Journal of the Japan Society of Erosion Control Engineering, Vol.56,No.3,2003

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The temporal variations of runoff processes of the debris flows at the Mizunashi river in Unzen volcano

Yukiyoshi TERAMOTO, Etsuro SHIMOKAWA, Takashi JITOUSONO

Abstract
From 1991 to 1995,the Mt. Unzen volcano had many pyroclastic flows; the flow deposits widely and thickly covered the flank of the volcano and created a radical alteration of the hydrologic and erosion regime of the areas. As a result, debris flows have frequently occurred in the rivers on the flank of the volcano and the topography of the basins has much changed. In this paper, we analyzed the temporal variations of runoff processes of the debris flows, which followed the pyroclastic flows observed from 1995 to 2000 at the Mizunashi River at the Unzen volcano, based on field surveys, interpretation of aerial photographs and runoff analysis of debris flows. The area contributed to occurrence of Horton's overland flow declined with time as the gully erosion area increase. The rate of sediment production from gully erosion became a peak between September 1995 and August 1996 and then declined with time. Temporal variations of sediment yield by debris flows show the same tendency as those of sediment production by gully erosion. The temporal variations of the runoff process of debris flows following eruptions were examined by runoff model. The results showed that a roughness coefficient, rainfall loss, infiltration capacity and the area that contributed to the occurrence of Horton's overland flow are important factors for reproduction of the runoff processes of debris flows. Occurrence frequency and the magnitude of the debris flows have decreased since the end of volcanic activity because of recovery of the infiltration capacity, decrease of Horton's overland flow in frequency, and the magnitude of the decrease of the area contributing to occurrence of Horton's overland flow.

Key words:Unzen volcano, pyroclastic flow, debris flow, runoff model, Horton's overland flow