Authored By: D. Kennard
Fireline intensity, also known as Byrams fireline intensity or frontal fire intensity, is the rate of heat energy released per unit time per unit length of fire front, regardless of the depth of the flame zone (Byram 1959). It is calculated as the product of available fuel energy and the fires rate of advance:
I = Hwr
where:
- I = Byrams fireline intensity (kW/m or Btu/s/ft)
- H = fuel low heat of combustion (kJ/kg), subject to reductions
- w = weight of fuel consumed per unit area in the active flaming zone (kg/m2)
- r = rate of spread (m/s)
Fireline intensity may vary by more than 1000-fold, or from 15 to at least 100,000 kW/m (Alexander 1982). However, most fire intensities seldom exceed 50,000 kW/m and most crown fires fall within the range of 10,000-30,000 kW/m. Low intensity surface fires are generally less than 550 kW/m. Fireline intensities above 4000 kW/m are generally characterized as "high intensity" (Alexander 1982). Because considerable burning can take place after passage of the flaming front, these figures do not describe the total energy or heat released (Alexander 1982).
The wide variation in fireline intensity is largely because of the potential variation in rate of spread. Rates of spread can vary 100-fold, while fuel consumption varies only 10-fold, and low heat of combustion varies so little from fuel to fuel (roughly 10%) that it can be thought of as a constant (Alexander 1982). Because of the importance of rate of spread, fireline intensity is greatly influenced by weather and topography (Brown and Davis 1973, DeBano et al. 1998).
While fireline intensity may be one of the best single descriptors of fire behavior (Alexander 1982), it is difficult to accurately measure the required variables in the field. In particular, it is difficult to distinguish fuel consumed in the active flaming zone (w) from fuel consumed after the flame front passes. As a consequece, w is often overestimated (Wade 1986). An easier way to estimate fireline intensity in the field is to determine fireline intensity using observed flame length. Although not entirely accurate, estimating fireline intensity indirectly from flame length will help avoid errors due to overestimating w when using Byrams equation. A more complete description of fire behavior would quote the rate of spread in addition to fireline intensity because identical fireline intensities can be arrived at by fires with different spread rates and fuel consumption (Van Wagner 1965, Alexander 1982).
Fireline intensity can be useful for comparing fires (Wade 1986). It can also be used to assess the effects of prescribed burns. For example, fireline intensity correlates well with crown damage and can actually be used to calculate lethal scorch height as well as expected temperatures at certain heights above surface fires. Other fire parameters are better for assessing damage to plants within and below the flame zone, however. For example, residence time or reaction intensity are more strongly correlated with basal stem damage than fireline intensity. Depth of burn or heat release per unit area are good indicators of damage to roots and stems belowground (Wade 1986).
Fireline intensity is also used to assess the difficulty of wildfire containment. For example, direct attack with hand tools and assured control of prescribed fires is possible when fireline intensity is less than 400-425 kW/m (Hodgson 1968, Chandler et al. 1983). Heavy mechanical equipment can usually control a fire if fireline intensity is below 1700 to 1750 kW/m (Forest Service 1978, Chandler et al. 1983). Spot fires can become serious at 2000 to 2100 kW/m (Hodgson 1968, Chandler et al. 1983) and fires are completely uncontrollable with fireline intensities above 3500-3700 kW/m (Forest Service 1978, Chandler et al. 1983).