Variations in sediment lithology of submarine flood deposits on the slope off Kumano River, Japan

Abstract

AbstractMajor river flooding is a typical geohazard frequently characterized by the transportation of large quantities of terrigenous material from the land to the sea. However, the types and patterns of flood material deposition and the influence on seafloor environments differ because of the diversity in geological and climatic conditions. In September 2011, the activity of typhoon 201112 produced severe rainfall in the central Kii Peninsula and caused a large flood along the Kumano River. The event was considered a once-in-a-century-scale flood. We examined the occurrence of flood deposits along the slope off the mouth of Kumano River in four surface sediment cores collected around 2 months after the 2011 flood. We found a thin and muddy event bed at the core top of three cores, with two or three event beds corresponding to past historical flood events. The sediment thickness, grain size and sedimentary structures for three historical flood deposits demonstrated significant variation. The thicknesses and the grain sizes of the three flood deposits from the same river showed no relationship with the maximum discharge from each flood. The differences in the types and fate of sediments discharge during the floods probably account for the variations.