Characteristics of Debris Flow at the Mizunashi River in Mt. Unzen

Unzen volcano began to erupt in November 1990 after 198 years of dormancy and was in violent activity. Continuous growth of lava dome and falls of lava rocks resulted in frequent pyroclastic flows. As a great volume of volcanic material deposited and scattered by the pyroclastic flows, debris flows frequently occurred along the Mizunashi River and damaged many houses in 1991-93. In 1995, the volcanic activity has become calm and no pyloclastic flow has been observed from March, 1995. With the change of the volcanic activity, the occurrence condition of debris flow is expected to change. The result of the data analysis indicates the abrupt change in the critical rainfall for occurrence of debris flow in July, 1993. The relation between rainfall and runoff of debris flow is also changed in the same period of the time.