Every summer, firefighters find themselves in the grip of dangerous forest fires. The consequences of these episodes are often tragic for fire fighters despite increasingly efficient equipment.
All countries where firefighters are fighting violent fires are concerned. This was particularly the case in the United States, where two major accidents – those of Mann Gulch and South Canyon – refer.
In the 1940s, the units of smoke jumpers are created. These extreme parachutists are considered the elite but, in 1949 in Mann Gulch (Montana), only three of them (out of about twenty) will survive a fire that suddenly ignites an entire valley. In 1994, a similar accident killed 14 firefighters in the South Canyon (Colorado). In these two cases, the victims intervened on foot, in steep and inaccessible terrain.
In France, the Mediterranean and Landes massifs are crisscrossed with trails: here, firefighters generally intervene on board fire vehicles. But accidents remain just as frequent and serious. There are thus more than 80 firefighters trapped since the 1980s. A third of them are dead. The others survived, but with serious sequelae, most often burns to the face and hands.
To better understand these accidents, we have analyzed various episodes that have occurred throughout the world, particularly in Europe and Australia where fire fighting methods are similar.
Increasingly efficient equipment
For fire fighters, the danger comes from both heat and smoke.
In drought conditions, the flames can indeed reach more than 10 meters in height in the forest. And certain plants, such as scrubland shrubs, release highly toxic gases when burned.
To deal with this, firefighters now have high-performance equipment. Their vehicles are protected by automatic spraying of the cabin and tires. Some trucks also have overpressure to prevent fumes from entering the cabin. The outfits have also evolved thanks to textiles which provide better thermal protection. Finally, the procedures and transmissions make it possible to quickly locate a crew in difficulty.
But, despite all these innovations, the number of accidents has not diminished. How to explain this situation ?
To do this, it is necessary to distinguish between the configurations that are particularly dangerous for the firefighters according to their position vis-à-vis the flames (in front, on the sides or at a distance).
Devastating fires are often guided by the wind, which gives them a main direction. It is therefore dangerous to be on this trajectory. This is what can happen to rescuers in particular when they seek to protect dwellings located in the axis of the fire. In the Mediterranean, the urban sprawl at the edge of the forest increases this vulnerability.
In Australia, the weather depends on the meeting between the oceanic air mass and that of the continental desert. Their displacement leads to wind shifts in a few minutes. The edges of fire are then reactivated for kilometers. These sudden changes in direction are the main cause of accidents. In Europe, the shifts are less brutal because of the mountain ranges which channel the winds.
The question of relief
If the relief makes it possible to contain the winds, it can also generate dangerous situations. There is thus a natural tendency of the fire to accelerate when it climbs a slope. Studies of modelization show for example that beyond 25 degrees (it is, for reference, the limit of what an all-terrain vehicle can cross), the flames stick to the ground. A truly eruption can then cover several hectares in a few seconds.
This does not mean that one is safe on a downward slope. Behind a ridge line, eddies of air, called vortex, can for example spread the fire laterally. The fire then rolls up, deviating from its initial trajectory. 21 cases of vehicle bombs under these conditions have recently been identified throughout the world.
Finally, under certain conditions, firefighters can be surrounded by flames hundreds of meters from the main source. This is the case during a heat wave after a prolonged drought; the fires become convective. The wind is no longer necessary. The spread is erratic and produces long distance swings. This summer's climatic conditions pose this risk in several mediterranean countries, including in France.
In extreme cases, the energy developed by the fire is such that it generates a pyrocumulus cloud. This storm cloud rises to the top of the troposphere, around 10 meters above sea level. It is composed of glowing particles and hot gases. When these elements come down, sometimes from miles away, it's a real firestorm that hits.
This phenomenon is common in Australia. In Europe, global warming increases the risk and a cloud of this type was, for example, observed by satellite above Portugal in June 2017.
Although today's firefighters' equipment is close to the optimum level of protection, other developments are still possible. They consist in better understanding the atypical spreads of fires; to train firefighters to recognize and anticipate these phenomena; to accept that firefighters stay out of scrubland and forests in extreme situations.
This last point, however, requires better preparation of vulnerable dwellings. the clearing is a regulatory obligation which contributes to this in France. Other countries go further and encourage each household to develop a backup plan. Beyond firefighters, all residents living in high-risk areas have their role to play.
Sebastien lahaye, Safe Cluster Project Coordinator (France), School of Advanced Studies (EPHE)
This article is republished from The Conversation under Creative Commons license. Read theoriginal article.