Monitoring channel morphology changes in a headwater stream with monocable-system photographing and structure from motion photogrammetry.

In humid forested mountains, headwater stream is important as a source of sediment in geomorphology as well as a habitat of aquatic organisms in geoechology. The authors monitored sediment dynamics in a forested headwater stream in Field Museum of Karasawayama, TUAT, with a new system of photographing along a small cableway. The system is composed of a SLR camera, a gimbal (camera stabilizer), and a zig-zag monocable system, which is used for yarding in forestry. The zig-zag monocable system was installed along a 15-m reach of the stream at a height of 2-3 m above the channel bed. The cableway was bended with pulleys at two bending sections of the stream. The photographs for the 15-m channel reach were taken from a suspended camera slowly moving along the cableway at a 2-second interval. Operation started from September 5 in 2017, and the channel bed was observed every month except for dry winter season. Fine resolution DEMs were constructed from the photographs with structure from motion photogrammetry software. Each DEM has a resolution of 5 mm.

Although the surveyed channel reach was almost stable during the observation period, a storm event with total rainfall of 143 mm in October, 2017 caused sediment transport in the surveyed channel reach. The fine-resolution DEMs revealed channel aggradation up to 25 cm immediately above a large woody debris in the middle section of the surveyed reach. Channel degradation of 5-15 cm was also observed immediately below this woody debris. The monocable system is useful for monitoring fine-resolution change of channel morphology in forested mountain basins.