Grassfires in Canada’s western prairies have been observed to propagate over multi-hour periods with sustained rates of spread in excess of 6.0 km/h. For example, on December 14, 1997, a human-caused grassfire starting west of Claresholm in southern Alberta advanced eastward some 35 km in the span of about four hours before being held up at Highway 2 near the town of Granum.
As a result of their rapid spread rate potential, grassfires advancing under extreme burning conditions (e.g., degree of curing >90%, fuel moisture content <6%) pose a serious threat to both firefighters and members of the general public in rural areas at certain times of the year. The formal capacity to predict fire spread in such situations e.g., a fire behaviour analyst or FBAN with REDapp or Prometheus software; fire suppression personnel with an FBP System “Red Book” or the Grassland Fire Behavior Pocket Card) and in turn issue timely emergency warnings may not be readily available more often than not.
Two Australian colleagues and I recently developed a simple rule of thumb for estimating the forward spread rate of wildfires in cured grasslands (Cruz, M.G.; Alexander, M.E.; Kilinc, M. 2022. Wildfire rates of spread in grasslands under critical burning conditions. Fire 5, 55) based on an analysis of a relatively large dataset (n = 58) of wildfire observations involving spread rates and 10-m open wind speeds ranging, respectively, from 1.6 to 17 km/h and 20 to 62 km/h.
The rule of thumb can be expressed as an equation:
Head Fire Rate of Spread ≈ 20% of the Average 10-m Open Wind Speed
For example, for 10-m open winds of 40 km/h, a fire’s rate of advance would be approximately 8 km/h.
Assumptions, limitations and potential uses of the rule of thumb will all be discussed.