[H-GM02] Geomorphology

[HGM02-P03] Terrace deposits in headwater basins in Hofu City, Yamaguchi Prefecture, Japan

*Shiori Matsumoto¹, Tsuyoshi Hattanji², Takuro Ogura³, Takahisa Furuichi⁴, Shoji Doshida⁵, Tanaka Yasushi⁶ (1.Graduate School of Science and Technology, University of Tsukuba, 2.Faculty of Life and Environment Sciences, University of Tsukuba, 3.Graduate School of Education, Hyogo University of Teacher Education, 4.Forestry and Forest Products Research Institute, 5.National Research Institute of Fire and Disaster, 6.Faculty of Letters, Komazawa University)

Keywords:bare land, radiocarbon dating, sediment transport

Since the Middle Ages, increasing human activities caused deforestation in many parts of the world. In Japan, bare land with poor or no vegetation appeared mainly in granitic mountains due to fuel wood harvesting. This paper focused on the past changes in sediment transport processes in two small basins in Hofu City, Yamaguchi Prefecture, where bare land had widely distributed until the early 20th century. There are small terraces about 100 m² in the headwater basins, separated into the lower and upper layers. The lower layer is about 2 m thick and composed of poorly sorted deposits containing boulders (>1 m in grain size) in a matrix of mainly fine gravels, which is supposed to be debris flow deposits. The estimation from the result of radiocarbon dating indicated that the debris flow was deposited in the 1442~1515 cal AD. The upper layer is about 1 m thick and composed of well sorted deposits with mainly granite sand (<1 mm in grain size, called masa in Japanese). Review of local historical literature revealed that a large amount of fuel wood had been cut around Hofu with the development of the salt manufacturing industry around 1700 AD, and that bare land in Hofu had been recorded around 1717 AD. Aggradation of river channel by massive sediment runoff due to deforestation causes deposition of the upper layers of the terraces after 1500 cal AD. Thus, there was likely a change in the sediment transport process from debris flow to bed load transport or concentrated flow of sand within the headwater basins.