Study on debris flow monitoring in Mt. Fuji

Abstract

The Osawa River, which has its headwaters at the top of Mt. Fuji, has one of the largest failure in Japan, the Osawa Failure, in its uppermost part of the catchment. The Osawa Failure produces more than 100,000m3 of sediment every year by erosion and collapse, which causes debris flows and slash avalanches. Since its opening in 1970, the Fuji Sabo Office has been observing the sediment dynamics of the Osawa River through field surveys, observation equipment and aerial photogrammetry, aerial laser survey, and Japanese pipe hydrophone. In 2021, the largest debris flows since the start of observations in the Osawa River occurred twice in March and August. In the March 21 debris flow, 416,000m3 of sediment flowed down from the Osawa Failure, and 482,000m3 was deposited in the alluvial fan. In the August 18 debris flow, 421,000m3 of sediment was deposited in the alluvial fan. Our observations from CCTV cameras, rain gauges, and other equipment installed in the Osawa River indicate that the rainfall characteristics during the two debris flows were different, and that the conditions for debris flows differed depending on the season.