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Crown Fire Initiation and Propagation

Introduction

In the publication “Conditions for the start and spread of crown fire”, C.E. Van Wagner (1977) identified that crown fire is the interaction between separate fuel layers in forested areas.

Further, he described two processes and defined models for estimating their potential:

  • Crown Fire Initiation is an indicator of the potential for surface fire to ignite tree crown and produce either passive or active crown fire. Inputs include the gap between the surface fuels and the tree crowns (Canopy Base Height – CBH), the foliar moisture content (FMC) of the tree crowns.  The result is a threshold surface fire intensity required to produce some crown fire.
  • Active Crown Fire Propagation (Crown Spread) is an indicator of the potential for continuous spread through the tree crowns. Inputs include only a characterization of the canopy fuel density in a single number. The result is a threshold rate of spread required to sustain a “solid crown flame…with associated horizontal spread.”

These models are very coarse due to the way they represent highly variable characteristics, canopy base height and canopy fuel density.  Because they are so variable, their inputs represent grand averages and may require adjustment in modeling efforts.

As shown in this matrix, the Crown Fire Initiation and Active Crown Fire Propagation models work together to estimate when fires will remain as surface, when they will produce torching, or passive crown fire, behavior in the canopy, and when they will progress to active crown fires.

Linking Surface and Crown Fire Behavior 

(Scott & Reinhardt, 2001)

Crown Fire Criteria

Use the model results from sections Crown Fire Initiation and Crown Fire Propagation to compare against estimates of surface and crown fire spread produced using anticipated environmental factors.

Crown Fire Initiation

These two graphs identify the height to live crowns (CBH) and the canopy foliar moisture content (FMC) as critical factors, resulting in the threshold surface fire intensity or flame length for evaluating of crown fire initiation. Use either of them to estimate minimum surface fireline intensity or flame length that will support at least passive crown fire.

Crown Fire Initiation (FLI)

Crown Fire Initiation (FL)

Active Crown Fire Propagation & Crowning Index (CI)

According to Van Wagner (1977), minimum threshold values for canopy fuel/bulk density (CBD) are necessary to sustain active crown fire at given spread rates.  And since there is only a single crown fire fuel model, that threshold spread rate can be converted to a threshold windspeed or “crowning index” (CI).

Crown Fire Propagation Table

Crown Fire Propagation Graph

Finney and Scott/Reinhardt Approaches to Crown Fire Behavior

  • Both approaches identify the threshold for predicting crown fire initiation and active crown fire spread using the same criteria, based on the Van Wagner Crown Fire Initiation and Propagation models.
  • BehavePlus identifies the fire type for a given scenario but does not provide separate estimated passive crown fire behavior characteristics (ROS, FL).   The user must select surface or crown fire characteristics from separate tables based on the fire type (surface or active) expected.
  • The Surface Fire Control model (implemented only in FLAMMAP, FARSITE, and FSPro) produces specific fire behavior characteristics for surface and active crown fire.  Passive crown fire is accounted for by modeling new spot fires at frequencies and distances estimated separately.
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