayamasawa River and Toua River) had a bank breach caused by the back water of Tokoro River. Although both tributaries were almost the same size and both junctions were very close, the situation of inundation were largely different. At the breaching point of the Sibayamasawa River, the inundated flow toward to the upstream side of the inland, while at the Toua River it was perpendicular to the channel.

In order to understand the cause of the difference in phenomena, it was tried to reproduce the breaching situation in both tributaries using the numerical simulation model “iRIC Nays 2D Breach”. From the result, it was revealed that the direction of the inundated flow changes greatly depending on the timing of the discharge change at the tributary when the bank failure occurs. At the time of bank breaching, when the discharge of the tributary is small, the inundated flow towards to the upstream direction. At the time of bank breaching, when the discharge of the tributary is large, the inundated flow towards to the perpendicular direction of the channel.