Topographical and geological characteristics of source areas of debris flows on hillslope covered
with pyroclastic-flow deposits for 20 years after the eruption of Mt. Unzen

Yasuyuki HIRAKAWA, Kazyuyuki OKANO, Toshiyasu UENO,
Daizo TSUTSUMI, Shusuke MIYATA and Yoshifumi SATOFUKA

Abstract

There are many cases where debris flows are generated frequently after a volcanic eruption, and the frequency of such occurrences gradually decreases with the laps of time. As a major reason for this, a change in the characteristics of runoff depending on volcanic ash with a low infiltration rate has been pointed out by previous works. However, even now, debris flows have been occurring for about once or twice a year for more than20years after the eruption of Mt. Unzen. The temporal change in the characteristics of runoff depending on volcanic ash cannot explain the actual conditions described above. Aiming at interpretation of the actual condition, we researched the topographical and geological characteristics of source areas of debris flow. Firstly, we confirmed that the post‐eruption infiltration rate had been recovered to a level similar to that before the eruption through conducting field measurements. Then, after conducting topographical analysis and making field observations of debris flows, we succeeded in pinpointing the location of recent major source area of debris flow. Based on the erosion conditions, topographical and geological characteristics and groundwater discharge at the pinpointed source area, we examined the major factors for the generation of debris flows. As a result, it was suggested that debris flows occurred mainly because of the existence of groundwater and differential erosion, which depended on the discontinuity of new pyroclastic‐flow deposits and pre‐eruptive ground surface below. Based on this finding, we proposed an initiation model of debris flow in current situation of the recent major source area of debris flows in Mt. Unzen. We also proposed a schematic diagram showing the temporal change of dominant factors of debris flows after a pyroclastic eruption.

Key words

debris flow, pyroclastic‐flow deposits, pre‐eruptive ground surface, groundwater, Mt. Unzen