Characteristics of juvenile pyroclastic materials affected on runoff and sediment discharge. |A Case Study of the 2011 Eruption of the Shinmoedake|

Takashi KOI, Yasuhiro FUJISAWA, Koji FUJITA, Nobuo ANYOJI, Shigetaka TAKIGUCHI and Mitsunori SUGIYAMA

We examined effects of juvenile pyroclastic materials produced by the 2011 eruption of the Shinmoedake Volcano in the Kirishima Mountains on runoff and sediment discharge. The four characteristics of juvenile pyroclastic materials are thickness, grain size, porosity, and solidification. Pyroclastic materials consist of fine ash and coarse pumice, distributed mainly around the east and southeast areas of Shinmoedake, respectively. The thickness of the fine ash and the pumice deposits is more than 1 cm (maximum 3 cm) and more than 10 cm, respectively. The pumice particles are vesicular. Solidification materials were not reportedly found in the ash fall, so it is unclear whether the ash fall contained any solidification materials. Hyetographs and hydrographs of the eight rainfall events corresponded well. Although direct runoff related to total rainfall of less than about 100 mm was low in all the watersheds, direct runoff related to total rainfall of more than about 200 mm increased in the two watersheds whose headwater is located in the Shinmoedake and whose runoff coefficient was higher than that of the other watersheds. The two watersheds with a high runoff coefficient had a relatively thick ash fall deposit because of their closer proximity to the Shinmoedake Volcano compared to the other watersheds. In addition, these two watersheds were almost completely covered by fine ash. These facts suggest that the infiltration capacity in the watersheds temporarily decreased, and hence, there was a high level of discharge during rainfall events. The sediment movement after the 2011 eruption of Shinmoedake was only due to the high rainfall intensity (>30 mm/h). Because the pyroclastic materials were mainly porous pumice and the deposit of fine ash was thin, decrease in infiltration capacity was small. Therefore, sediment discharge was not triggered by low rainfall intensity events. To evaluate the potential for sediment discharge after an eruption, it is essential to investigate the infiltration capacity and the thickness of the ash fall, as well as the characteristics of juvenile pyroclastic materials considered in this study.