Abstract:
Numerical simulations of the pyroclastic flows of the 1991-92 eruption of Unzen Volcano, Kyushu, Japan were performed to evaluate runout distances, disaster areas, flow velocities and arrival times of the flow. Three gravity current models were used for the simulations: the energy line model, the energy cone model and the Bingham flow model. Using the energy cone model, runout distances and possible disaster areas were mapped as a function of various effective friction coefficients. This energy cone model was revised not to reach the shadow zone due to topographic barrier. Relationship between the volume of the pyroclastic flow and the effective friction coefficient was investigated. Average velocities of five pyroclastic flows (May 24 8:07; May 26 11:13; May 26 14:28; May 29 15:02; May 29 15:31, 1991) were measured using video images. Average velocities of these pyroclastic flows were up to 42m/s. Flow velocities and arrival times for these pyrocalstic flows were simulated using both the energy line and Bingham flow models. The flow velocity calculated by the energy line model was larger than the measured average velocity. In contrast, the flow velocity and the arrival time calculated by the Bingham flow model with the appropriate parameters (density, flow thickness, yield strength, viscosity and drag coefficient) agreed with the measured values except in steep slope region.