火山灰被覆に伴う侵食と土砂流出

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This study sought to clarify the temporal changes caused through erosion processes and runoff processes of rainwater, and the resulting sediment discharge; along with factors governing them on slopes made up of volcanic ash caused by volcanic activities on Sakurajima volcano, and on slopes made up of volcanic ash and pyroclastic flow deposits caused by volcanic activities on Unzen volcano. Moreover, the effects of revegetation on the processes of rainwater and sediment discharge were examined. In chapter one, the author reviewed past studies, and the purpose of this study. In chapter two, the author investigated the effects of volcanic ash on runoff processes, the area contributing to the occurrence of Hortonian overland flow, and sediment yield and discharge caused by sheet erosion on Sakurajima volcano. Results of analyses using a runoff model show that ash-covering caused a decrease of rainfall loss and Manning's coefficient of roughness, and an increase of the area contributing to the occurrence of Hortonian overland flow, surface runoff and sediment discharge. The area contributing to the occurrence of Hortonian overland flow, the infiltration capacity and Manning's coefficient of roughness have been increased by the armoring of the surface layer of volcanic ash. In chapter three, the author investigated the effects of revegetation on the occurrence of Hortonian overland flow and surface erosion processes on Sakurajima volcano. It was found that sediment discharge on revegetated slopes was much less than that on bare land slopes. The infiltration capacity and Manning's coefficient of roughness were increased by incremental plant cover and plant body weight. Moreover, results of analyses using a runoff model and a sediment discharge formula showed that revegetation caused a decrease of the area contributing to the occurrence of Hortonian overland flow. In chapter four, the author investigated the temporal changes of runoff processes of debris flows, which followed the pyroclastic flows observed from 1995 to 2000 along the Mizunashi River at Unzen volcano. Temporal changes of the runoff process of debris flows following eruptions were examined using a runoff model. The results showed that the roughness coefficient, rainfall loss, infiltration capacity and the area contributing to the occurrence of Hortonian overland flow are important factors for the reproduction of the runoff processes of debris flows. Occurrence frequency and magnitude of debris flows have decreased since volcanic activity ended because of the recovery of the infiltration capacity, a decrease in frequency and magnitude of Hortonian overland flow, and a decrease of the area contributing to the occurrence of Hortonian overland flow. In chapter five, the author investigated the temporal changes in the Yue River caused through the processes of erosion and sediment discharge that followed the eruption of Unzen volcano. Temporal changes in the processes involved in the debris flows that followed the eruption were recreated by constructing a runoff model for the Mizunashi river basin at Unzen volcano.
2005-12-01

著者

寺本行芳
鹿児島大学農学部

火山灰被覆に伴う侵食と土砂流出

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