INVESTIGATION OF THE CHARACTERISTICS OF FLOOD FLOWS IN THE CHIKUGO RIVER SYSTEM BASED ON WATER-LEVEL INFORMATION DURING THE HEAVY RAINFALL IN JULY 2020

Abstract

The heavy rainfall in July 2020 caused extensive damage to the Chikugo River system. In this study, we summarized the rainfall and damage in the system. We also conducted a one-dimensional flood flow analysis based on observed water levels. The analysis covered the Chikugo, Kusu, Kuma, Shode, and Kagetsu Rivers in the upper Chikugo River basin, which were inundated by river water. The results showed the following: (1) The amount of heavy rainfall was almost the same as the design magnitude. (2) In the Chikugo River basin, inundation by river water and inundation inside a levee occurred, and road and railroad bridges were washed away. (3) The flow rate and water level at Senjo, Ogase, Kobuchi, Kuma, Kagetsu, and Arase stations were estimated. The water levels of the Chikugo, Shode, and Kusu Rivers rose to or exceeded the design high-water level. The flow rate in the sections where the water level rose to the design high-water level was at or above the flow rate specified in the river improvement plan.