(NOTE: the information below, a new way to determine the size of a wildland firefighter’s safety zone, was revised again in July, 2014. Read the new guidelines HERE, and do not use the method below.)
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New guidelines are being proposed for the safety zones where wildland firefighters may be forced to take refuge from an approaching wildfire. In a safety zone a firefighter should be able to survive without being injured from exposure to the radiant and convective heat from the fire, and would not have to deploy and enter a fire shelter.
Bret Butler, who works in the Fire Behavior Research Work Unit at the Fire Sciences Laboratory in Missoula, Montana, has been working with this subject for years and continues to fine tune the data through additional research. The previous recommendations about the size of a safety zone, or the safe separation distance between the firefighter and the fire, were based on flame height.
At the Large Fire Conference in Missoula this week he unveiled new proposed guidelines for the Safe Separation Distance between personnel and the approaching fire. Instead of the flame height, the system uses a constant number (eight), a value from a table, and the height of the vegetation that will be burning. The calculation goes like this:
Example: if the wind is 10, the slope is 15%, and fire intensity is Medium, that gives you a “2”. Plugging that in to the formula, and assuming the height of the vegetation is 50 feet:
8 x 2 x 50 = 800 feet Safe Separation Distance
While this recommended system is still considered preliminary, Bret suggests that it be used this fire season. He said he anticipates that further information will refine the rule, but is confident that this is better than the simple rule currently in place, which is based on flame height.
(NOTE: if you want a copy of the table above, click on it to open it in a window of its own, then click on Print in your internet browser.
How many people does this Safety zone calculation account for?
David, it is my understanding that the calculation tells you the recommended separation distance between firefighters and the fire. As long as all firefighters are no closer than that distance, there should be no limit on the number of firefighters.
Good reminder. I wonder if it would work as well if we only used the High Intensity numbers. It seems as if you rarely have to get into a safety zone until the fire gets to that range. I don’t plan for safety zones as a best case location 😉
So did he define what low thru high fire intensity was?