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International Journal of Wildland Fire International Journal of Wildland Fire Society
Journal of the International Association of Wildland Fire
RESEARCH ARTICLE

An examination of fire spread thresholds in discontinuous fuel bedsA

Mark A. Finney A B , Jack D. Cohen A , Isaac C. Grenfell A and Kara M. Yedinak A
+ Author Affiliations
- Author Affiliations

A USDA Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory, Fire Behavior, 5775 Highway 10 West, Missoula, MT 59808, USA.

B Corresponding autor. Email: mfinney@fs.fed.us

International Journal of Wildland Fire 19(2) 163-170 https://doi.org/10.1071/WF07177
Submitted: 21 December 2007  Accepted: 11 November 2008   Published: 31 March 2010

Abstract

Many fuel beds, especially live vegetation canopies (conifer forests, shrub fields, bunch-grasses) contain gaps between vegetation clumps. Fires burning in these fuel types often display thresholds for spread that are observed to depend on environmental factors like wind, slope, and fuel moisture content. To investigate threshold spread behaviours, we conducted a set of laboratory burn experiments in artificial fuel beds where gap structure, depth, and slope were controlled. Results revealed that fire spread was limited by gap distance and that the threshold distance for spread was increased for deeper fuel beds and steeper slopes. The reasons for this behaviour were found using a high-speed thermal camera. Flame movements recorded by the camera at 120 Hz suggested fuel particles experience intermittent bathing of non-steady flames before ignition and that fuel particles across the gap ignited only after direct flame contact. The images also showed that the flame profile within the fuel bed expands with height, producing greater horizontal flame displacement in deeper beds. Slope, thus, enhances spread by increasing the effective depth in the uphill direction, which produces wider flames, and thereby increases the potential flame contact. This information suggests that fire spread across discontinuous fuel beds is dependent on the vertical flame profile geometry within the fuel bed and the statistical properties of flame characteristics.


Acknowledgements

The authors are indebted to Bob Schuette who led the design and construction of laboratory equipment, Paul Sopko for help with instrumentation and environmental conditioning, and AJ Hershman and DK Paige and numerous technicians who tirelessly built the fuel beds for the experiments.


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A The manuscript was produced by US Government employees on official time and is therefore in the public domain and subject to copyright laws in the US.

B Tradenames are provided for informational purposes only and do not constitute endorsement by the USDA.