The 10% wind speed rule of thumb for estimating a wildfire’s forward rate of spread in forests and shrublands

Abstract

Key message: The collective analysis of a relatively large number of wildfire observations documented in conifer forests, dry eucalypt forests and temperate shrublands revealed that the forward rate of fire spread is roughly 10% of the average 10-m open wind speed, provided both are expressed in the same units (e.g. km h−1). Context: Knowledge of a wildfire’s forward spread rate is a prerequisite for defining adequate fire suppression strategies and to ensure timely public warnings. Aims: We wanted to investigate the possibility that a simple relationship exists that could be used as a first approximation for quickly estimating a wildfire’s spread rate simply from the open wind speed alone. Methods: We analysed data from a number of high-intensity wildfire observations (n = 118) documented in temperate shrublands, Australian dry eucalypt forests and North American conifer forests to examine the suitability and soundness of a relationship between wind speed and rate of fire spread. We also contrasted the performance of the best function against established fire spread rate models for the three fuel types. Results: The resulting rule of thumb is that the forward rate of spread of wildfires burning in forests and shrublands in relatively dry conditions is approximately equal to 10% of the average 10-m open wind speed, where both values are expressed in the same units. Conclusion: The rule of thumb gives the most accurate results for dry fuel and high wind speed conditions with reduced bias and mean relative errors lower than 50%. Under these conditions, the error statistics are comparable to those obtained by the established fire spread rate models. The rule is not applicable to grasslands.