EFFECTS OF THE TREE DESTRUCTION AND DEBRIS DAM FORMATION AT THE GRAVEL-BED BARS IN KUMAGAYA IN THE ARAKAWA RIVER ON MAXIMUM WATER DEPTH AND FLOOD HYDROGRAPH AT FLOOD EVENT DUE TO TYPHOON HAGIBIS

Abstract

 At the flood event due to Typhoon Hagibis, large amounts of trees on the forested gravel bars were destructed in Kumagaya, on the alluvial fan in the Arakawa River. Part of the trees were trapped in the downstream bars and formed debris dams. The resistance change by debris dam is supposed to affect not only the formation of repelling current but flood propagation to downstream. A two-dimensional flood simulation model with the destruction and trapping driftwoods was applied to the site, to clarify what kind of differences were caused due to the destructive flood. The maximum velocity was decreased on the gravel bar where debris dams was formed although the velocity increased by the destruction in the low channel. The tree destruction situation itself improved in the sheltered area of the forest by the debris dam. From the point of the maximum water depth at the embankment, the trapping effects is law in case of the Arakawa River that has a wide cross section, but it also reduced the peek discharge around 10m³/s.