was being consumed in the fire front. The per- 

 centages apply to the size classes and loading 

 as previously described. 



Laboratory work (7) has shown that the 

 percent of ground fuel burned in the flame 

 front decreases as windspeed increases. We 

 estimated 20-percent ground fuel consump- 

 tion in our calculations. The difference be- 

 tween this and the field-determined amount 

 consumed is assumed to have been burned by 

 sporadic flaming and glowing combustion fol- 

 lowing passage of the main fire front. 



INTENSITY 



Fire intensity values were calculated and 

 are given in the Log at hourly intervals, 

 along with values for maximum energy release 

 from the total front. These, too, are average 

 values corresponding to the time and distance 

 intervals. 



As explained earlier, physical constants for 

 loading and moisture content of strata were 

 derived. These constants established the 

 amount of fuel on the site that could contrib- 

 ute energy to the fire. The average energy 

 content of fuel was set at 8,500 B.t.u./lb.; the 

 heat required to remove the moisture and 

 raise the fuel to ignition in each fuel level was 

 then subtracted to arrive at an energy release 

 potential (B.t.u./lb.). Values of the energy 

 potential per unit were calculated by multi- 

 plying the loading, corrected to pounds per 

 square foot, by the energy release potential. 



For any time interval of interest, the energy 

 per unit area was combined with the percent 

 consumption values to obtain the energy re- 

 leased per unit area. Fire intensity and total 

 energy release rate were calculated by use of 

 the rate-of-spread determinations and applica- 

 tion of laboratory findings for residence time 

 (time the flaming zone exists at one point). ^ 

 The physical constants for the three fuel 

 levels are given in table 1. 



The values for energy release rate are shown 

 in figure 11, for a 12-hour period that includes 

 the fire's major run. To arrive at our values, 

 the residence time for the fire-controlling fuel 

 level was combined with a rate of spread to 

 establish a flame depth. The active combustion 

 area was calculated by multiplying the fire 

 edge length by the flame depth. The combina- 

 tion of the combustion area and the energy 

 released per unit area divided by the residence 

 time yielded the total energy release rate per 

 unit time (B.t.u./sec). 



Fire intensity in B.t.u. per second per foot 

 of fireline (fire edge) was calculated after 

 Byram (2, chap. 3, p. 79). Values were ob- 

 tained by combining the summed energy per 

 unit area and the rate of spread for a time 

 interval. The fire intensity values (fig. 12) ap- 

 pear reasonable when compared with the 

 limits established by Byram. He gives an 



Anderson, Hal E. Heat transfer and fire spread. U.S.D.A. 

 Forest Serv., Intermountain Forest and Range Exp. Sta., 

 Ogden, Utah. (In preparation.) 



Table 1 . — Estimated fuel constants for the Sundance Fire area. 



Fuel 

 strata 



Loading 



Moisture 

 content 



Residence 

 time 



Energy release 

 potential' 



Energy per 

 unit area 





Ton/acre 



Percent 



Min. 



B.t.u./lb. 



B.t.u./sq.ft. 



Ground 













litter 



20 (max.) 



12 



1.0 



7,675 



7,060 





1 (min.) 



12 



1.0 



7,675 



384 



Brush 



2.7 (ave.) 



60 



2.0 



7,718 



957 



Crown 



5.25 (max.) 



145 



0.33 



6,773 



1,630 





2.04 (min.) 



145 



0.33 



6,773 



636 



Corrected for moisture and heat to ignition. 



12 



