Estimating how quickly a wildfire can spread is crucial for effective emergency response and community safety. When dealing with severe burning conditions – high winds and critically dry fuels – a simple rule of thumb can provide a rapid initial assessment. This article explores the “10% rule,” which suggests that a wildfire’s forward spread rate is roughly 10% of the prevailing open wind speed. We’ll delve into the science behind this rule and its accuracy in predicting wildfire behavior.
Wildfires pose significant threats, especially when fueled by strong winds. Understanding How Fast Can Fire Travel With Wind is essential for mitigating their impact. Let’s examine the factors influencing wildfire spread and the effectiveness of simplified prediction methods.
Evaluating the 10% Rule: An Overview
The 10% rule of thumb, as described by Alexander and Cruz (2019), offers a quick way to estimate a wildfire’s forward spread rate during severe burning conditions. This rule suggests that the rate of spread is approximately 10% of the open wind speed, regardless of the unit system used. For example, with a wind speed of 40 km/h (25 mi/h), the estimated wildfire spread rate would be about 4 km/h (2.5 mi/h).
This estimation method is particularly valuable when time is limited, and a more detailed prediction is not immediately feasible.
Data Sources for Analysis
To assess the predictive accuracy of the 10% rule, researchers analyzed data from two independent wildfire databases, encompassing a total of 350 wildfire runs. The analysis focused on wildfires spreading under strong wind speeds (>30 km/h or 19 mi/h) and low fine dead fuel moisture contents (<7%). After applying these criteria, the dataset was reduced to 88 fire runs.
- Australian Eucalypt Forests: 30 runs from native eucalypt forests in southern Australia. Fire spread rates ranged from 0.8-8.0 km/h (0.5-5.0 mi/h) with corresponding wind speeds of 30-100 km/h (19-62 mi/h).
- BONFIRE Global Fire Database: 58 fire runs in shrublands, eucalypt forests, and conifer forests. Fire spread rates varied from 0.55-12.5 km/h (0.3-7.8 mi/h), with wind speeds between 30-80 km/h (19-50 mi/h).
Key Findings on Wildfire Spread Rate and Wind Speed
The evaluation of the 10% rule against these wildfire datasets revealed several important insights:
- Comparable Accuracy: The rule’s predictive accuracy is on par with other empirical fire spread models used in wildfire studies. No significant differences were observed in error trends across shrublands, conifer forests, and eucalypt forests.
- Over-Prediction Bias: The rule tends to overestimate spread rates for fires moving slower than 2.0 km/h (1.2 mi/h). This bias was also noted in the original analysis that led to the rule’s development.
- Optimal Performance: The 10% rule works best for wildfires spreading at rates exceeding 2.0 km/h (1.2 mi/h). Most fires in this category were predicted within a ±35% error band. Prediction error, as a percentage of the observed spread rate, decreased as the fire spread rate increased.
Rognac Fire in France
Image showing the Rognac Fire’s rapid spread through shrubland fuels, highlighting the influence of wind on wildfire propagation.
The Dominant Role of Wind Speed in Wildfire Behavior
The study by Cruz et al. (2020) underscores the significant influence of wind speed on wildfire forward spread, especially when fuels are critically dry. Under these conditions, wildfires can rapidly intensify, posing unexpected challenges for emergency responders and communities.
Strong winds cause the convective plume to tilt, potentially reducing interactions between the flame front and the plume. This decoupling can decrease uncertainty associated with surface weather conditions. This highlights how fast can fire travel with wind under specific environmental conditions.
Limitations and Appropriate Use of the 10% Rule
The 10% rule is intended for quick, initial estimates of wildfire spread rates when more comprehensive methods are impractical. Its application is most effective in dry, windy conditions associated with fast-spreading wildfires. However, it can significantly overestimate spread rates when wind speeds are low to moderate (<20 km/h or 12 mi/h) and fine dead fuel moisture content is high (>7%).
Error variation with observed fire spread rate
Graph illustrating the relationship between error percentage and observed fire spread rate, showing decreased error at higher spread rates when using the 10% rule of thumb.
Conclusion: Wildfire Dynamics and Prediction
Understanding how fast can fire travel with wind is crucial for predicting wildfire behavior and mitigating its impact. The 10% rule provides a valuable, albeit simplified, method for estimating wildfire spread rates in severe burning conditions. While it has limitations, particularly in low-wind situations, it serves as a useful tool for initial assessments. Emergency response agencies can leverage this rule to quickly gauge potential fire behavior, aiding in timely decision-making and resource allocation. Further research and refined models will continue to improve our ability to predict and manage wildfires effectively.
References
Alexander, M.E. and M.G. Cruz. 2019. A rule of thumb for estimating a wildfire’s forward spread rate. Wildfire 28(5): 36-39.
Cruz, M.G. and M.E. Alexander. 2013. Uncertainty associated with model predictions of surface and crown fire rates of spread. Environmental Modelling & Software 47: 16-28.
Cruz, M.G. and M.E. Alexander. 2019. The 10% wind speed rule of thumb for estimating a wildfire’s forward rate of spread in forests and shrublands. Annals of Forest Science 76(2): 44. 11 p.
Cruz, M.G., M.E. Alexander, P.M. Fernandes, M. Kilinc and Â. Sil. 2020. Evaluating the 10% wind speed rule of thumb for estimating a wildfire’s forward rate of spread against an extensive independent set of observations. Environmental Modelling & Software 133: 104818. 15 p.
Fernandes, P.M., Â. Sil, C.G. Rossa, D. Ascoli, M.G. Cruz and M.E. Alexander. 2020. Characterizing fire behavior across the globe. In: Hood, S., S. Drury, T. Steelman and R. Steffens (Editors), The Fire Continuum – Preparing for the Future of Wildland Fire: Proceedings of the Fire Continuum Conference, 2018 May 21-24, Missoula, MT. Proceedings RMRS-P-78. Fort Collins, CO: US Department of Agriculture, Forest Service, Rocky Mountain Research Station. pp. 258-263.
Harris, S., W. Anderson, M. Kilinc and L. Fogarty. 2011. Establishing a link between the power of fire and community loss: the first step towards developing a bushfire severity scale. Report 89. Melbourne, VIC: Victorian Government, Department of Sustainability and Environment. 75 p.
Kilinc, M., W. Anderson and B. Price. 2012. The applicability of bushfire behaviour models in Australia. Technical Report 1. Melbourne, VIC: Victorian Government, Department of Sustainability and Environment, DSE Schedule 5: Fire Severity Rating Project. 60 p.
