二次元 LiDAR による桜島・有村川における土石流の転波列の観測

抄録

We studied roll waves on the Arimura River, Sakurajima Island, Kagoshima, Japan; using a 2D-LiDAR and a debris flow force plate. There are very few examples of time-series measurements of cross-sectional profiles of roll waves in the field. There have been 100 debris flow occurrences in the Arimura River between 2013 and 2023. Of the 33 cases with large peak discharge rates, 23 exhibited roll waves, and 6 of the remaining 10 cases showed no occurrence and in the remaining cases the debris flow was not visible due to fog or night conditions. The study of the roll waves observed on the Arimura River on July 4, 2017 showed a dimensionless wave speed c’ of between 0.76 and 1.41 and a Froude number Fr between 0.94 and 2.65, which was in line with the conditions for the occurrence of roll waves in previous reports. The period of the roll wave ranged from 7 to 22 s, and maximum wave height h_b was 3.4 times higher than minimum wave height h_f and up to 1.9 times higher than mean depth h_m. These results are also consistent with theories and other observations. Based on the relationship between the velocity coefficient U₀/U_＊ and relative water depth H₀/d for stony debris flow, we considered the flow in the roll waves to be a dilatant fluid. In this case, the measurement accuracy of 1-second intervals made it possible to measure roll waves. These observation data are expected to be useful as the verification and validation data for debris flow countermeasures and debris flow numerical analysis models.