Fundamentals - Wildland Fuel Fundamentals and Applications - page 18

Table 2.1 Fuel characteristics and properties used as input parameters for basic fire behavior
equations at the three scales of fuelbed description
Scale Symbol Parameter Notes

Fuel
particle

d Particle diameter Often stratefied into classes

SAVR Surface-area-to-volume

ratio (m 2 /m 3 )

Called SAVR in this topic but is also called
σ in many fire texts

Particle density (kg m −3 ) Generally 500 kg m −3 (32 lb ft −3 ) ρ p

FMC Moisture content (fraction) Dry weight basis kg moisture per kg wood

S e Effective mineral content

(fraction)

Generally 0.010 (kg minerals - kg silica)
per kg wood

S T Total mineral content

(fraction)

Generally 0.0555 kg minerals per kg wood

h Heat content Often 18586 J kg −1 (8000 BTU lb −1 )

Fuel loading (kg m −2 ) Fuel

component

W Oven-dried fuel weight; a highly dynamic

input

ρ b Component bulk density

(kg m −3 )

Generally an integrated average across the
fuel component and includes air space

M x Dead fuel moisture of

extinction (fraction)

Live fuel moisture of extinction is not in
the basic model; another highly dynamic
input

δ Surface fuel layer depth (m) Mean fuelbed value

β Surface fuel layer packing

ratio (dimensionless)

See Table 2.2 for estimation

Critically needed was an estimate of how fast a fire burned, called rate of spread
( R ), because this was identified as an important characteristic in firefighter deaths
(Barrows 1951 ). An additional estimate of how hot a fire burned, called fire line
intensity ( I ) or the rate of heat release per unit length of the fire front, was needed
to determine when a fire is too hot to fight. Byram ( 1959 ) defined fire line intensity,
I , as:

hW S

I =

c
60

(2.4)

where h is the heat yield of the fuel (kJ kg −1 ), S is the forward rate of spread of
the fire (m min −1 ), and W c is the weight of fuel consumed in flaming combustion
(kg m −2 ). The number 60 is a conversion factor so that the units for I are kW m −1
(kJ m −1 min −1 ). Fuel weight consumed ( W c ) depends on initial fuel loading ( W ;
kg m −2 dry weight). Both loading ( W) and heat yield of fuel ( h , often called heat
content) are the first two important fuel properties for predicting fire behavior
(Table 2.1 ). Linking fire intensity ( I ) with spread rate ( R ) provided a means to eval-
uate the potential to suppress the fire using the fire characteristics chart (Andrews
and Rothermel 1982 ).
Rothermel ( 1972 ) and his team used results of this previous work to create the
quasi-empirical mathematical model that is now integrated into a wide variety of
US fire behavior prediction systems, such as BEHAVE (Andrews 2014 ), FARSITE
(Finney 1998 ), and FIREHARM (Keane et al. 2010 ). This model has been exten-
sively modified, adjusted, and refined (Albini 1976 ; Andrews 1986 ), but the main