EFFECT OF VOLCANIC ASH THICKNESS ON DEBRIS FLOW CHARACTERISTICS AND RISK ASSESSMENT IN THE ARIMURA RIVER BASIN OF SAKURAJIMA VOLCANO

Sakurajima, one of Japan’s most active volcanos, is characterized by its transient topography, which is influenced by continuous eruptions that supply volcanic ash, thus altering surface permeability and initiating lahars. The term lahar, which includes any flow containing volcanic material, rock debris and water, can be attributed to debris flows and hyperconcentrated flows, depending on the amount of material carried by the flow. Lahar characteristics change with the temporal variation of surface permeability due to volcanic ash deposition and erosion. Due to the influence of the variation in volcanic ash thickness on the initiation of lahars, it is of great importance to investigate the relationship between surficial properties and lahar initiation mechanisms. Therefore, in this study, sixty-two lahars occurred in Arimura River basin of Sakurajima volcano has been investigated between 2015 and 2020. Utilizing X Band Multi-Parameter (XMP) radars, precipitation data was acquired to assess the role of precipitation in lahar formation. Lahar sediment runoff features and flow characteristics were analyzed using Load, Vibration, Pressure (LVP) sensors and kinematic wave models developed specifically for this study. The lahars were categorized as regular and irregular, showing changes in discharge and sediment concentrations over time. Irregular lahars were found to carry larger and higher amounts of material and have different properties compared to regular ones. Concurrently, the topographic impact of volcanic ash thickness was explored. Using Rainfall Intensity-Duration (ID) curves, volcanic ash thickness and crater activity were found to have a direct influence on lahar initiation. Categorizing all lahars, the study identified a critical monthly ash fall thickness of 0.1 cm, with an under-detection rate of only 9% and a false alarm rate of 26%. Not only the monthly ash fall, but also the temporal change of volcanic ash accumulation on the surface was investigated by using the Discharge Dependent Erosion (DDE) method developed specifically for this study and Universal Soil Loss Equation (USLE). It is concluded that the effect of erosion and deposition plays an important role on the permeability of the ground surface, even if the monthly ash fall thickness has a certain effect. Risk assessment charts were developed based on volcanic ash thickness, crater activities, ID curves, peak discharge values of lahars and rock block sizes transported in the flow. These charts were successfully validated to a large extent by the lahars that occurred in 2014 and 2021.