Phil Dennison of the University of Utah and Bret Butler at the Missoula Fire Lab are working on ways to better characterize safety zones designated by firefighting crews. There is currently very little information available about how safety zones are actually used in practice. The researchers want to determine if current guidelines are practical when implemented by firefighters in the field.
Mr. Dennison and Mr. Butler have developed a form they would like wildfire personnel to complete this summer when they designate an area to be a safety zone. You can download it here (74k pdf file).
The calculation of the size of a safety zone is somewhat complex for a firefighter in the heat of battle, and these various guidelines can only be used if the firefighter on the ground is carrying the latest written directions about how to do the math. While it is laudable that researchers are working to improve the safety zone guidelines, changing them every two to four months is too confusing.
In the video (webinar) below the new revision is discussed in detail in the one hour and 15 minute presentation, including questions. (A three-minute executive summary version would be very much appreciated.) This new November, 2014 version of the “Preliminary Proposed Safety Zone Rule” appears at 44:00. The fact that it is called both preliminary and proposed leads us to believe there will be still more changes in the near future.
Below is the description of the December 2, 2014 webinar, presented by Mr. Butler.
Current safety zone guidelines for wildland firefighters are based on the assumption of flat ground, no wind, and radiative heating only. Recent measurements in grass, shrub and crown fires indicate that convective heating can be significant especially when wind or slope are present. Measurements and computer modeling supports this finding and suggests that convective energy transport should be considered when assessing safety zone effectiveness any time wind or slope is present. The results of the research are presented along with recommendations for modifications to current safety zone guides.
In his continuing efforts to improve the recommended standards for wildland firefighters’ safety zones, researcher Bret Butler has released a revised version based on additional research. Dr. Butler developed the guidelines that had been used for years which were based on the height of the flames, but in May, 2014 released a new recommendation that was based on height of the vegetation, wind speed, slope, fire intensity, and a constant number. This new July, 2014 version replaces the one that was released in May.
A safety zone is an area where wildland firefighters may be forced to take refuge from an approaching wildfire. There, 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.
The latest version of the guidelines released a few days ago is based on height of the vegetation, wind speed, slope, and the same constant number (8). It removes a factor that could be a little subjective or difficult to quantify accurately in the field, fire intensity.
The new system, like the one unveiled in May, calculates the Safe Separation Distance (SSD) between the fire and the firefighters. To determine the SSD, using the table above multiply the constant number (8) times the number from the table (Slope-Wind Factor) times the height of the vegetation.
Example for 15 mph wind, 24% slope, 6-foot vegetation:
The Safe Separation Distance is 8 x 3 x 6 = 144 feet
Dr. Butlers’ Additional Considerations:
For a 20-person crew, add 10 feet of radius and for a vehicle add another 5 feet of radius.
The area in red requires large natural openings or construction by mechanized equipment.
The proposed rule is to be used for flat ground rather than the existing flame height rule.
Also consider additional lookouts on the ground and in the air to monitor fire activity with early egress to escape routes and safety zones.
At 30% or greater slopes, hot gases tend to stay close to the ground.
Dr. Butler’s disclaimer: This proposed safety zone rule should be considered preliminary because it is based on limited data and analysis and subject to increase or decrease based on additional data. It is presented for release this fire season with the intent of increasing firefighter safety and reducing risk of injury. It is likely that an updated rule will be released in the next year.
(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.)
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.