THE EFFECTS OF LEVEE BREACHING AT THE IRUMAGAWA RIVER BASIN IN TYPHOON HAGIBIS 2019 ON THE FLOOD VOLUME FROM THE RIVERS AND DISCHARGE-HYDROGRAPH

Abstract

 Typhoon Hagibis 2019 recorded heavy rainfall at a lot of area in east Japan. As a result, levee breaching occurred at four places in the Tokigawa River and two places in the Oppegawa River. When the river channel management is planned, the inundation between the river and the flood plain should be understood to discuss the change of the high-risk area. Therefore, the effects of the levee breaching of the Tokigawa River and the Oppegawa River on flood volume and discharge-hydrograph of downstream were considered. The levee-breach timing and time it took for the levee to be washed away on the 0.0k right levee of the Oppegawa River greatly affected the inundated volume. Considering the flood volume from the Otanigawa River, which is a branch of the Oppegawa River, the flood volume in some simulation case was similar to that of the actual case. Condition of the simulation case was that the levee-breach time was 24:00 on October 12, and it took 3 hours for the levee to be wash away. On the other hand, the effect of the levee breach on the downstream peak discharge was small. In Typhoon Hagibis, the duration of rainfall was longer than the time until flood reached downstream; therefore peak discharge continued long after when levee breaching occurred.