Initial growth of fires in eucalypt litter, from ignition to steady-state rate of spread: laboratory studies

Abstract

As part of an investigation of wildfire growth and acceleration, the initial growth of incipient fires burning in uniform dry eucalypt forest (Eucalyptus rossii, E. macrorhyncha) litter fuel of 1.2 kg m−2 was studied in a combustion wind tunnel with a fuel bed width of 1.5 m. Fifty-eight fires of three ignition patterns (point, 400-mm line and 800-mm line) were carried out at two air speeds (1.25 and 2.0 m s−1) and two dead fuel moisture content (FMC) groups for each air speed (≤7.5% and >7.5% oven-dry weight for the low air speed and ≤5% and >5% for the high air speed). The fraction of steady-state rate of spread reached as a function of time was determined and fitted to two theoretical fire growth models from the literature. The best model suggests the times for a point ignition fire to reach steady-state spread rate were ~38 and 50 min under the higher FMC for 2.0 m s−1 and 1.25 m s−1 air speeds, respectively, and ~25 min under the lower FMC for 2.0 m s−1. Future work will extend these results to field-scale fire behaviour, which will help improve operational response to wildfire outbreaks and planning of ignition patterns for prescribed burning.