10:45 AM - 11:00 AM
[SVC27-01] Ash Fall Hazard Map using Newly Developed Simulation Model
Keywords:Ash Fall Simulation, Hazard Map, Taisho Eruption, Sakurajima volcano
In 2020, Sakurajima volcano has recorded 432 times eruptions by counting cases of which column heights are more than 1000m, according to Kagoshima Local Meteorological Observatory. It should be noted that such an active volcano is located at just 6km west of the central city of Kagoshima having 600,000 population. In recent years, accumulation of magma in the underground pool is observed to be comparable to the 1914 Taisho eruption volume, and therefore, we are highly conscious of the precaution against large scale eruption in near future.
Not limited to large scale eruptions, smaller but more frequent eruptions may cause serious damages against infrastructure. In May and August 2020, 10 and 20 flights connecting to Kagoshima Airport respectively, were reported to be canceled due to ash fall from the eruptions. The Kagoshima Airport has many domestic flights to connect remote islands; some of them are also volcanic islands. Thus, it is important in this area to ensure the safety of aviation from the impact of ash fall.
The Research and Education Center for Natural Hazards, Kagoshima University, has been studying methodology for predicting ash fall transport in real time, and evaluating expected disaster risks to improve the resilience in Kagoshima area. In this presentation, ash fall hazard map using newly developed simulation model will be reported.
The simulation model was based on a two-dimensional convection-diffusion equation including a settling transport term of pyroclastic material, and a source term. The eruption column acts as the source term, where we adopted Suzuki(1983) model to describe the vertical mass distribution. The particle size distribution was assumed to be comprised of 11 classes ranging from -5 to 5 on the Φ scale, which was equivalent to 32mm to 0.03125mm.
In the numerical algorithm, the advection term and the diffusion term were solved by the upwind difference scheme and the central difference scheme, respectively. The computational domain was taken horizontally about 150km square which covers the entire mainland of Kagoshima prefecture and taken vertically from 1500m to 20000m in the altitude. The number of computational grids was 169×169×38, which is about 1 million points totally. To reduce the computation time, non-equidistant grids were applied for the horizontal plane. As the results, the horizontal grid spacing around Sakurajima became 500m, meanwhile the spacing near the edge of the domain was 1330m. In the vertical direction, a constant grid spacing of 500m was taken, and the volume of ash fall passing through the grid of 1500m altitude was assumed to be accumulated on the ground.
The ash fall hazard map was made by the following scenarios.
1) The same magnitude of Taisho eruption is assumed to occur at 9 am every day in 2020.
2) The temporal variation of eruption column height is given as the same as Taisho eruption case. The eruption lasts for 39 hours.
3) The total amount of eruption volume is given as 600million m3 equal to the Taisho case. The total amount is distributed to the instantaneous amount by referring the above assumed temporal variation.
4) Kagoshima Aerological Observatory has conducted observations on wind velocity and direction daily at 9am and 9pm. Using these data, the spatially interpolated dataset on the wind speeds of the east-west and north-south components at every 500m altitude is constructed.
5) The simulation is carried out while updating the eruption height every hour, and the wind speed every 12 hours. The computational duration of one case simulation is set as 48 hours.
6) Computations are performed for 366 days from January 1 to December 31, 2020. For each calculation point, the maximum depth of ash deposition in one year period is analyzed.
The ash fall hazard map 2020 reveals that there are several vulnerable areas such as the coastal area of central Kagoshima city, where the deposition depths exceed 1m.
Not limited to large scale eruptions, smaller but more frequent eruptions may cause serious damages against infrastructure. In May and August 2020, 10 and 20 flights connecting to Kagoshima Airport respectively, were reported to be canceled due to ash fall from the eruptions. The Kagoshima Airport has many domestic flights to connect remote islands; some of them are also volcanic islands. Thus, it is important in this area to ensure the safety of aviation from the impact of ash fall.
The Research and Education Center for Natural Hazards, Kagoshima University, has been studying methodology for predicting ash fall transport in real time, and evaluating expected disaster risks to improve the resilience in Kagoshima area. In this presentation, ash fall hazard map using newly developed simulation model will be reported.
The simulation model was based on a two-dimensional convection-diffusion equation including a settling transport term of pyroclastic material, and a source term. The eruption column acts as the source term, where we adopted Suzuki(1983) model to describe the vertical mass distribution. The particle size distribution was assumed to be comprised of 11 classes ranging from -5 to 5 on the Φ scale, which was equivalent to 32mm to 0.03125mm.
In the numerical algorithm, the advection term and the diffusion term were solved by the upwind difference scheme and the central difference scheme, respectively. The computational domain was taken horizontally about 150km square which covers the entire mainland of Kagoshima prefecture and taken vertically from 1500m to 20000m in the altitude. The number of computational grids was 169×169×38, which is about 1 million points totally. To reduce the computation time, non-equidistant grids were applied for the horizontal plane. As the results, the horizontal grid spacing around Sakurajima became 500m, meanwhile the spacing near the edge of the domain was 1330m. In the vertical direction, a constant grid spacing of 500m was taken, and the volume of ash fall passing through the grid of 1500m altitude was assumed to be accumulated on the ground.
The ash fall hazard map was made by the following scenarios.
1) The same magnitude of Taisho eruption is assumed to occur at 9 am every day in 2020.
2) The temporal variation of eruption column height is given as the same as Taisho eruption case. The eruption lasts for 39 hours.
3) The total amount of eruption volume is given as 600million m3 equal to the Taisho case. The total amount is distributed to the instantaneous amount by referring the above assumed temporal variation.
4) Kagoshima Aerological Observatory has conducted observations on wind velocity and direction daily at 9am and 9pm. Using these data, the spatially interpolated dataset on the wind speeds of the east-west and north-south components at every 500m altitude is constructed.
5) The simulation is carried out while updating the eruption height every hour, and the wind speed every 12 hours. The computational duration of one case simulation is set as 48 hours.
6) Computations are performed for 366 days from January 1 to December 31, 2020. For each calculation point, the maximum depth of ash deposition in one year period is analyzed.
The ash fall hazard map 2020 reveals that there are several vulnerable areas such as the coastal area of central Kagoshima city, where the deposition depths exceed 1m.