KILLER DUST: ASH FALL MODELLING AND HAZARDS
This program has been developed to enable the rapid calculation of the ash fall from a volcanic eruption. It is designed for Civil defense used when it is feared that a damaging volcanic eruption might be about to occur, or if an eruption has actually started. The information required about the eruption is the site, time of the eruption, and the height of the eruption cloud.
This program ASHFALL, which calculates ash thicknesses for a given volcanic eruption and wind conditions, was used in the calculation of ash fall on mount versuvius (79 eruption of Vesuvius). The program requires large 3-dimensional arrays, and multi-hour runs on a VAX computer for each particle size.
- The program starts with a volcanic eruption column that produces a certain total weight of ash.
- An eruption column contains turbulent hot gases, moving much faster than the settling velocities of the particles being considered here, so that the ash will not settle out until it leaves the column.
- The heat of the gases makes them buoyant, and their initial up wards velocity is aided by their buoyancy until they reach an altitude (Zbuoy), at which their density equals that of the surrounding air.
- Above this height the gases are heavier than the surrounding air, so they lose momentum and the upward velocity decreases, becoming zero at the top of the column.
- The maximum concentration of volcanic gas and ash is at Zbuoy
- Given the total volume (V) of the eruption, and the height of the eruption cloud, the ash distribution with height (V'(z)), is assumed to follow the Suzuki distribution.
- where Z is the height of the eruption cloud, and
- k is a constant of integration, a function of A
The Suzuki constant A defines the relationship between Z and Zbuoy. A typical value of 5(Macedonio et al., 1988) means that Zbuoy= 0.8Z, i.e. the highest ash concentration is found at 80% of Z.
The amount of ash leaving the column at any height is assumed to be proportional to the concentration at that height. This is the case if the eruption column continues for a considerable period, or decays slowly. From the ash distribution in the column for each eruptive event, the ashfall distribution is obtained by tracing how the ash falling out of the column is influenced by wind.
The program ASHFALL.EXE calculates the expected thickness of ashfall as a function of position. The ash distribution depends on both volcanic and Meteorological factors.
- Eruption column position and height: If a volcanic eruption is occurring, the position and height of the eruption column are the parameters which can most easily be observed, either from the ground, from aircraft, or from satellites. Once an eruption column has developed it may be difficult to see its maximum height from the ground.
- Total erupted mass: It has been observed that the column height (Z) is positively correlated with both the rate of eruption, and the total size of the eruption. Carey & Sigurdsson (1989) suggested that for Plinian eruptions of a wide range of sizes, there was a close relationship between the maximum sustained height of the eruption column and the total erupted mass (M). Their relationship was
Ash Size Distribution: The ash size distribution is not likely to be known until the ash has landed, which is obviously too late to be useful. Therefore in any practical case, an ash size distribution derived from previous eruptions of the same volcano, or other eruptions with similar characteristics, will have to be Once ash particles leave the eruption column and start falling, their settling velocity will very soon reach the terminal velocity, at which the drag force equals the gravitational force. The terminal velocity of ash particles increases with particle size and density, but cannot be calculated by a simple formula in the range of interest for volcanic ash studies.
Wind direction & velocity: The initial estimates of meteorological conditions would normally be obtained from the New Zealand Meteorological Service ionosondes. Currently the upper wind profiles are measured daily at Kaitaia, New Plymouth, Gisborne, and Paraparaumu.
These profiles generally cover heights up to about 20 km. In an emergency situation, it is likely that doppler radar systems for wind measurement could be deployed near the area of the eruption to provide better local wind information.
ASH FALL HAZARDS
Ash falls are the most widespread of all volcanic hazards – may cover 1000-10,000 Km2 with >10cm of ash during a large eruption. Fine ash may spread across continents (Mt St. Helens, 1980) or even encircle the globe (Pinatubo, 1991). Hazards are closest to the vent, decreasing with increasing distance.
- About 11,000 deaths have been attributed to ash fall since 1600 A.D. (probably an overestimate). Most caused by collapsing roofs due to weight of ash (worse if wet).
- Damages or kills crops (even a few cm).
- Causes respiratory problems.
- Pollutes water supplies and drainage.
- Disrupts transportation and lifestyle.
- Can poison grazing animals (fluorosis).
- Damage aircraft, with the potential to cause fatal crashes.