Abstract
In August, 1979, a large-scale mudflow occurred at Hora valley near Tochio area of Kamitakara village (Gifu Pref.), then the area suffered from severe disaster.
In this report, inferred causes and conditions of occurrence of mudflow based on the field investigations at the stricken area are described.
At first, geological constitution and their features of Tochio area were reviewed. Secondary, the author attempted to conjecture conditions of running down of the mudflow based on the detail observations of the mudflow deposit. At last, limitation of rainfall condition on occurrence of mudflow was estimated from hydrological data and past record on the typical disaster at the area.
Results obtained from this investigation are as follows:
1) The debris which was produced by collapse of Nohi-rhyolite at the uppermost-reaches of the valley might run down taking deposit of the channel into the flow.
2) A part of the mudflow began to accumulate at the place where valley suddenly broaden, and remaining part of it run down further and accumulated at the lowermost-reaches in Tochio area.
3) Mean velocity of the mudflow was estimated as follows:
In the valley………6.0-6.5m/sec
At the place where valley suddenly broaden……… 4m/sec
At Tochio area………1.6-2.5m/sec
4) Critical precipitation for occurrence of mudflow at the valley was estimated. It is about 30 mm/hour for intensity of effective precipitation.
