Author(s): Tomoko Kyuka; Yasuyuki Shimizu; Yoshiaki Ishida
Linked Author(s): Yasuyuki Shimizu, Tomoko Kyuka
Keywords: Channel migration; Sediment accumulation; Record-breaking rainfall; Bedrock exposure; Pekerebetsu River
Abstract: In August 2016, Hokkaido region of Japan was exposed to record-breaking precipitation by four typhoons and a rain front within a month. The Pekerebetsu River, which is a tributary of the Tokachi River in the eastern part of Hokkaido, a comparatively small river, was the worst affected after facing the last typhoon. The width of the river expanded from two to five times during an overnight of 31 Aug. 2016; most bridges and several houses fell into the river because of channel migration during the last flooding event. In this study, the authors conducted several field surveys and followed by performing numerical calculations to understand the large-scale channel deformation, erosion and deposition phenomenon in the Pekerebetsu River under the extreme large-scale discharge, which was over the designed discharge of the Pekerebetsu River. The results of field survey showed that different disaster situations were observed in a short distance around 4.7 km. In the upstream section, severe bed degradation and bedrock exposure were developed. In contrast, sediment deposited within the original main flow channel and the flow path significantly migrated with bank erosion in the downstream section, near the residential area. The main factor of this phenomenon, numerical analysis explained that (1) bed scouring and degradation in the upstream section was gradually developed and propagated to the downstream because of sediment shortage from the upstream. (2) The situation dramatically changed in the upstream area of the slope gradient change point. At this point, sediment accumulated and it stopped propagation of the bed degradation. (3) The accumulated sediment became a source of a large amount of sediment to the downstream section during a series of flooding events. It became a trigger of channel aggradation in the original flow path and remarkable channel migration due to the flow deflection toward the outer banks of channel curvature.
Year: 2018