On the fire line, wildland firefighters are required to have a safety zone: an area of refuge that offers safety without the use of fire shelters. The current guideline recommends a safety zone be large enough to provide a separation distance from the flames that is four times the flame height. However, this guideline doesn’t account for the interactions of topography, convection (wind), and radiation, and their combined effects upon wildfire behavior. It’s a limitation that Bret Butler, a Mechanical Engineer with the U.S. Forest Service Rocky Mountain Research Station (RMRS), recognized when the guideline was proposed in 1998. He was a member of the team who developed the current safety zone rule of thumb that was initiated by their analysis of 1994’s South Canyon Creek fire that claimed the lives of 14 wildland firefighters.
“The analysis behind the guideline only considered radiation as causing injuries, not convection because convection isn’t critical unless you’re really close to a flame,” Butler explained. “Where the guideline fails is when a firefighter’s safety zone is on a slope above the wildfire since hot gases flow along the ground ahead of the flames or when the safety zone is downwind of the fire.”
Twenty years later, Butler and his colleagues have now accumulated the knowledge needed to quantify the interactions of slope and convection, as well as wind speed and fuel type on fire behavior. The team collected data in diverse vegetation, weather, and geographical settings. Out in the field, sensors were deployed in the paths of wildfires. At the Insurance Institute for Business & Home Safety (IBHS) Research Center in South Carolina, which is a specially designed research facility where full-size houses are subjected to hazardous conditions such as 140-mile-hour windstorms and wildfire ember storms, Butler burned pine needle beds at windspeeds exceeding 10 miles an hour to study how fire reacted to increased wind speeds. This type of observation hadn’t previously been possible in a laboratory setting.
From the Wildland Fire Lessons Learned Center Incident Review database, Butler’s colleague Wesley Page, who is also affiliated with the RMRS, analyzed 65 wildland firefighter entrapments, which occurred between 1937 and 2015, to determine if there was a relationship between the weather, fuel type, topography, and the use of a fire shelter, and resulting injuries or fatalities.
From these data, Butler and Page developed a statistical model that calculated a firefighter’s safety zone based upon fuel type, landscape topography, or weather patterns, and probability of fatal injuries. Depending upon the fuel type, the model proposed safety zones ranging from twice to up to 16 times greater than the current guidelines to reduce the probability of a fatal injury in the event a firefighter doesn’t have use of a fire shelter. “However, in many cases where you have fires burning under substantial winds and around slopes safety zones are non-existent,” Butler explained. Steep slopes also appeared to reduce the effectiveness of fire shelters in preventing injuries.
Just in time for the upcoming fire season is the Wildfire Safety Evaluator (WiSE), an app that Technosylva developed in collaboration with Butler based upon his safety zone model. The app is available through the Google Play store and soon through Apple app store, which means firefighters can determine their safety zone in real time using their smart phone or other portable device while on the fire line.
Butler is already planning the next iteration that will include a crucial yet overlooked safety detail of safety zones: escape route travel times. “We can’t consider safety zones without considering the escape routes that firefighters take to reach their safety zone,” he cautioned.
Butler, Bret W. 2014. Wildland firefighter safety zones: A review of past science and summary of future needs. International Journal of Wildland Fire. 23(3): 295-308.
Campbell, Michael J.; Dennison, Philip E.; Butler, Bret W. 2017. A LiDAR-based analysis of the effects of slope, vegetation density, and ground surface roughness on travel rates for wildland firefighter escape route mapping. International Journal of Wildland Fire.