THE RATE-OF-SPREAD MODEL 



Rothermel's rate-of-spread model is summarized by the following equation: 



^R^^^ ^ *w ^ *s^ 

 Rate of spread, R = (1) 



b ^ig 



where : 



I = reaction intensity or the rate of energy release in the flame front of a fire 

 R 



B.t.u./ft.^- 



mm. 



C = propagating flux ratio--the proportion of 1^^ required for propagation of a 

 no-wind fire, dimensionless 



w 



multiplying factor for the effect of wind, dimensionless 



(J)^ = multiplying factor for the effect of slope, dimensionless 



= bulk density of fuel, Ib./ft.^ 



e = effective heating number which expresses the proportion of fuel that is 

 heated to ignition temperature dimensionless 



Q. = heat of preignition, the heat required to bring a unit weight to ignition, 

 B.t.u./lb. 



These terms are computed from a set of rather complex equations. This set, which 

 is summarized in equation (1), constitutes the mathematical model. The basic fuel and 

 environmental input variables for the model are shown in table 1. 



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