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Firescience.gov Friday Flash News

Issue 96  |  July 11, 2014

CRITICAL New Wildland Firefighter Safety Zone Research 

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Critical New Wildland Firefighter
Safety Zone Research


The new calculation is
based on vegetation height rather rather than flame height
2011 Las Conchas Fire, New Mexico  
Photo: Kari Greer - National Interagency Fire Center

For many years, the Joint Fire Science Program has funded the wildfire safety zone work of Bret Butler who is a research engineer at the Missoula Fire Sciences Laboratory. Bret developed flat terrain safety zone recommendations for firefighters.  As a result of his initial work, the JFSP funded Bret’s additional research which focuses on safe separation distances on slopes. 

Although results are preliminary and subject to change,
 this new research should be used to
provide an extra margin of safety for all wildland fire personnel. 

U.S. Forest Service scientists Russ Parsons and Ruddy Mell collaborated on the project, providing assistance formulating and running the computer simulations that form the basis of the modified rule.

Safe Separation Distance (SSD)

SSD = 8 x Slope-Wind Factor x Height of Surrounding Vegetation
Firefighters: 
Please watch for future releases and check
the date of the table you are using.
ADDITIONAL CONSIDERATIONS
  1. For a 20-person crew, add 10 feet of radius and for a vehicle add another 5 feet of radius.
  2. The area in red requires large natural openings or construction by mechanized equipment.
  3. The proposed rule is to be used for flat ground rather than the existing flame height rule.
  4. Also consider additional lookouts on the ground and in the air to monitor fire activity with early egress to escape routes and safety zones.
  5. At 30% or greater slopes, hot gases tend to stay close to the ground.
In the two examples below, with slope, wind, and vegetation height remaining the same, the calculated safe separation distance of the proposed rule is double the distance of the existing rule

The difference is due to the
influence of wind and slope on fire intensity.
Example from Existing Safety Zone Rule
Based on flat terrain with no wind
  • Flames are 6 feet tall, wind speed is 10 miles per hour, slope is 20 percent and sagebrush is 3 feet tall.
  • Radius of the safety zone = 4 x flame height = 4 x 6 feet = 24 feet
Example from Preliminary Proposed New Safety Zone Rule
Based on vegetation height not flame height
  • Wind speed is 10 miles per hour, slope is 20 percent, and sagebrush is 3 feet tall
  • Slope-wind factor from the table is 2
  • Radius of the safety zone = 8 x slope-wind factor x vegetation height = 8 x 2 x 3 feet = 48 feet.
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. 

Read the article in the International Journal of Wildland Fire:

Wildland Firefighter Safety Zones: A Review of Past Science and
Summary of Future Needs
 
Monitor www.firelab.org and firescience.gov for updates
                                     
Photo: Setting up project sensors in Alaska. Courtesy Bret W. Butler, Missoula Fire Sciences Lab
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