[H-TT13] 高精細地形表層情報と人新世におけるコネクティビティ

[HTT13-P03] Conditions for topographic changes in shallow landslide scars due to rainfall events: High-resolution analysis based on UAV-SfM approach

鈴木 花七海¹、*八反地 剛²、小倉 拓郎³、古市 剛久⁴、田中 靖⁵、土志田 正二⁶ (1.筑波大学大学院理工情報生命学術院、2.筑波大学生命環境系、3.兵庫教育大学学校教育研究科、4.森林総合研究所、5.駒澤大学文学部、6.消防庁消防研究センター)

キーワード：侵食、水路頭、土砂輸送、山地源流域

This study focuses on elevation change in two shallow landslide scars on granite slopes in Hiroshima Prefecture, Japan, to elucidate the conditions of erosion and deposition at landslide scars. Site A is a shallow landslide scar with an area of 6,100 m², a depth of 2.0 m, and an average slope of 30°. Channel diverges at the center of the landslide scar of Site A. Site B is a shallow landslide scar with an area of 4,000 m², a depth of 2.1 m, and an average slope of 10°. The cause of the landslides at Sites A and B are the heavy rainfall events in 2014 and 2018, respectively. Unmanned Aerial Vehicles (UAVs) were used for taking aerial photographs from December 2019 to October 2022 in Site A, and from April to October 2021 in Site B. High-resolution orthomosaics and Digital Elevation Models (DEMs) created from the UAV images were used to analyze elevation changes, the distribution of vegetation, and grain size of displaced gravels. In 2021 summer, heavy rainfall events with recurrence periods of about 10 years occurred. Elevation changes due to erosion and deposition exceeded 1 m in the north tributary and the main channel of Site A, and debris with grain size larger than 50 cm was transported during this event. The elevation changes due to erosion and deposition were less than 30 cm in the south tributary channel of Site A. The elevation changes due to erosion in Site B were less than 20 cm, and only gravels with a grain size smaller than 25 cm were transported. According to these results, heavy rainfall events occurring about every ten years can maintain the channel morphology along the north tributary and the main channel of Site A. In contrast, the stream channel was stable even during the rainfall events with a recurrence period of about 10 years in Site B and the south tributary channel of Site A. Analysis of microtopograpy before the heavy rainfall event in 2014 revealed that channel heads were located upstream of the surveyed section of the northern tributary, while there was no channels in the southern tributary of Site A. In Site B, a channel head was located at the center of the landslide scar. These observed trends were consistent with the channel network before the event in 2014, suggesting that the channel in the northern tributary of Site A will be maintained, while the channel morphology formed by the event in 2014 will become ambiguous in the southern tributary of Site A and the upstream section of Site B.