Studies on the Effects of Volcanic Activity on Runoff

火山侧面的水文和侵蚀状况将在火山爆发和火山碎屑流之后发生根本改变。本研究的目的是研究火山爆发后的一些现象，即，火山灰覆盖，入渗速率降低，地表径流增加，泥石流发生。主要研究结果如下：1.火山碎屑流发生后，云仙火山山坡的渗透率迅速降低，随着时间的推移，渗透率逐渐上升.为了研究火山灰堆积物覆盖的山坡上产生地表径流的降雨条件和地表径流特征，以及火山灰活动的时间变化对其的影响，在云仙火山和樱岛火山的山坡上设置了地表径流观测设施。火山灰覆盖坡面的地表径流与10 min内超过1.0-1.5 mm的雨强同步启动.为了定量研究泥石流的发生和径流机制，在云仙火山和樱岛火山的山坡上建立了泥石流观测设施。云仙火山在火山碎屑流之后发生了大规模的泥石流，泥石流的规模随着时间的推移而减小。在樱岛火山，泥石流的规模取决于火山活动，在相同的降雨条件下，频繁喷发的泥石流规模大于较少喷发的泥石流规模。

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.