Changes in slope erosion and sedimentation rates and its relations with the amount of ash fall and debris flow occurrence based on Lidar data from 2010 to 2018 in the Arimura River basin of Sakurajima Volcano, Japan
Sakiko TETSUKA, Shin‐ichiro HAYASHI and Yasuo ISHII
At Sakurajima Volcano, debris flows frequently occur under the changes of volcanic ash fall thickness by continuous eruptions. To clarify effect of the volcanic ash fall on debris flow occurrence, we investigate relations with slope erosion and sedimentation rates, the amount of rainfall, ash fall and debris flow occurrence. The slope erosion and sedimentation rates were estimated from the difference value of Lidar data in the upper Arimura River basin of Sakurajima from 2010 to 2018. As a result of investigation, the erosion and sedimentation mainly occurred in the uppermost basin (basin 1 to 4). In basin1where relatively old pyroclastic materials had dissected, sediment runoff occurred due to the expansion of developed gully on slope and the erosion of riverbed sediments, regardless of the amount of ash fall. In basins 2‐4 where latest pyroclastic materials deposited, erosion rate on the slope increased with the development of gully on the slope after the amount of ash fall decreased. The scale of debris flow was estimated from the height of broken wire sensor in lower Arimura River. Although the scale of debris flows tended to be large in the period when the riverbed erosion occurred, it tended to be small in the period when the gully develop on the slopes where latest pyroclastic materials deposited. This result suggests that characteristics of sediment runoff with continuous volcanic eruptions depends not only on the amount of ash fall, but also on the type of erosion in the basin, the materials of eroded sediments, and the volume of deposits in the basin.
Lidar data, gully erosion, riverbed erosion, ash fall, Sakurajima