Canopy Fuels - Wildland Fuel Fundamentals and Applications - page 58

CBH was defined as the vertical distance between the ground surface and the base
of the live crown fuels, but, many have advocated that it should be expanded to the
canopy-scale instead of an individual tree so that it may include all types of “ladder”
fuels including small trees, large shrubs, and attached dead branches (Alexander
1988 ; FCFDG 1992 ). The model keys the type of crown fire depending on the
threshold for active crown fire spread rate RAC (Alexander 1988 ):

= 3.

(4.2)

RAC

CBD

where CBD is the canopy bulk density (kg m −3 ) and the 3.0 coefficient is an empiri-
cal constant defining the critical mass flow rate through the crown layer for con-
tinuous flame (0.05 kg m −2 s −1 ) and a conversion factor (60 s min −1 ). Van Wagner
( 1977 ) estimated CBD by computing canopy biomass (kg) of all trees in a unit area
and dividing it by the canopy volume (unit area times the canopy depth defined as
the canopy height - CH, m - minus CBH ). One of three types of crown fire (passive,
active, and independent) is then keyed using the I o and RAC values. A passive crown
fire must meet the following conditions:

I = I , but R < RAC (4.3)

b o C actual

where I b is the surface fireline intensity (see Chap. 2.2) and R C actual is the active
crown fire spread rate (m min −1 ) calculated from the following using the FCFDG
( 1992 ) approach:

(4.4)

R =+ R CFBR R ( − )

C actual C max

where R is the rate of spread (m min −1 ) of the surface fire (see Chap. 2.2) and CFB
is crown fraction burned, a scalar from 0 to 0.9 computed from RAC and a critical
surface fire spread rate ( R o ) (Finney 1998 ; Scott and Reinhardt 2001 ). The variable
R C max is the maximum crown fire spread rate (m min −1 ) that many estimate using the
Rothermel ( 1991 ) algorithm where R C max is computed from a correlation with the
forward surface fire spread rate for the Anderson ( 1982 ) US fuel model 10 using a
0.4 wind reduction factor ( R 10 m min −1 ) from the following relationship:

R

C max = 334

. R

(4.5)

10

The rate of spread of a passive crown fire is assumed to be equal to that of the sur-
face fire.
If passive conditions aren't met, then conditions for active crown fire must meet
the following criteria:

I ≥ I and R ≥ RAC

(4.6)

b o C actual