Description

In 2019 we described the development of a rule of thumb for estimating a wildfire’s forward rate of spread in cases when burning conditions are severe (i.e., namely when wind speeds are high and fuels are critically dry) and the time available to prepare a more exacting prediction is limited. The rule of thumb was based on data obtained from wildfire runs that had occurred around the world in forests (conifer- and eucalypt-dominated) and shrublands. The rule of thumb states that the rate of fire spread is considered to be roughly 10% of the prevailing 10-m (33-ft) open wind speed, independent of the unit system used. For example, given an open wind speed of 25 km/h (16 mi/h), the estimated wildfire spread rate would be approximately 2.5 km/h (1.6 mi/h).

In 2020 we undertook an evaluation study to analyze the predictive accuracy of the 10% rule of thumb based on independent datasets. Documented observations were obtained for 88 wildfire runs associated with strong wind speeds (i.e., >30 km/h or 19 mi/h) and low fine dead fuel moisture contents (i.e., <7%). The rates of fire spread and corresponding wind speeds ranged from 0.55-12.5 km/h (0.3-7.8 mi/h) and 30-100 km/h (19-62 mi/h), respectively. The results of the evaluation study have substantiated the strong control that wind speed exerts on the forward spread rate of wildfires when fuels are critically dry and winds are strong. These burning conditions produce the type of fires that typically surprise emergency response agencies and communities as a result of their fast spread rates and corresponding high fireline intensities. The 10% wind speed rule of thumb was shown to work at least as well as other operational fire spread models.