THE EFFECTS OF LEVEE-BREACH TIMING AT THE UPSTREAM OF THE TOKIGAWA RIVER AND THE JUNCTION SECTION OF THE TOKIGAWA AND OPPEGAWA RIVERS ON THE FLOOD VOLUME FROM THE RIVERS

Abstract

 Typhoon Hagibis 2019 recorded the highest precipitation in history, at a lot of area in east Japan. Levee breaching occurred at 7 places in the Arakawa River system. Risk migration to downstream should be discussed when river channel management is conducted. For this purpose, it is necessary to understand the inundation between the river and the flood plain. As a first step, the effects of the levee breaching of the Tokigawa River and the timing on flood volume were considered.

 It was suggested that the levee at junction section of the Tokigawa River and Oppegawa River gradually scoured, considering from the change of water level hydrograph by changing levee breaching timing. In addition, it can be considered that left levee breaching at the Tokigawa River at 6.5 KP occurred after 22:00 on October 12 considering from the change of flood volume by changing levee breaching timing. Since the inundation on the right side of the Tokigawa River in the management section of Saitama Prefecture was caused by the inundation from the Kasumi-tei (open levee), therefore, levee breaching at the 1.4 KP right levee had little effect on the inundation level.