Long-distance spotting potential of bark strips of a ribbon gum (Eucalyptus viminalis)

Firebrands of ribbon bark eucalypt are notorious for igniting spotfires many kilometres ahead of a bushfire. However, no research to date has demonstrated that this bark type can sustain combustion at its terminal velocity for the travel time required. Fifty samples of shed bark of Eucalyptus viminalis of three distinct morphologies were ignited at one end and burned tethered in a vertical wind tunnel at air velocities approximating their terminal velocity. Mean terminal velocity and burnout time for ‘flat plates’, ‘simple cylinders’ and ‘internally convoluted cylinders’ were 5.4 m s–1 and 251 s; 5.2 m s–1 and 122 s; and 5.8 m s–1 and 429 s. The corresponding maximum burnout times were 785 s, 353 s and 1304 s. One internally convoluted cylinder flamed continuously and consumed its length of 368 mm in 271 s. The maximum burnout time for the internally convoluted cylinders is commensurate with a potential spotting distance exceeding 20 km given a mean wind speed during transport of 60 km h–1. This is the first study in which combustion times exceeding a few minutes have been recorded for this bark morphology, and thus provides some corroboration of the notoriety for long-distance spotting.