11:30 AM - 11:45 AM
[SVC33-09] Analysis of D-F Diagram Using Ashfall Simulation: Impact of Proximal Observational Data on Ashfall Distribution Assessment
Keywords:Pyroclastic fall, D-F diagram, Isopach
The dispersal of volcanic ash from explosive eruptions is strongly influenced by parameters such as eruption rate and initial grain size distribution. This study employs numerical experiments using the newly developed TWiCE model to quantitatively assess the impact of these parameters on ash distribution. Simulations were conducted for sub-Plinian and Plinian eruptions, systematically varying eruption rate and initial grain size distribution. The results were analyzed using the D-F diagram, a classification method proposed by Walker (1973), where the maximum mass loading at the vent is defined as Smax. The parameters D and F represent the area enclosed by the 0.1Smax contour and the proportion of particle sizes smaller than 1 mm along the dispersion axis at 0.01Smax, respectively.
The numerical experiments revealed that an increase in eruption rate led to a corresponding increase in the D value; however, the computed values remained within the sub-Plinian range. Even when eruption rates and initial grain size distributions characteristic of Plinian eruptions were applied, the D value did not reach the expected range for Plinian eruptions.
This discrepancy may stem from missing observational data near the vent, where measurements are difficult due to safety concerns and topographical constraints. Field observations often fail to account for increased deposition near the vent, potentially leading to an underestimation of the actual deposit distribution in the D-F diagram. In contrast, numerical simulations incorporate data from near-vent regions, producing results that may differ from actual observations.
Beyond data limitations, Bursik et al. (1992) suggested that in large eruptions, particle recycling through entrainment can prevent efficient detachment from the eruption column. Since the TWiCE model does not account for this recycling effect, deposition near the vent may be overestimated, further contributing to the underestimation of the D value.
Additionally, increasing the "Plume Thickness" parameter led to higher D values, bringing them within the Plinian eruption range. Plume Thickness controls the vertical extent of the eruption column, with larger values indicating extended particle transport. Increasing Plume Thickness resulted in a smaller slope in the source magnitude distribution (SMD), suppressing particle settling, thereby increasing the D value and decreasing the F value. However, the required Plume Thickness values reached several kilometers, necessitating further investigation to assess their compatibility with actual plume heights.
This study quantitatively evaluated the effects of eruption rate and initial grain size distribution on ash dispersal using numerical simulations with TWiCE. The findings highlight the influence of Plume Thickness and missing observational data on the D value. Future research will focus on validating model outputs with observed plume thicknesses and refining simulation accuracy.
The numerical experiments revealed that an increase in eruption rate led to a corresponding increase in the D value; however, the computed values remained within the sub-Plinian range. Even when eruption rates and initial grain size distributions characteristic of Plinian eruptions were applied, the D value did not reach the expected range for Plinian eruptions.
This discrepancy may stem from missing observational data near the vent, where measurements are difficult due to safety concerns and topographical constraints. Field observations often fail to account for increased deposition near the vent, potentially leading to an underestimation of the actual deposit distribution in the D-F diagram. In contrast, numerical simulations incorporate data from near-vent regions, producing results that may differ from actual observations.
Beyond data limitations, Bursik et al. (1992) suggested that in large eruptions, particle recycling through entrainment can prevent efficient detachment from the eruption column. Since the TWiCE model does not account for this recycling effect, deposition near the vent may be overestimated, further contributing to the underestimation of the D value.
Additionally, increasing the "Plume Thickness" parameter led to higher D values, bringing them within the Plinian eruption range. Plume Thickness controls the vertical extent of the eruption column, with larger values indicating extended particle transport. Increasing Plume Thickness resulted in a smaller slope in the source magnitude distribution (SMD), suppressing particle settling, thereby increasing the D value and decreasing the F value. However, the required Plume Thickness values reached several kilometers, necessitating further investigation to assess their compatibility with actual plume heights.
This study quantitatively evaluated the effects of eruption rate and initial grain size distribution on ash dispersal using numerical simulations with TWiCE. The findings highlight the influence of Plume Thickness and missing observational data on the D value. Future research will focus on validating model outputs with observed plume thicknesses and refining simulation accuracy.