Estimation of debris-flow motion by field surveys

The 6 December 1996 debris flow killed 14 workers and injured 9 persons at the construction site of Gamaharazawa Gully, Japan. The cause of the debris flow was landsliding. No data were available about the motion of the major surge of the debris flow. Nonetheless, evidence of mud-line elevation of debris flow was found on the winding channel which was left by the surge. Analysis of super-elevation sites in the channel indicates that the largest surge had velocities of 5-11 m/sec and peak discharges of 200-500 m(3)/sec on the fan. This debris flow can be evaluated as a typical muddy debris flow by size distribution of solid materials and rheology of the flow. Synthetic analysis of a combination of data such as residual remains, topographic features in stream channels, meteorological data, and interviews with eyewitnesses are effective for reconstructing motion of the flow.