2018 年 71 巻 2 号 p. 22-33
It is important to understand the characteristics of sediment transport in gravel bed rivers for sediment disaster prevention. During a storm event due to Typhoon Lionrock (2016), large sediment discharge occurred in Pekerebetsu Creek, Hokkaido, Japan, where periglacial slopes were extensively observed. This study aims to find relationships between the sediment discharge and its control factors such as sediment supply and deposition, topographic features and bed materials. Digital aerial photographs were taken immediately after the event and were analyzed through comparison both with other aerial photographs taken of the same area and a 10 m digital terrain model simulated before the event. The supplied and deposited sediment volumes along the main and tributary channels were measured at the catchment scale. Small-scale debris flows were detected in the headwater, which developed into large scale flow due to the erosion of periglacial slope deposits with fine materials (<2 mm) at 35-45%. Sediment deposition and bank erosion along the main channel were caused by the debris or flood flows, and the river bed was 2-15 times wider than that before the event. Consequently, the supplied sediment volume from the river bed was dominant and accounted for 73.4% of the entire supplied sediment volume, in contrast to Ribira Creek where the supplied sediment volume from the river bed was 17.2%. Sediment discharge from the debris flow was estimated at 780,000 ㎥. Sediment estimated at 400,000 ㎥ was discharged at the outlet of the study catchment while sediment estimated at 630,000 ㎥ was trapped by the two erosion control dams. In the sediment sampling survey, fine materials (<2 mm) were selectively transported downstream, although a lot of coarse materials (≥2 mm) were deposited in the catchment.
