Rainstorm control of the scale and the type of debris flow was analyzed using the 14 sets of rainstorm and debris]flow data obtained from 1980 through 2005 at the Kamikamihorizawa Creek of Mount Yakedake. The principal component analysis on the parameters of debris flow: its frontal velocity, peak velocity, peak flow depth, peak discharge and total discharge, combined with the correlation analysis among these parameters and the pre]event rainfall intensities indicates that these 14 debris flows should be divided into three groups. The flow in the first group shows the higher velocity and the larger total]discharge, and have a massive and turbulent boulder]dam filled with muddy matrix, while the flow in the second group shows the lower velocity and the smaller total]discharge, and a massive boulder]dam scarcely filled with muddy matrix, and an alternation of halt and re]start of the dam, and the flows in the third group take the lower velocity and the larger total]discharge. The analyses assert that the combination of the rainfall in longer time duration such as 24 hours and the rainfall in shorter time duration such as 10 minutes before the events controls this debris]flow grouping, namely the larger rainfall in longer time span generates flows in the 1 st group and the smaller rainfall in longer time span with the higher rainfall in the shorter time span generate ones in the 2 nd. Ones in the third group are generated by the larger rainfall in longer time span with the smaller rainfall in the shorter time span. The characterized relationship between rainstorm condition and debris]flow parameters might be applicable not only to the similar volcanic slopes but also to the torrents in other geologic regions, and be significant both to mitigation of debris]flow hazards and control of fluvial sediments.

Key wordsFdebris flow, rainfall condition, boulder dam, principal component analysis, Mount Yakedake