Firebrand characteristics of the stringy bark of messmate (Eucalyptus obliqua) investigated using non-tethered samples

Abstract

The fragile, fibrous bark of messmate (Eucalyptus obliqua) has been implicated in Victorian bushfires in which significant loss of life and property occurred. The airflow of a vertical wind tunnel was modified in order to allow the observation of non-tethered samples. The duration of flaming combustion (flameout time) and total combustion time during flight (burnout time), and mass at burnout, were measured for 192 samples between 0.4 and 8.3 g, and impact with the walls subjectively categorised. Of 146 samples not significantly affected by impact, 55 had flameout times between 30 and 70 s (maximum 75 s) and 131 had burnout times ≤180 s (maximum 420 s). Eleven samples had significantly longer burnout times apparently because combustion occurred internally or from one end, and 18 reflamed during their latter, glowing phase, one more than 4 min after ignition. This in-flight combustion behaviour, and the quantities of loosely attached, weathered flakes on long-unburnt trunks, is consistent with the species notoriety for intense spotting to distances of a few kilometres. Modification of wind tunnel airflow in order to study non-tethered firebrands and sample reflaming during flight has not been previously reported.