5:15 PM - 6:30 PM
[HGM03-P07] Observation of riverbed degradation process in the Tottabetsu River, eastern Hokkaido, using ALB and drone aerial photography
Keywords:soft rock, riverbed degradation, ALB, drone aerial photography, sediment deposit
This study presents the results of observations of the riverbed degradation process progressing in a bedrock river, by using ALB (Airborne Laser Bathymetry) and drone aerial photography. The study reach was 1.2 km (average gradient 0.44°) of the Tottabetsu River in eastern Hokkaido, Japan. A flood in August 2016 with a discharge of 700 m3/s (2.4 km upstream of the study reach), caused a large amount of sediment inflow into the reach. Subsequently, intense channel bed incision has started. Presently, the alternating layers of silt rocks and lignite belonging to the Pleistocene marine deposit (Osarushinai Formation) is exposed in places on the channel beds. The uniaxial compressive strength of the silt rock was 8.1 MPa, being defined as “soft rock” according to the criteria by JSCE (1992). Point bars are also developed. The grain size of sediment that composes them (D50) was 2.2 mm. Formation of potholes and the smooth surface of the exposed rocks indicates that the abrasion of suspended load played a major role on river erosion as shown by Whipple et al. (2000).
Photographical image was taken by drone (Mavic2 Pro and Inspire2) once a month from 2018 to 2020, except during the winter season, and ALB was conducted twice, in September 2019 and June 2020. Knickpoints consisting of bedrock were recognized in May 2019 and September 2019. In the former, the bedrock formed the point gradually eroded after retreating by 6 m in two months, completely disappearing in September 2019. The latter retreated 1.5 m in one month while lowering the riverbed by 0.5-0.8 m. This knickpoint also disappeared by June 2020.
During the observation period, meandering progressed with the development of point bars. From September 2018 to October 2020, banks retreated by 7.2m, 4.5m, and 19.6m at three sections, respectively. The size of the pool at the edge of the concave banks did not change significantly, while the riverbed at the deepest point lowered by 0.12m, 0.93m, and 0.07m from September 2019 to June 2020. With the development of point bars, the deposition of sediment in the channel near the bars caused the formation of knickpoints. The riverbed just downstream these knickpoints exposed soft rocks, and significant erosion by flowing water occurred. As the drop between the knickpoint and the pool below increased, sediment that formed the knickpoints flowed downstream. As a result of the expansion of the depositional area, one knickpoint moved 5 m downstream in 3 months, while the other knickpoint disappeared. The riverbed elevation of the pools directly downstream these knickpoints lowered by 0.17-0.37 m from September 2019 to June 2020. In the 0.3 km section where meandering was not developed, a narrow channel thread developed by May 2019 due to undercutting of part of the bedrock on the riverbed. This channel bed was covered by sediment from upstream two months later. This deposition was accompanied by lateral erosion, which lowered the average cross-sectional riverbed elevation by 0.3 m.
The observations conducted in this study suggest that geomorphic changes caused by the deposition of sediment accelerate the progression of riverbed degradation in soft rock rivers.
Photographical image was taken by drone (Mavic2 Pro and Inspire2) once a month from 2018 to 2020, except during the winter season, and ALB was conducted twice, in September 2019 and June 2020. Knickpoints consisting of bedrock were recognized in May 2019 and September 2019. In the former, the bedrock formed the point gradually eroded after retreating by 6 m in two months, completely disappearing in September 2019. The latter retreated 1.5 m in one month while lowering the riverbed by 0.5-0.8 m. This knickpoint also disappeared by June 2020.
During the observation period, meandering progressed with the development of point bars. From September 2018 to October 2020, banks retreated by 7.2m, 4.5m, and 19.6m at three sections, respectively. The size of the pool at the edge of the concave banks did not change significantly, while the riverbed at the deepest point lowered by 0.12m, 0.93m, and 0.07m from September 2019 to June 2020. With the development of point bars, the deposition of sediment in the channel near the bars caused the formation of knickpoints. The riverbed just downstream these knickpoints exposed soft rocks, and significant erosion by flowing water occurred. As the drop between the knickpoint and the pool below increased, sediment that formed the knickpoints flowed downstream. As a result of the expansion of the depositional area, one knickpoint moved 5 m downstream in 3 months, while the other knickpoint disappeared. The riverbed elevation of the pools directly downstream these knickpoints lowered by 0.17-0.37 m from September 2019 to June 2020. In the 0.3 km section where meandering was not developed, a narrow channel thread developed by May 2019 due to undercutting of part of the bedrock on the riverbed. This channel bed was covered by sediment from upstream two months later. This deposition was accompanied by lateral erosion, which lowered the average cross-sectional riverbed elevation by 0.3 m.
The observations conducted in this study suggest that geomorphic changes caused by the deposition of sediment accelerate the progression of riverbed degradation in soft rock rivers.