A numerical model for simulation of tephra transport and deposition: Applications to May 18, 1980, Mount St. Helens eruption

Abstract

A numerical model for the computation of tephra fall accumulation resulting from Plinian or sub‐Plinian eruptions is presented. Mass accumulation at the ground level is found by solving a continuity equation which describes the transport of tephra in the atmosphere. The treatment incorporates horizontal advection due to wind, vertical gravitational settling, and dispersion due to atmospheric turbulence. Aspects of the parameters which enter the model, such as settling velocity, diffusion coefficients, deposition velocity, and source shape, are considered and discussed. Particular attention is devoted to settling velocity dependence on particle size and density, to deposition velocity behavior in weak and strong wind fields, and to eddy diffusion coefficient dependence on atmospheric conditions, particle size, and density. The results for some theoretical cases are presented to illustrate the behavior of the model. The validity of the model is tested by comparing observed and calculated deposits for the May 1980 Mount St. Helens eruption. The model predicts a double thickness maximum, the first near the volcano, the second at a distance of about 300 km, as was observed in the actual deposit.

A review of volcanic ash aggregation

“…In particular, in order to simulate the Campanian Y-5 ash layer, Cornell et al (1983) Armienti et al (1988) considered the same data and required particles smaller than 90 m to fall at speed of 0.55 m s -1 . However, Carey and Sigurdsson (1982) were not able to reproduce the deposition of fine ash observed in the proximal area (Hobbs et al, 1981;Carey and Sigurdsson, 1982).…”

“…These models include analytical solutions, used widely for investigations of particle sedimentation and for hazard assessments (e.g., Armienti et al, 1988;Bonadonna et al, 2005a;Bursik et al, 1992a and b;Connor et al, 2001;Connor and Connor, 2006;Glaze and Self, 1991;Hurst and Turner, 1999;Koyaguchi and Ohno, 2001a;Macedonio et al, 2005;Suzuki, 1983), and numerical models for real-time forecast of plume evolution and sedimentation (e.g., Costa et al, 2006;Searcy et al 1998). Both types have been validated with field data and are now used regularly for both these purposes.…”

An example of enhanced tephra deposition driven by topographically induced atmospheric turbulence

“…Suzuki 1983;Woods 1988;Bonadonna et al 1998), are widely used to predict patterns of tephra deposition or to estimate eruption parameters from deposit measurements (e.g. Armienti et al 1988;Connor et al 2001;Costa et al 2006). To improve the predictive capability of tephra dispersal models, it is important to capture the full spectrum of processes that characterise tephra sedimentation.…”

Volcanic eruption plume top topography and heights as determined from photoclinometric analysis of satellite data

“…Once an eruption column has reached its maximum height, the plume material may behave as a gravity current [Bursik et al, 1992] seeking its neutral buoyancy level, and subsequently be transported laterally by the prevailing winds [Armienti et al, 1988;Glaze and Self, 1991]. The morphology of the upper surface of the volcanic plume can provide clues needed to better understand the dynamics of the gravity current behavior as well as the influence of the ambient atmosphere on the plume transport.…”