Volumetric changes in volcanic mudflow occurred in 1926 at Mt. Tokachi was successfully estimated using both sediment remnants in the investigation area and historical archives. The aim of this study is to clarify the flow and sedimentation process of the 1926 mudflow. We carried out the equilibrium experiment to confirm the separation of the fluid and solid phase under the similar condition of 1926 mudflow deposition. Moreover, we analyzed the found document that indicates the detail distribution and the thickness in particular point of the mudflow just after mudflow occurrence. Cumulative volume of mudflow deposit in the flooding area located in lower half reach of the Furano river was estimated at 3,200,000m3. Discharge yielded from source area was calculated at 6,500,000m3 including original collapse of crater, melting water from the foot slope and erosion sediment along the river course, and has grown up at 6,900,000m3 with plus 400,000m3 from transport area. At last, the 3,200,000m3 of mudflow deposit was deposited in the flooding area and the residual mudflow estimated to flashed over downstream. Knowledge developed by the calculation of mudflow volume from source to flooding area allowed drawing the longitudinal change in mudflow volume.

Key wordsFvolcanic mudflow, flow and sedimentation process, field investigation data analysis, sediment remnants, longitudinal change in mudflow volume