The effect of the aerodynamic behaviour of flakes of jarrah and karri bark on their potential as firebrands.

Spotting, the process by which new fires are ignited ahead of bushfires by firebrands transported by convection and wind, is a significant problem for fire suppression, and potentially, for fire crew safety. The magnitude of the potential problems caused by spotting is determined by many factors, notably spotting distance and spotfire numbers. This paper explains the notoriety of two Western Australian forest eucalypts, jarrah (E. marginata) and karri (E. diversicolor), in terms of bark aerodynamic characteristics and likely firebrand yield. Terminal velocity, the equilibrium falling velocity, and potentially, gliding behaviour, determine how high a particle is likely to be lofted for given convection strength, and how far it will travel for a given height and wind conditions. Particles with low terminal velocities can potentially be lofted to greater heights and transported longer distances than those with greater terminal velocities. The gliding and spin behaviour of shed flakes of bark were observed during tower drops, and their terminal velocities derived from fall time. Terminal velocity varied between 2.5 and 8 m s and is shown to be a function of the square root of surface density (mass/projected area) of the sample, the amount of spin during free-fall, and bark shape. Bark flakes which showed rapid spin had terminal velocities up to 18% less than those of non-spinning flakes. The measurements indicate that many of these flakes could be lofted in the convection plumes of low to medium-intensity fires, such as those with fire-front intensities between 0.5 and 2.5 MW m. Aerodynamic characteristics which would make these bark flakes effective firebrands appear to be their low terminal velocities, rather than their ability to glide. Observed differences in spotting behaviour between the two species are their spotting densities and maximum spotting distances. These differences are not wholly explained by their measured differences in free-fall behaviour, but will more completely be explained by differences in the numbers of detachable flakes, their ease of ignition and their combustion characteristics during flight.