Effects of volcanic ash deposition on surface erosion and runoff in Sakurajima, Japan

Given the large quantity of volcanic ash that falls continuously on basins nears volcanoes, debris flows can be easily triggered by low amounts of rainfall. Debris flows occur because the infiltration capacity of slopes is reduced by the accumulation of ash; consequently, a large proportion of any rainfall ends up as surface runoff, which in turn promotes sediment erosion. In order to better understand debris flows, it is therefore necessary to clarify the characteristics of surface runoff on volcanic slopes. Further, the erosion patterns after repeated rainfall events in such areas also need to be clarified. In December 2011, the authors selected a study slope (ca.1.8 m²) in the Arimura River catchment on the southeastern slopes of Sakurajima Volcano, one of the most active volcanos in Japan. The site had a bare slope due to volcanic ash that had accumulated after previous eruptions. To estimate the infiltration capacity of the study site, the volume of surface runoff (excluding the sediment volume) and sediment transport were measured separately, continuously and automatically (Fig. 1). The results showed that the total surface runoff was relatively large at the study site when a rainfall event was accompanied by debris flow. Extensive rill erosion was evident on the slope during the observation period. When the ash deposition rate was high, the surface of the rills surfaces was covered with volcanic ash. However, when the ash deposition rate was low due to fewer ash emissions and frequent rainfall, rills were not covered with volcanic ash and were exposed at the surface. We examined the rainfall conditions that were necessary for the occurrence of surface runoff on the slope under two scenarios. The lower limits of the apparent infiltration capacity of the slope during periods with high and low daily ash deposition rates ranged between 11 and 14 mm/h, and 15 and 25 mm/h, respectively. Assuming that the infiltration capacity of rills is relatively high compared to that of interrill surfaces, the infiltration capacity of the study slope appeared to increase when daily ash deposition rates were low.