Author(s): Hiroshi Takebayashi; Shinji Egashira; Masaharu Fujita
Linked Author(s): Hiroshi Takebayashi
Keywords: Mudflow; Numerical analysis; Izu Oshima; Turbulence flow; Volcanic island; Volcanic sediment
Abstract: Landslides and mudflows occurred on the western part of the island of Izu Oshima in Japan on October 16,2013. I zu Oshima is a volcanic island whose land surface is covered by volcanic ash sediment to a depth of approximately 1 m. As a result, mudflow with high sediment concentration was formed on the day. A laminar layer forms in a debris flow from the bed to the fluid surface. In contrast, laminar flow is restricted to near the bed in a mudflow, and a turbulent flow forms on the laminar flow layer. Thus, the equilibrium slope of a mudflow is smaller than that of a debris flow. In this study, a horizontal two-dimensional mudflow model considering both laminar and turbulent flows is developed. The model is subsequently applied to the mudflow that occurred on Izu Oshima and the applicability of the model and the flow characteristics of the mudflow are discussed. The differences in the horizontal flow areas between the simulated results and the field data are compared; it is found that the outline of the horizontal shape of the flow areas is reproduced well. Furthermore, the horizontal distribution of the erosion and deposition area is reproduced well by the numerical analysis except in the Kandachi residential area. The Kandachi area was found to be an erosional area in the field observations; however, sediment was found to be deposited there in the numerical analysis. The 1.5 h of heavy rain (over 100 mm/h) on the day after the mudflow is considered to account for the discrepancy, i. e., the deposited sediment was eroded by the persistent heavy rain. The mudflow arrived in the residential district in 80 s. The average velocity and depth of the mudflow exceeded 14 m/s and 1 m, respectively, and the fluid power was sufficiently large to collapse houses.
Year: 2017