抄録
Shinmoe-dake in Kirishima volcano erupted violently on January 26, 2011 and spewed a large amount of volcanic ash and pumice over surrounding area, especially eastward and southeastward, in the following several days. We observed the states of the tephra fall deposits and traces of surface runoff at a lot of hillslopes around Mount Takachiho in May and June, 2011. The observation aimed at making clear some features of surface runoff along hillslopes covered with tephra fall by the eruptions. The result shows that traces of surface runoff are likely to occur on hillslopes where the surface is covered with volcanic ash densely. On the other hand, there are many forest slopes where no trace is found in spite of deposition of volcanic ash. According to visual observation, we observe that fallen leaves had formed gap structures into original surface layer and keep the surface permeable. The structure seemed to make it difficult to cover the surface of the ground with thin volcanic ash fall deposit. However, the permeability-holding effect of the gap structures seemed to be dismissed by the thick deposit of pyroclastic materials. It might be thought there are some necessary conditions to generate surface runoff. We estimate that one of them is existence of volcanic ash in the surface layer and another is thick deposit of pyroclastic materials. We also compared the distribution of traces of surface runoff with distribution of torrents with significant sediment discharge during rainfall in June and September, 2011. They are coincident to each other. In order to evaluate the post-eruption sediment discharge from the volcanically disturbed watersheds, however, more quantitative studies on hillslope hydrology and sediment dynamics are necessary.
