Hurst, A.W. 1994 ASHFALL - a computer program for estimating volcanic ash fallout : report and users guide. Lower Hutt: Institute of Geological & Nuclear Sciences Institute of Geological & Nuclear Sciences science report 94/23 22 p.
Abstract: This program has been developed to enable the rapid calculation of the ashfall from a volcanic eruption. It is designed for Civil Defence use when it is feared that a damaging volcanic eruption might be about to occur, or if an eruption has actually started. It is also suitable for the estimation of volcanic hazards and for studies of past eruptions, by making it possible to calculate the ash distributions produced by a range of possible eruptions, under the various likely wind conditions. The program calculates the likely distribution of wind-borne ash. The intention is to give timely warning of areas likely to be affected by an eruption, so that if necessary these areas can be evacuated. It is assumed that the immediate vicinity of an erupting volcano will be evacuated in any case, so the local hazards from larger particles and bombs, i.e. anything greater than a centimetre in diameter, are not considered in this program. The program also does not consider the effect of either lahars or pyroclastic flows, which may travel considerable distances from large eruptions. Calculation of these hazards is affected by the detailed topography around the volcano, whereas ashfall calculations are little affected by topography. The information required about the eruption is the site and time of the eruption, and the height of the eruption cloud. The height is an important parameter, as it defines the extent to which ash will be spread, and also gives an indication of the rate at which the volcano is erupting. At the start of an eruption, an estimate based on eruption column height is likely to be the best available information on the eruption rate. Wind direction and speed information are also needed. Because upper atmosphere winds are often different from the low level winds, it is essential to get a complete profile of wind versus height up to the top of the eruption column. It is also desirable to have forecasts of expected changes in the wind pattern. The programs run on an IBM-compatible PC, and for reasonable results the minimum configuration is an 80386 with co-processor. A monochrome screen is adequate, although a colour screen makes it easier to distinguish the ash contours from other details. (auth.)