Occurrence Frequency and Sedimentation Properties of Debris Flows in Ishihara District, Hofu City, Yamaguchi Prefecture

Abstract

On July 21, 2009, the torrential rain resulted in debris flow at more than 480 mountain streams in Hofu City, Yamaguchi Prefecture. In the present study, the layered samples were collected using a Geo-slicer at the location of the alluvial flow generation area in Ishihara, which was one of the districts where a remarkable debris flow disasters occurred. It is located in the flooding area of the Saba river, which flows south within the central part of Hofu City. The soil layers consist primarily of river/lake deposits, and the thin layers of debris flow deposits are sandwiched. In addition, we estimated the formation age of carbide in the old and new debris flow deposit by radiocarbon dating (accelerator mass spectrometry, AMS). Debris flows that have reached the flooding deposit area have occurred at least four times after A.D.1230, including the debris flow in A.D.2009 in the Ishihara area.