[H-GM02] Geomorphology

[HGM02-P02] Detecting high-resolution variation of a forested headwater stream by a low frequency flood: An approach from cableway-system photographing

*八反地 剛¹、青野 涼音²、吉原 直志³、五味 高志⁴、松本 武⁴、坂下 渉⁵、恩田 裕一¹ (1.筑波大学生命環境系、2.筑波大学生命環境学群地球学類、3.産業技術総合研究所地質調査総合センター、4.東京農工大学農学研究院、5.森林総合研究所震災復興・放射性物質研究拠点)

キーワード：河床、低頻度大規模イベント、土砂運搬、ステップ

Geomorphic response of headwater stream to storms with various magnitude was quantitatively analyzed in K2 stream at Field Museum of Karasawayama, TUAT, Japan. A new system for photogrammetry composed of a SLR camera, a camera stabilizer, and a circular cableway system was installed along a 15-m reach of the stream at a height of 2–3 m above the channel bed. DEMs with a resolution of 5 mm were constructed from the photographs with SfM software. The observation period is from September 2017 to October 2021, including the flood event caused by Typhoon No.19 in 2019, of which the recurrence interval of rainfall was about 200 years. In addition, the movement of debris on the channel bed by the 2019 flood event was evaluated from comparison of photograph and hydraulic calculations. Elevation change occurred at 43% of the total channel bed for the 2019 event. High tractive force by the flood caused significant geomorphic changes, particularly for the steep downstream section with a step topography. In contrast, elevation change occurred at only 11–14% of the total channel bed for the year 2021. Key boulders with a diameter of about 50 cm composing step topography did not move even by the 2019 flood event with recurrence interval of 200 years. This fact implies that the core structure of the stream would be formed by mass movement such as debris flow and maintained during the high-magnitude low frequency flood event.