2015 WOR Day 2: New Research on Safety Zones

Week of Remembrance June 30-July 6, 2015

First, a Fire Behavior 101 refresher:  You can warm yourself around the sides of a campfire for quite some time; that’s radiant heat.  If you hold your hands over the top of the fire, you’ll get burned relatively quickly; that’s convective heat.

Basically, wind or slope can tip the flames over, so that the convective heat is no longer going straight up, but is now aimed more along the ground, sending the heat and hot gasses much further ahead.  This causes pre heating of the fuels, faster fire spread and greater fire intensities.  You’ll need a larger Safety Zone if that fire is coming towards you.

The current equation for safety zone size in the IRPG (page 8) is:

4 x Flame Height = Safe Separation Distance

To make estimations of flame height though, you either have to use past fire behavior observations or use your experience to guess what the fire may do in the future.  After a decade of research, Bret Butler, at the Missoula Technology and Development Center, suggests removing the uncertainty and guesswork that comes with estimating flame height by taking the general rule of thumb: Flame Height = 2 x Vegetation Height

…and substituting that Flame Height equation into the original IRPG equation, to give:

4 x 2 x Vegetation Height = Safe Separation Distance, which simplified is:

8 x Vegetation Height = Safe Separation Distance

But remember, that’s still for radiant heat only, on flat ground, with no wind. To take into account the convective heat from slope or wind, Butler’s research suggests that a “Slope Wind Factor” is needed in the equation:

8 x Vegetation Height x Slope Wind Factor = Safe Separation Distance

But what is the Slope Wind Factor? Current research is indicating that the Slope Wind Factor is between 1 and 10; with Butler arguing it may be closer to between 1 and 5. Butler’s ongoing research is focused on answering that question by gathering sensor data on fires, running computer simulations, and refining the models…Stay tuned.

In the meantime, utilize the calculations on page 8 of your IRPG to help you determine a bare minimum size for your safety zone with the understanding that slope and wind need to be considered in your decision making.

But remember, a safety zone is only good if you can get there…join us tomorrow for some thoughts on Escape Routes.

The “Take 5@2” safety messages are a cooperative project of 6 Minutes for Safety, the Wildland Fire Safety Training Annual Refresher (WFSTAR), NIFC External Affairs, the NWCG Leadership Committee, the NWCG Risk Management Committee, and the Wildland Fire Lessons Learned Center.