Briefing: Firefighters on the front lines of wildfires
By Peter Aldhous
The tragic deaths of 19 firefighters at Yarnell Hill in Arizona have put a sharp focus on the dangers faced by the teams deployed to fight the intense wildfires that ravage the western US each summer. New Scientist asks what can be done to keep them safer
How unusual is the Yarnell Hill tragedy?
Losing so many firefighters to a single wildfire is exceedingly rare. This is the biggest loss of life since 1933, when 29 firefighters perished in a blaze at Griffith Park in Los Angeles. But fatalities are common&colon; between 1990 and 2006, 310 wildland firefighters died in fire-related accidents. Of those, 64 perished after being trapped by the fire itself.
What happened in this case?
It seems from preliminary reports that the firefighters were engulfed by the advancing fire-front. Some managed to hide beneath personal “fire shelters” – which look like small tents – that each firefighter carries into the field, but these were unable to protect them.
So do we need to design better fire shelters?
That’s already happened once, in the late 1990s. Earlier shelters consisted of flame-retardant fibreglass bearing a single layer of aluminium foil, to reflect heat. But in experimental fires, this foil melted if it came into direct contact with flames for 10 seconds or so, says Bret Butler of the US Forest Service in Missoula, Montana, who led the research.
Today’s shelters have multiple layers of foil and flame-retardant fabric, and can provide protection for the couple of minutes it takes most wildfires to pass through. But if fires move more slowly, or are especially intense, they can fail. Adding further layers of protective material is not an option, says Butler, as the shelters would then become too heavy for firefighters to carry.
What other safety measures are available?
Shelters are intended only as a back-up for when things go very badly wrong. Firefighters’ primary safety strategy is to establish a safety zone away from flammable vegetation to which they can retreat if they are unable to control the flames – and to make sure that there’s a practical escape route to get there.
Misjudging escape-route viability has been a factor in previous tragedies, notably the deaths of 14 firefighters in the South Canyon Fire on Storm King Mountain, Colorado, in 1994. Their escape route led up a steep slope, and fire is known to move more rapidly uphill than on level ground – especially when fanned by strong winds. An official report into the incident concluded that shortly before the retreating firefighters were overrun, the fire was advancing at speeds approaching 3 metres per second.
But if firefighters get to their safety zones, they should be OK?
Not necessarily. The current rule of thumb, laid down in a document called the Incident Response Pocket Guide, is that safety zones should be established so that firefighters can maintain a distance between themselves and a fire of four times the flame height, which means finding or clearing a sufficiently large area free of flammable vegetation.
This is based on models of fire behaviour that assume the main factor involved is the spread of radiative heat. But Butler’s latest, unpublished research shows that convection – the transfer of heat via the movement of air – poses a significant additional risk. While we usually think of convection in terms of air rising vertically, in wildfires convective flows can also hug the ground, especially on slopes. “There’s a lot of flow horizontally,” Butler says.
Butler is now devising new guidelines for the pocket guide that will ask firefighters to account for factors like slope and vegetation type when calculating the size of their safety zones. “I’m trying to come up with a system that firefighters can use simply and easily,” he says. One idea is a smartphone app to make the necessary calculations.
What lessons will be drawn from the latest tragedy?
That may depend on the scope of the official investigations. One prominent critic of inquiries into previous tragedies is Ted Putnam, a psychologist formerly with the US Forest Service who also served as a “smokejumper” – the elite firefighters who parachute in to combat major wildfires.
Putnam argues that there is typically too much focus on factors such as wind and topography, and too little on errors in decision-making by the people involved. He suggests that human-factors experts need to conduct individual interviews with survivors from the field, and those in their chain of command, to see if errors were made – and how they might be avoided in future.
“If you don’t send in the right people, and send them immediately, you lose information,” Putnam warns.