Journal of the Japan Society of Erosion Control Engineering,
Vol.55,No.6,p.3-12,2003
Physical properties of volcanic ash deposits in Miyakejima, Mt. Usu and Sakurajima
\Analysis of physical properties of ash deposits in relation to mudflow occurrences\
Yasuhiro NOMURA, Ken'ichirou KOSUGI, Takahisa MIZUYAMA
Abstract
After volcanic eruptions, mudflows are reportedly triggered even by small rainfalls, which have never caused mudflows before the eruptions. This study investigated the influence of volcanic ash deposits on the mudflow occurrence by the saturated hydraulic conductivity test, the water sprinkling test and the viscosity test. Laboratory experiments were carried out on volcanic ash deposits collected from Miyakejima, Mt. Usu and Sakurajima. Measured saturated hydraulic conductivity values were 0.61, 20.5 and 172.4mm/h for the ash deposits in Miyakejima, Mt. Usu and Sakurajima, respectively. Results of the water sprinkling test showed that the infiltration rate of the volcanic ash deposits in Miyakejima and Mt. Usu immediately decreased to the final infiltration rate of about 1 and 41mm/h, respectively. On the other hand, the infiltration rate of the ash deposit in Sakurajima did not decrease below 160mm/h. The viscosity test revealed that the ash deposit in Mt. Usu had much higher yield]viscosity and kinetic]viscosity values than the ash deposits in Miyakejima and Sakurajima. Based on the measured physical properties of the ash deposits, we inferred the possibility of the mudflow in each region. A larger number of mudflows are expected at Miyakejima because of the low permeability and the small viscosity of the ash deposit. At Mt. Usu, occurrence of the surface runoff is expected under heavy storm events because of the relatively low permeability. However, the mudflow may seldom occur since the viscosity of the ash deposit is high. In stead of the relatively high permeability of the ash deposits in Sakurajima, the small viscosity of the ash deposit can trigger the mudflow. It was found that these presumptions corresponded well with the reported mudflow occurrences in Miyakejima, Mt. Usu and Sakurajima.
Key words:volcanic ash deposit, mudflow, permeability, viscosity