Backfires? Please explain.

Phil Pilgrim (PhiPi)

Okay, two of life's abiding mysteries (the automobile differential and, to a lesser extent, the flush toilet) I can grasp. But backfires (i.e. fighting fire with fire) still elude my understanding. I've been watching the news about the wildfires in Arizona and New Mexico, where firefighters are seen lighting fires to "burn off the fuel" in order to "starve" the wildfires. But isn't burning fuel what the wildfires do? And isn't a deliberately-set backfire just as likely to rage out of control as the fire you're supposed to be fighting? I don't get it. How can this possibly help? Somebody who understand this stuff: please fill me in.

Thanks,
-Phil

Ron Czapala

I believe the intent is to burn an area wide enough that when the approaching fire line reaches it, it burns out - provided it can't jump the gap to more combustible material.

Phil Pilgrim (PhiPi)

Ron Czapala wrote: »

I believe the intent is to burn an area wide enough that when the approaching fire line reaches it, it burns out - provided it can't jump the gap to more combustible material.

One side of me kinda gets that, but the other side goes, "Now wait a minute! If you light a fire in dry tinder close to the property you're trying to save, isn't the danger just as great as letting the main burn do the same thing? How is setting a fire any better?"

-Phil

Ron Czapala

I guess it is more of a controlled burn and is extinguished after the desired area is cleared.

http://www.wisegeek.com/in-firefighting-what-is-a-backfire.htm

A backfire or back burn is a fire which is set deliberately in the path of an oncoming fire. As the backfire burns, it consumes fuel, thereby depriving the primary fire of tinder when it reaches the site of the backfire. When the technique is executed correctly, it stops a wildfire in its tracks, or confines it, making it much easier to control. Many firefighters are trained in the art of calculating and placing a backfire when they are taught techniques for fighting wildfires.
The technique is one among an assortment of so-called “fire breaks,” all of which are intended to confine a large wildland fire. Various styles of firebreaks are used, ranging from backfires to simple trenches which are intended to arrest the primary fire. When the decision to set a backfire is placed, it is an acknowledgment that the primary fire is getting out of control, and that it needs to be arrested before it becomes significantly larger.

Phil Pilgrim (PhiPi)

Yes, but ... given that a wildfire can be started by a single carelessly-flicked cigarette or an incompletely extinguished campfire, how is a "controlled burn" even possible in such tinder-dry conditions? To me it's like saying, "We're going to flood this area to protect you from the encroaching floodwaters over yonder."

-Phil

kwinn

A large fire creates a large updraft. The surrounding air moves in towards the fire so the back fires move towards the wild fire. Of course you need to clear a small area of the highly flammable materials before starting the back fire and be prepared to put out any unwanted small fires.

Phil Pilgrim (PhiPi)

Thanks, kwinn. Now it's at least starting to make sense. I assume, though, that this technique is not possible when the prevailing wind countervails and exceeds that created by the updraft, right?

-Phil

localroger

The backfires are started small and kept small. They are totally controlled. They don't spread because they aren't raging wildfires; they are controlled burns done with full suppression gear on scene. The idea is that when the actual raging wildfire arrives, which is too large to control by any available technique, it will meet a lack of fuel. It's a proven technique and it works quite well when it's possible.

Zoot

I live in New Mexico where often the air is filled with smoke -- like a heavy fog, only it's like being right next to a campfire when the wind is in your face and you can't turn away from it.

Backfires and controlled burns are an art as much as a methodology. The former is usually set on one side of a fireline after a long swath of earth is cleared (by hand). Basically the fire teams hack a dirt road about twenty feet wide in a long line. Then they burn from the side of the "road" towards the oncoming wildfire. The idea is that the fire won't cross the line (no consumables) and the out of control aspect of the moving fire is reduced. When you hear that a fire is "contained" along with a percentage, it does not mean the fire is "out" by that much -- the fire is usually burning for weeks. It means that they have hacked fire lines along the entire perimeter of the wildfire so it's boxed in. Obviously, when you've got 120,000+ acres burning -- as we do now -- that is quite a feat.

Controlled burns are usually done off fire season -- the idea is to burn off the stuff when you are ready, and under ideal weather conditions (low temps, high humidity, no wind), so that a lightning strike, camp fire, downed power line, etc. won't set everything aflame during the Summer. Controlled burns are much trickier, and in fact, the last time that Los Alamos and points north burned (the Cerro Grande Fire) was due to a controlled burn that got out of control.

If you are really curious, here's some links to photos. We can see the plumes of smoke from the mountaintops when we drive around Albuquerque.

http://www.flickr.com/photos/bytegirl/5885934101/in/photostream
http://www.flickr.com/photos/pixiebat/5894269217/
http://www.flickr.com/photos/28731120@N05/5892715099/
http://www.flickr.com/photos/wakingsky/5892578803/
http://www.flickr.com/photos/wakingsky/5892578811/
http://www.abqjournal.com/pix/2011losconchas/483420mp062911k.jpg
the below is the fire as seen from the ISS
http://vallescaldera.com/wp-content/uploads/photos/LasConchasFireFromISS.jpg

Zoot

I assume, though, that this technique is not possible when the prevailing wind countervails and exceeds that created by the updraft, right?

If the fire is out of control and moving (esp. towards communities, infrastructure, etc), then they cut lines and hope for the best -- what happens when winds are high is that the fire "crowns" -- hot embers at the very top of the fire blow across the fireline and set fire to the tops of trees on the other side. Usually they try to cut the firelines where the vegetation and topography help prevent that, but only so much can be done. And naturally they try to use existing breaks wherever they can (forest roads, rivers/streams, paved state roads, etc).

Tubular

We had some bad fires here a couple of years ago. Thanks to heaps of media coverage, and a royal commission, we all know much more about the hope and hopelessness of controlling and measuring these fires.

One interesting thing - the bushfire intensity is measured in kilowatts per metre (across the fire front). The firefighters reckon they can control (have at least some impact on) fires up to 2500~4000 kW/metre. the black saturday fires peaked at about 100,000 kW/metre and with a firefront up to 100m tall. So if you can imagine a vertical stack of 100,000 bar radiator elements, stacked vertically, you're getting some feel for the intensity.

This is what 10,000 (10%) looks like... more from this page

Jimbo30

Beyond our realm of understanding. Just got to be black magic or something.

PJAllen

You're skepticism isn't unwarranted. Several years ago, Sandia was endangered as the result of a "controlled burn" that went awry.
The best laid schemes o' mice an' men. Gang aft agley, An' lea'e us nought but grief an' pain...