Temporal changes in lahar sediment run-off characteristics and run-off coefficients in the Arimura River basin of Sakurajima volcano, Japan

The rapid flowing nature of lahars can cause serious hazards in the proximal to distal areas of volcanoes. The accumulation of ash can form an impermeable layer on the ground and prevent the infiltration of precipitation. The surface water thus formed erodes the ash layer and causes the surface permeability to rise again. Over time, this accumulation and erosion sequence changes the lahar risk. Large numbers of lahars are observed each year around Sakurajima, which is known as one of the most active volcanoes in Japan. In this study, the temporal change of lahar sediment run-off properties and run-off coefficients have been investigated between 2015 and 2020 in the Arimura River basin of Sakurajima volcano, Japan. X Band Multi-Parameter (XMP) radars are used to gather precipitation data in the basin. The sediment data have been obtained by Load, Vibration, Pressure (LVP) sensors, and a kinematic wave model is adopted to calculate surface run-off. Lahars are categorized as regular and irregular considering the correspondence of measured and calculated waveforms of the hydrographs based on the kinematic wave model. Analyses reveal that run-off coefficient changes depend on the volcanic activity and decreases after an irregular lahar compared with a previous regular lahar. LVP sensor analysis results show that sediment concentrations of the lahars change in time. It is found that irregular and regular lahars are distinct in terms of velocity and sediment concentrations. It is estimated that irregular lahars carry larger material than regular lahars based on the vibration data. This study underlines that the properties of lahars change over time under the influence of ash fall thickness, and these changes can be detected in detail with our kinematic wave model and LVP sensors.