The processes of erosion and sediment discharge at the Yue river in Unzen volcano

Abstract

To clarify the temporal changes in the Yue River caused through the processes of erosion and sediment discharge that followed the eruption of the Unzen volcano, field surveys, interpretation of aerial photographs, and runoff analysis of debris flows were carried out. It was found that the area contributing to the occurrence of debris flow declined with time as the area of gully erosion had increased. It was also found that the rate of sediment yield from gully erosion and sediment discharge by debris flows had peaked during October 1992 and August 1993, and then declined with time, though there was an increase during September 1996 and September 1997 after the occurrence of pyroclastic flows. Temporal changes in the processes involved in the debris flows following the eruption were recreated by constructing a runoff model for the Mizunashi river basin in the Unzen volcano. The results showed that this model is applicable to the Yue river basin. The frequency and the magnitude of debris flow occurrences decreased after October 1997 because of the recovery of the infiltration rate, the decrease in the frequency and the magnitude of the Hortonian overland flow, and the decrease in the area contributing to the Hortonian overland flow.