[H-DS11] Geohazards in humid, tectonically active countries and their precursors

[HDS11-P04] Sediment production area in the radial valley and the actual situations of sediment production there from 2010 to 2018 in the upper reaches of the Arimura River, Sakurajima Volcano, Japan

*Yasushi Sano¹, Takeshi Shimizu¹, Koji Ishida¹, Naoki Imamori¹ (1.Public Works Research Institute)

Keywords:Sakurajima Volcano, aerial LiDAR survey data, GIS, radial valley, sediment production

Sakurajima is one of the world’s most active volcanoes that has been erupting explosively since 1955. As of the end of January 2022, the rivers in Sakurajima have been prone to debris flow. This occurs even with a small amount of rainfall compared with watersheds, which have not been affected by the volcanic activity (Osumi Office of River and Highway, Kyushu Regional Development Bureau, and Ministry of Land, Infrastructure, Transport and Tourism 2022). The erosion and sedimentation in valley sections have been found to affect the occurrence and magnitude of debris flows. In volcanic areas after the end of an eruptive activity and basins with intense sediment transport, erosion and sedimentation were repeatedly observed at almost the same locations in the valley (Hirakawa et al., 2019; De Hass et al., 2020). However, the actual situation of sediment production in valley sections of volcanoes with continuous eruptions and continuous pyroclastic supply is not fully understood. In this study, we investigated the characteristics of sediment production in the upper reaches of the Arimura River upstream from the Arimura River No. 3 sabo dam, which are found on the southeast side of Sakurajima, using 9 years of aerial LiDAR survey data that were acquired at 1-year intervals from October 2010 to October 2018. The aerial LiDAR survey data were obtained from the Japanese Ministry of Land, Infrastructure, Transport and Tourism. The survey was conducted using differential analysis of eight periods in the aerial LiDAR survey data. The sections of the radial valley where sediment transport areas with a depth of erosion or deposition of 1.0 m and an area of 10 m² were particularly concentrated were selected. The longitudinal profiles of the selected sections were then prepared to confirm the observed riverbed fluctuations. In addition, microtopography in the radial valley of the extracted sections was deciphered based on the concept of geomorphic species by referring to the slope and 3-D elevation maps. This allowed the actual situation of sediment production in the area was sorted out from the changes in the microtopography. We confirmed that erosion and sedimentation were repeated at almost the same location in the radial valley section from about 250 m to about 320 m above sea level at the foot of the volcano from 2010 to 2018. It was also confirmed that the erosion and sedimentation were repeated in different radial valley sections and adjacent alluvial cones located in the section from about 310 m to about 350 m above sea level at the foot of the volcano. The microtopographic changes associated with sediment transport in the former radial valley section were caused by changes in the riverbed at the bottom of the valley and the disappearance and reformation of small terraces that were formed on the side banks. In contrast, the microtopographic changes associated with sediment transport in the latter section of the radial valley and adjacent alluvial cone were caused by fluvial changes in the radial valley and the expansion and contraction of the alluvial cone. These were dependent on the number of eruptions, amounts of ash fall, and sediment erosion and deposition in the pyroclastic cone, which has been upstream of the alluvial cone since 2013. Our results indicated that the sediment erosion and deposition were repeated in the radial valley section found at the foot of the volcano in the upper Arimura River basin in Sakurajima; however, the actual situation of sediment production differed in each section.

Acknowledgements
We would like to express our sincere gratitude to the Osumi Office of River and Highway, Kyushu Regional Development Bureau, and Japanese Ministry of Land, Infrastructure, Transport and Tourism for providing us with valuable data, such as aerial laser survey, eruption and ash fall, and debris flow observation data.