土石流の総合的観測その6. 1979年焼岳上々堀沢における観測

Abstract

Systematic observation of debris flow was carried out at valley Kamikamihori on the easternslope of Mt. Yakedake in 1979. A synthetic observation system equipped with many instru-mentations was set in operation which had been devised and put to practical use in these ten years.In the source area, topographic changes of slopes were surveyed besides the hydrologicalobservations. In the channel reach, measurements were frequently carried out to reveal thegeomorphological processes in the valley bed, besides the observation of flowing state of debrisflow by the system.Special effort was made to study the depositional process of the debris flows. In order toreveal the dynamical aspects of debris flow in flowing, retarding and stopping stages, many typesof visual recording systems were set in the whole area of the fan. For the investigation of thedepositional features, leveling survey and polar coordinate survey have been frequently carriedout before and after debris flows.With heavy rainfalls, this year, seven debris flows occurred, among which the one on Aug.22 was the largest in the last ten years. Following items are discussed on the basis of observedfacts; (a) relationship between rainfall intensity and runoff at the time when debris flow occurrs, (b) exact calculation of gully runoff by the kinematic-wave method, ? violent erosion in therill and gully, (d) strength and frequency characteristics of ground vibration due to debris flows, (e) lateral inclination of the head part of debris flow, (f) seasonal changes in the valley profile, (g) dependence of debris accumulation behind the dams on the individual scale of debris flow, (h) arrival of a large debris flow to gentle slope with an inclination of 1-2°, (I) development ofthe fan due to the repeated deposition of debris lobes, (j) erosional and depositional effects bydebris flow and sand flow over the fan, (k) a low infiltration loss of water in the debris flow atthe fan.