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A tephra transport and dispersion model considering volcanic ash fingers

Kosei Takishita 1, Yujiro J. Suzuki2, Takafumi Maruishi1

  • Affiliations: 1National Research Institute for Earth Science and Disaster Resilience, Tsukuba, Japan; 2Earthquake Research Institute, The University of Tokyo, Tokyo, Japan

  • Presentation type: Talk

  • Presentation time: Thursday 09:00 - 09:15, Room S160

  • Programme No: 3.12.3

  • Theme 3 > Session 12

Abstract

Gravitational instabilities driven by particle settling at the base of volcanic plumes generate highly concentrated currents of volcanic ash, known as "ash fingers" (e.g., Carazzo and Jellinek, 2013). When the falling velocity of ash fingers, vf, exceeds the terminal settling velocity of individual particles, vt, ash fingers enhance the ash sedimentation rate. To accurately reproduce and predict ash arrival times and sediment distributions, volcanic ash transport and dispersion models (VATDMs) must account for the finger effects (Takishita et al., 2024). In this study, we incorporated the influence of ash finger formation on particle velocities into a VATDM based on the FALL3D framework (Folch et al., 2020). The finger velocity, vf, was calculated as the function of the characteristic particle velocity and particle concentration using an empirical formula derived from laboratory experiments (Hoyal et al., 1999). The characteristic particle velocity was determined as the mass-weighted mean vt across all size bins. Particles were assumed to behave as part of ash fingers and settle at vf, when vf exceeds vt in each size bin (every 0.5 φ). We simulated the volcanic ash load distribution over a 15×15 km area for two Vulcanian eruptions with 2.5 and 4.7 km plume heights. Particles smaller than 3 φ were more efficiently sedimented by ash fingers compared to scenarios that ignored finger effects. Applying this model to eruptions of Sakurajima volcano, Japan, we discuss the impact of incorporating ash fingers on estimating the spatial distribution and arrival time of volcanic ash deposits.