Category:Fire Intensity

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Frontal Fire Intensity - The rate of heat energy release per unit time per unit length of fire front. Flame size is its main visual manifestation. Frontal fire intensity is a major determinant of certain fire effects and difficulty of control. Numerically, it is equal to the product of the net heat of combustion, quantity of fuel consumed in the flaming front, and linear rate of spread. Recommended SI unit is kilowatts per metre (kW/m).

(CIFFC Glossary of Forest Fire Management Terms)


'Perhaps the single most valid characteristic of a fire's general behavior and direct impact on above ground vegetation is "fire intensity" as described by Byram (1959).' (Alexander 1982, p.350)


Contents

Fire Behaviour Terminology

Development of a system for quantitative assessment of fire characteristics and adoption by the fire community has advanced fire observations from a 'subjective, qualitative system of descriptions' (Alexander 1982,p.349) to a universal language of numerically communicating fire behaviour within the realms of fire assessment and management and incorporating into safe work procedures on the fireline.

'Fire intensity (also termed frontal fire intensity and line fire intensity) has become one of the standard guages by which fire managers estimate the difficulty of controlling a fire and select appropriate suppression action.' (Van Wagner et al. 1992,p.38).

Byram's Fire Intensity Equation

Byram's fire intensity equation (1959) is often cited in fire behaviour research journals as a fundamental building block for determining fire intensity.

The formula developed by Byram is I = Hwr where:

I = Fire Intensity (kW/m)

H = Fuel low heat of combustion (kJ/kg)

w = Weight of fuel consumed per unit area (kg/m 2)

r = Rate of Spread (m/sec) (Van Wagner 1992,p.38)

Byram's fire intensity equation can be adapted to use flame length as a variable to calculate fire intensity in the equation:

I = 258 * FL 2.17 where:

I = Fire Intensity and FL = Flame Length (Rothermel and Deeming 1980,p.3).

For field observations, this equation has been rounded to a rough rule of thumb I = 300 L2 where:

I = fire intensity and L = Flame Length (Beck et al. 2002)

Estimating Flame Length

Flame Length - The length of flames measured along their axis at the fire front; the distance between the flame height tip and the midpoint of the flame depth at the ground surface. Flame length is an approximate indicator of frontal fire intensity. Recommended SI unit is metres (m)

(CIFFC Glossary of Forest Fire Management Terms)

Average flame length should be estimated in field observations over a reasonable time period to take into acount the large variance in flame length. (Rothermel and Deeming 1980, p.2) More accurate estimations can be made with visual indicators such as steel posts with marked graduations placed in the burn. Photos and video will be useful in estimating the average flame length.

Flame Height - The average maximum vertical extension of flames at the fire front; occasional flashes that rise above the general level of flames are not considered. Recommended SI unit is metres (m).

(CIFFC Glossary of Forest Fire Management Terms)


When using flame height to estimate fire intensity, the following trigonometric equations can be used to calculate flame length as a function of flame height.

Fire Intensity graphic.png


L = hF / sin A

L = hF / cos AT


where:

L = Flame Length

hF = flame height

A = flame angle (degrees)

AT = flame tilt angle (degrees)

(Alexander 1982)


Graphic reproduced from Alexander 1982



Fire Intensity Class

Fire intensity classes are conceptually introduced in Hirsch (1996,p.79) where general fire behaviour descrptions based on head fire intensity are associated with a range of fire intensity values (kW/m). The concept of fire intensity class is formally adopted in Taylor 1997 as one of the key outputs for each rate of spread/fire intensity class table of the Field guide to the Canadian Forest Fire Behaviour Prediction (FBP) System. Each fire intensity class (1 to 6) is assigned a range of fire intensity values (kW/m).

Fire Intensity Class or Head Fire Intensity Class has become a commonly accepted terminology for communicating observed fire behaviour or predicting fire behaviour.

An analogous system for communicating fire behaviour by numerical rank is used by the British Columbia Wildfire Management Branch. The Fire Intensity Rank System assigns a numberical rank (1 to 6) to an established set of fire behaviour visual indicators and rate of spread.

Research Applications

Baxter (2008) applied Byram's fire intensity equation to determine differences in fire behaviour (fire intensity) generated in a series of experimental burns in grass fuel plots with varying fuel treatments (fall mowed, freshly mowed or standing). Using data collected (rate of spread and fuel loads), the fire intensity for each burn plot was calculated with Byram's fire intensity equation:

I = 300 w (kg/m2) r (m/min) where:

I = fire intensity (kW/m)

w = fuel load (kg/m2)

r = head fire rate of spread in m/min.

The value of 300 is derived by dividing 18,000 kJ/kg (an assumed constant value for the low heat of combustion) by 60 so that rate of spread can be expresed in m/min rather thatn m/sec. (Hirsch 1996)

Stocks (1989) used Byram's fire intensity equation to determine frontal fire intensity and qualifies the fuel load consumed value (w) used in the equation with the assumption that 'all fuel consumption was to have taken place in the flaming front and not as a result of smoldering or glowing combustion'. (p.784)

Fire Intensity in experimental burns using various less flammable ground cover species as fuel types is determined in Baxter (2010), p.10 by estimating flame length and inputting these values in the formula I = 300 L2 where:

I = Fire Intensity and L = Flame Length

References

Alexander, M.E. 1982. Calculating and interpreting forest fire intensities. Can. J. Bot. 60: 349-357.

Baxter, G. 2008. Effectiveness of Mowing Grass to Reduce potential fire behaviour in linear corridors FPInnovations Wildland Fire Operations Research Group

Baxter, G. 2010 Burn trial of a lowland site vegetated by less-flammable native speciesFPInnovations Wildland Fire Operations Research Group

Beck,J.A.; Alexander,M.E; Harvey,S.D.; Beaver,A.K. 2002. Forecasting diurnal variations in fire intensity to enhance wildland firefighter safety.International Journal of Wildland Fire 11(4) 173 - 182

British Columbia Forests and Range Wildfire Management Branch

Forestry Canada Fire Danger Group. (1992) Development and structure of the Canadian Forest Fire Behavior Prediction System. Information Report ST-X-3. Forestry Canada, Science and Sustainable Development Directorate, Ottawa, Ontario, 63 pp.

Hirsch, K.G. 1996. Canadian forest Fire Behavior Prediction (FBP) System: user's guide. Nat. Resour. Can., Can. For. Serv., North. For. Cent., Edmonton, Alberta. Spec. Rep.7.

Rothermel, R.C. and Deeming, J.E. 1980. Measuring and Interpreting Fire Behavior For Correlation with Fire Effects. USDA Forest Service General Technical Report INT-93 November 1980.

Stocks,B.J. 1989. Fire Behaviour in mature jack pine. Nat. Resour. Can., Can. For. Serv.

Taylor,S.W.; Pike,R.G.; Alexander,M.E. 1997. Field Guide to the Canadian Forest fire Behavior Prediction (FBP) System. Nat. Resour. Can., Can. For. Serv., north. For. Cent., Edmonton, Alberta. Spec. Rep.11

(Van Wagner et al. 1992. Development and Structure of the Canadian Forest Fire Behavior Prediction System. Forestry Canada Fire Danger Group Information Report ST-X-3)

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