The role of volcanic ash thickness on the hydraulic conductivity of the ground and the initiation of debris flows

Lahars are hazardous mixtures of rock and water that flow rapidly in the form of debris flows or mudflows. The continuous eruptions at Sakurajima supply volcanic ash to form on the surface, reducing the permeability of the ground. As the surface permeability decreases, surface runoff occurs and eventually lahar starts. Therefore, Sakurajima Island suffers from lahars triggered by heavy rains every year. Sheet erosion by surface runoff decreases the thickness of volcanic ash on the ground and subsequently increases hydraulic conductivity. This cycle continues due to the eruptions of the Sakurajima volcano and the lahars formed by heavy rains. Depending on the lahar type and amount of surface runoff, the flow density and block sizes carried in the lahar vary. In this study, the relationship between the volcanic ash thickness and the hydraulic conductivity of the ground was investigated in the Arimura River basin of Sakurajima volcano between 2015 and 2020. Monthly volcanic ash thickness was measured 2 km from the craters. The dynamic change of volcanic ash thickness due to accumulation and erosion was investigated by the Universal Soil Loss Equation and the newly developed Discharge Dependent Erosion methods. Block sizes of the lahars were estimated by vibrographs and CCTV camera. The hydraulic conductivity of the ground was calculated with a Kinematic Wave Model developed specifically for this study. Based on the relationship between volcanic ash thickness and hydraulic conductivity, a risk assessment chart was created considering the block sizes carried by the lahars.