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.LBtcalc <- function(FUELTYPE, LB, HR, CFB) {
#############################################################################
# Description:
# Computes the Length to Breadth ratio of an elliptically shaped fire at
# elapsed time since ignition. Equations are from listed FCFDG (1992) and
# Wotton et. al. (2009), and are marked as such.
#
# All variables names are laid out in the same manner as Forestry Canada
# Fire Danger Group (FCFDG) (1992). Development and Structure of the
# Canadian Forest Fire Behavior Prediction System." Technical Report
# ST-X-3, Forestry Canada, Ottawa, Ontario.
#
# Wotton, B.M., Alexander, M.E., Taylor, S.W. 2009. Updates and revisions to
# the 1992 Canadian forest fire behavior prediction system. Nat. Resour.
# Can., Can. For. Serv., Great Lakes For. Cent., Sault Ste. Marie, Ontario,
# Canada. Information Report GLC-X-10, 45p.
#
# Args:
# FUELTYPE: The Fire Behaviour Prediction FuelType
# LB: Length to Breadth ratio
# HR: Time since ignition (hours)
# CFB: Crown Fraction Burned
# Returns:
# LBt: Length to Breadth ratio at time since ignition
#
#############################################################################
#
#Eq. 72 (FCFDG 1992) - alpha constant value, dependent on fuel type
alpha <- ifelse(FUELTYPE %in% c("C1", "O1A", "O1B", "S1", "S2", "S3", "D1"),
0.115,
0.115 - 18.8 * (CFB**2.5) * exp(-8* CFB))
#Eq. 81 (Wotton et.al. 2009) - LB at time since ignition
LBt <- (LB - 1) * (1 - exp(-alpha * HR)) + 1
return(LBt)
}
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