STUDY ON DRIFTWOOD DISCHARGED FROM THE KUMA RIVER INTO THE YATSUSHIRO SEA DURING A HEAVY RAIN DISASTER IN JULY 2020

Abstract

 A large amount of driftwood and debris were discharged from the Kuma River into the Yatsushiro Sea and drifted to the coast of the Yatsushiro Sea during the heavy rains that occurred in the southern part of Kumamoto Prefecture in July 2020. According to the report released by the Kyushu Regional Development Bureau of the Ministry of Land, Infrastructure, Transport and Tourism (MLIT), the collection amount of drifting debris from the Yatsushiro Sea in July 2020 is approximately twelve times as much as the amount of annual drifting debris removed from the Yatsushiro Sea in 2019. These drifted materials are detrimental to the natural environment and fishing industry as well as a fear of secondary disasters caused by typhoons and high waves. The purpose of this study is to clarify the advection process of driftwood generated by heavy rainfall disasters in order to easily provide useful knowledge for predicting and responding to damage from driftwood without requiring complicated calculation processing or input operation. In this study, the spatial distribution of driftwood in the bay is calculated by numerical analysis, assuming that the driftwood moves in accordance with an advection-diffusion equation. In addition, wind effect items were added to advection terms and diffusion terms of the advection-diffusion equation in consideration of the influence of dispersion in wind velocity on driftwood movement.