Long‐term stream dynamics of Inamata Valley on field surveys and photographic observations of fixed spots for 1978‐2020
Reiko AKIYAMA and Masamu ANIYA
The purpose of this paper is to report long‐term stream dynamics based on photographic observations of fixed spots in Inamata Valley in Yamanashi prefecture, Japan. We focused on erosion and sedimentation caused by large discharges in 1982 and 2020. In this valley field survey was initiated in 1978 and continued almost annually from 1982 to 2003. Additional surveys were conducted in 2008 and 2020. The fixed spots for photographic record and observation were set on easily recognizable terrain features such as check dams and stream junctions. As a result, sedimentation of more than 10 m on streambed was recognized in some places in 1982. The successive photographs showed a downward trend of the streambed with repeated erosions and sedimentations in the downstream area. The streambed lowering lessened around 1990 and by 1995 it almost stopped. During the same period, however, streambed fluctuations on the order of several meters were still occurring in the upstream area. Since 1997, erosion has accelerated and the streambed had dropped to the same level as that in 1978 at downstream near Inamata‐Bridge due to the construction of the Inamata third check dam (H=50m), which was filled up by 2008. After the catastrophic landsliding and debris flows in 2020, streambed conditions of the downstream area have become similar to those in 1982 despite the new third check dam, while the upstream area became completely different than it had been before. Successive photographs of fixed spots enabled comprehensive interpretations of the streambed changes along with field surveys, based upon minute changes such as grain size, large gravel shifting and burying and exposure of falls, which are difficult to detect with LiDAR or satellite images. It is desirable to continue field observations using photographs of fixed spots in order to closely track changes over time after the 2020 sediment discharge.
stream dynamics, field survey and photographs, fixed spots, erosion and sedimentation