FIELD SURVEY OF DRIFTWOODS GENERATION IN THE KUMA RIVER AND THE CHIKUGO RIVER DUE TO THE HEAVY RAIN IN JULY, 2020 AND EVALUATION OF DRIFTWOOD DISASTER RISK IN THE KAWABE RIVER

Abstract

 In this paper, we attempted to conduct field survey of driftwoods generation in the Kuma River basin and the Chikugo River basin, where were damaged by the floods due to the heavy rain event in July, 2020. We focused on the upper area of Ushifusa Dam and several tributaries in the Kuma River and the upper area of Shimouke and Matsubara Dams in the Chikugo River. We carried out a photogrammetric survey using UAV (drone). The driftwoods generation was estimated from the measurement data and the tree species dataset in each slope failure site. Total amount of driftwoods from our survey was much smaller than the estimation by the public administration. In addition, the logistic regression model optimized the heavy rain in 2018 was applied to the heavy rain in the Kuma River basin in 2020, to express the driftwoods generation. As a result, it was shown that there was small possibility to generate a lot of driftwoods in the upper area of Kawabe-gawa Dam which was planned to construct after the floods in 2020.