京都・滋賀における2013年台風18号の豪雨の特徴と桂川流域被害調査

Abstract

Typhoon Man-yi caused heavy rainfall in Kyoto and Shiga area during September 15-16, 2013. Since sea water temperature around the Japanese Archipelago was higher than usual by 1-2 ℃ in the summer season, both the typhoon and rainy front delivered extraordinary water vapor to Kyoto and Shiga area. Rainfall intensity in short periods was not so high but longer ones such as 12, 24 and 48 hrs reached almost equal to the maximum records. Total average rainfall in the Yodo river basin was up to 269mm which is estimated more than 200 years return period. During the flood, several multipurpose dams located upstream of the Yodo river basin had effectively controlled inflow discharges coordinately. Especially in the Katsura river basin, Hiyoshi dam operated from 1998 by the Japan Water Agency had drastically cut the peak flow discharge from 1, 694m3/s to 148m3/s. This remarkable flood control operation was based on advanced rainfall monitoring and runoff prediction systems. Cooperated decision making by the Kinki Regional Management Bureau, MLIT and the Japan Water Agency had successfully guided dam operation based on downstream flood flow predictions. Without this operation, more severe flood damages may have occurred in the downstream reaches of the Katsura river including Kameoka, Arashiyama and Kyoto city areas. Additionally, huge drift woods up to 13, 500m3 have been accumulated in Hiyoshi dam reservoir during the flood. They have been delivered both from upstream mountains and river channels. Without trapping by the dam, they have been directly discharged downstream and might have increased flood damages by clogging at crossing bridges and flowing into inundated area. Moreover, flood peak cut by Hiyoshi dam might have decreased additional production of drift woods from the downstream river channel. These multiple functions should be studied further to clarify the total benefit of flood control dams."