Abstract
Unzen Volcano began to erupt in November 1990 after 198 years of dormancy and has remained active. Continuous growth of a lava dome and falls of lava rocks have resulted in frequent pyroclastic flows. Since a great volume of volcanic material has been deposited and scattered by the pyroclastic flows, debris flows have frequently occurred along the rivers around Mount Unzen, particularly along the Mizunashi River. On June 30, 1991, August 8, 1992, and April 28, 1993, large debris flows occurred in the Mizunashi River and caused severe damage in the down reach. Over a hundred houses were destroyed by each debris flow, however, no one was killed or injured as the debris flows flooded within the evacuated area. On June 3, 1991, however, 43 lives were lost as a result of a large pyroclastic flow. On June 8 and September 15, 1991, larger pyroclastic flows occurred and burned many houses along the Mizunashi River. As Unzen Volcano is still active, the possibilities of debris flows and corresponding risk to residents have risen up at the wider areas in addition to the risk of pyroclastic flows themselves.
