[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

*Tsuyoshi Hattanji¹, Suzune Aono², Naofumi Yoshihara³, Takashi Gomi⁴, Takeshi Matsumoto⁴, Wataru Sakashita⁵, Yuichi Onda¹ (1.Faculty of Life and Environmental Sciences, University of Tsukuba, 2.College of Geoscience, University of Tsukuba, 3.Geological Survey of Japan, AIST, 4.Graduate School of Agriculture, Tokyo University of Agriculture and Technology, 5.Center for Forest Restoration and Radioecology, FFPRI)

Keywords:channel bed, high-magnitude low-frequency event, sediment transport, step topography

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.