Studies on the Effects of Volcanic Activity on Runoff

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The hydrological and erosional regime of the flank of the volcano will be radically altered following the eruption and the pyroclastic flow. The purpose of this study is to examine a few phenomena following volcanic eruption, i.e., covering by tephra, lowering of infiltration rate, increasing in surface flow and occurrence of debris flow. The results are summarized as follows :1. The infiltration rate on the mountainsides of Unzen Volcano lowered rapidly after the occurrence of pyroclastic flows, and it has come gradually up with time.2. In order to research the rainfall conditions causing surface flow and the characteristics of surface runoff on mountainsides covered by ash fall deposits, and the effects of the temporal variation of ash fall activity on them, observation facilities of surface flow were established on the mountainsides of Unzen Volcano and Sakurajima Volcano. The surface runoff on tephra-covered hillslope was launched simultaneously with the rainfall intensity exceeding 1.0-1.5 mm in ten minutes.3. In order to research the occurrence and the runoff mechanism of debris flow quantitatively, the observation facilities of debris flow were established on the mountainsides of Unzen Volcano and Sakurajima Volcano. in Unzen Volcano, the large-scale debris flows occurred -after the pyroclastic flows and the magnitudes of debris flows have decreased with time. In Sakurajima Volcano, the magnitudes of debris flows depend on the volcanic activity and they are larger during the more frequent eruptions than during the fewer eruptions under the same conditions of rainfall.