Quantitative Evaluation of Sediment Dynamics in the Pankenushi River Basin, Saru River System, Hokkaido, Japan, Following Heavy Rain in August 2016

This study provides a quantitative assessment of sediment dynamics associated with newly formed landslides and river channels in the Pankenushi River basin (95.8 km 2 ), part of the Saru River system in Hokkaido, Japan. The analysis focuses on the impacts of the torrential rainfalls induced by four typhoons, including Typhoon Lionrock, which occurred between August 17 and 31, 2016. Using satellite imagery and laser profiler (LP) measurements, the study quantifies the spatial extent and volumetric changes in sediment dynamics. The areas affected by newly formed landslides and river channel disturbances were identified by analyzing changes in the normalized difference vegetation index calculated using images from before and after the heavy rainfall events. In the middle and lower reaches of the river, where channel disturbance was pronounced, LP measurements conducted in 2012 and 2019 were used to estimate topographic changes resulting from the rainfall and runoff. Sediment erosion volume was estimated at approximately 880,000 m 3 , approximately three times greater than the volume of channel deposits (310,000 m 3 ) and about five times greater than the volume supplied by shallow landslides in the upper basin. These findings indicate that riverbank erosion, in addition to landslides, plays a substantial role in sediment runoff during extreme rainfall events, such as that in August 2016.