Modelling volcanic ash clouds

Maptek Vulcan™ modelling and visualisation tools proved effective in studying the behaviour of volcanic ash clouds for improving hazard management.

Explosive volcanic eruptions cause local, regional and global hazards. Where the tephra settles during and after an eruption impacts aviation, agriculture and habitation.

By modelling ash clouds we can better understand their general anatomy and behaviour. Since eruption columns are opaque, we can’t see what is going on inside them. What are the global SO2 and CO2 loads, and how does this affect farming? Where is the greatest risk for aviation routes? These are all important questions.

Being able to predict where the ash goes during the first 8 hours after an eruption would allow information to be fed directly to the aviation industry for adjusting flight schedules. Emergency services could devise and implement evacuation plans. Advances in visualisation technology provide opportunities for better modelling, aiding communication and improving reaction time.

Standard integral models invoke a bullseye pattern - big ash inside, small ash outside. When affected by wind, the bullseye is stretched and becomes elongate.

Nomograms, graphical tables describing these ash deposition patterns using simple models, are widely used to interpret prehistoric eruptions. Unfortunately, the simple models used to build the nomograms are inadequate for bent, wind-affected plumes.

Recent advances in modelling employ multi-phase, physics-based simulations. ATHAM, the Active Tracer High-resolution Atmospheric Model originally from the labs of Graf and Herzog, uses computational fluid dynamics to model the eruption columnin 3D as a multiphase fluid flow, accounting for changing dynamics in a very complex system.

Our large study area 100 km in three dimensions meant radical filtering of the vast amount of data. ATHAM data was imported into Maptek Vulcan™ to take advantage of the 3D modelling and visualisation capabilities, and generate block models and grade shells.

Visualising the model in 3D is key to understanding what the data actually means.

Using Vulcan we combined files, filtering out zero concentrations to visualise a short event, allowing direct comparison with real eruption plumes. Showing how a plume behaves in windy conditions allowed us to talk meaningfully with risk mitigation experts.

To simulate the eruption with variable wind conditions takes hours to capture minutes of simulation. With more time, mappable regimes of flow can be generated. Cross-sections can be produced to show gradations of ash concentration.
Seeing the morphology and concentrations change over time provides valuable information on the amount of ash, where it is in the atmosphere, and therefore where flights can operate safely.

The modelled data can be merged with topography and hazard maps. We can easily show where accumulated tephra intersects with evacuation routes. Assessing risk factors and communicating this information clearly is vital for emergency crews and preparatory mitigation planning.

Thanks to Dr Shannon Nawotniak, Idaho State University
Presented at 2012 North America Users Conference



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On average, 20 volcanoes are erupting at any given time. More than 1500 volcanoes have been active in the past decade; 600 volcanoes have historic eruptions and 50-70 volcanoes erupt each year. Some volcanoes are active for many years with little impact, while others have short duration but very great impact.

Mt St Helens in the USA ejected 1km3 of material in 1980. Mt Pinatubo, in the Philippines in 1991, was 10 times the size of Mt St Helens in terms of ejected mass. Mt Tambora in Indonesia erupted in 1815, disgorging 160km3 of matter. By comparison, Yellowstone Caldera ‘supervolcano’ covered 30-50% of the United States with 1000km3 of ash 600,000 years ago.

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