Flow]direction changes in the structure of deposits and grain size distribution of hot ash clouds from the pyroclastic flows that occurred on June 3, 1991(Kitakamikoba district) and June 24, 1993(Senbongi district) in the Unzen Volcano were investigated. Changes in the flow velocity, eddy scale, and the eddy rotational speed of the hot ash clouds were also revealed by VTR] analysis.
The typical structure of a flow unit of the hot ash cloud deposits showed the alternation of a sand layer and an ash layer. Only sand layers were identified in the area where the hot ash clouds separated from the main body of pyroclastic flow. The depth and grain size of the sand layer decreased rapidly in the lower area where the hot ash clouds separated. The ranges of the critical velocities of traction and suspension for the 60% grain size of the hot ash cloud deposits of the Senbongi district were calculated to be 4.5 to 9.5 m/sec and 6.7 to 13.6 m/sec, respectively, using Bagnold's critical friction velocity equation and the logarithmic distribution law (flow velocity: 18 to 37 m/sec).
Above results might be expected to offer that hot ash cloud flow structure is the combination of a high] density sand suspension prevailing layer as a lower part and a low] density ash suspension layer as a upper part.

Key wordsFhot ash cloud, deposits survey, VTR]analysis, critical velocity of traction and suspension, flow structure