Understanding Critical Conditions for Wind-Driven Fire Spread Over a Fuel Gap*

As wildfires increase in size and frequency, accurate predictions of fire behavior are needed to protect the lives of firefighters and mitigate losses in the wildland-urban interface (communities bordering natural lands). When responding to wildfires, firefighter safety zones are needed to provide a "fuel gap," ensuring safer operating conditions for firefighters in the event of unexpected fire behavior changes. Determining the appropriate size of these gaps to prevent injury under varying wind, fuel, and slope conditions is difficult because the relationship between heating and ignition is nonlinear and not well characterized. In this study, a series of spreading fire experiments were performed and analyzed at the USFS Missoula Fire Sciences Laboratory (wind speeds 0.5 - 1 m/s) and at the Insurance Institute of Business & Home Safety (wind speeds 6 - 9 m/s) to understand the critical conditions necessary for a spreading fire to "jump" across a fuel gap. These critical conditions are found to vary with wind speed and fuel properties, which together affect the size, geometry, and downstream heating of the fire approaching the gap. A scaling analysis based on the balance between momentum forces from the wind and buoyant forces from the flame described changes in flame geometry and downstream heating. This is correlated with the probability of ignition downstream. The coupling between fluid dynamics ahead of the flame and downstream ignition is further investigated and discussed.