焼岳上々堀沢における土石流の発生と規模の検討

Abstract

The behavior of water in the gully bottom near debris-flow source area in the Kamikami-hori valley of Mount Yakedake has been observed in order to investigate the exact relation-ship between the water movements and debris-flow occurrences. It was confirmed thatdebris flows were generated by rapid appearances of large water surges when deposits layersalready acquired a high water-content from preceeding rainfalls, but still without beingtotally saturated by water. From the analysis of rainfall data over 14 years, the surfacerunoff of water at the source point of debris flow critical to the occurrence of debris flow wasfound to change not only over years but also seasonally, owing to the changes in the topogra-phic condition of gully and in the property of gully-bed deposits. The studies on the relation-ship between the scale of debris flows in terms of their peak flow rate or total volume and thesurface runoff of water, showed the fact that the scale of debris flows were controlled first bythe rainfall intensity in short time duration and the peak flow rate of surface runoff, andsecondly affected significantly by the depositional condition in the valley.