FOREST FDI's - Fire Behaviour Relationships

An index of one (1) means that fires either will not burn, or burn so slowly that control presents little difficulty. An index of one hundred (100) means that fires will burn so fast and hot that control is virtually impossible.

The meter is designed for general fire danger forecasting purposes and is based on the expected behaviour of fires burning for an extended period in high eucalypt forest carrying a fuel quantity of 12.5 tonnes per hectare and travelling over level to undulating topography. (Fuel Quantity is expressed in tonnes per hectare of combustible material less than 6 millimetres in diameter.)

The behaviour of individual fires can be predicted with reasonable accuracy providing the effect of fuel quantity and slope is taken into account.

The rate of perimeter spread is generally three times the rate of forward progress but may increase to a factor of four on a large irregular fire.

Fires travel upslope with the prevailing wind faster than on level ground. A five degree slope increases spread by 33 per cent; a ten degree slope by a factor of two and a twenty degree slope by a factor of four. Corresponding reductions occur on downslopes.

Fires in low quality eucalypt forest tend to spread at faster rates than those shown, due to greater wind movement near the ground. However, the spotting potential is generally lower.

The meter can be used to determine broad control burning conditions although a high degree of precision should not be expected. Burning at an index greater than twelve (12) would be very risky except in very light fuel types.

The rate of forward spread refers to a moving flame front which is only affected by relatively short distance spotting. When long distance spotting occurs, the rate of spread may be greater than indicated. The spotting distances given in the table below are for fuel types containing a high proportion of fibrous-barked eucalypts. Gum-type eucalypts will only throw long distance spot fires after a crown fire develops.

Atmospheric instability is not included as a factor affecting fire behaviour. However the rates of spread indicated are for generally Unstable conditions and may be less under conditions of stability.


FUEL
QUANTITY
(tonnes/hectares)
FIRE
BEHAVIOUR
FIRE DANGER INDEX
5 10 15 20 25 30 40 50 60 70 80 90 100
5 R(km/hr)
H(m)
S(km)
0.03
0.3
-
0.06
0.6
-
0.09
1.0
-
0.12
1.5
0.1
0.14
2.0
0.2
0.17
2.5
0.3
0.23
3.0
0.6
0.28
3.5
0.8
0.34
4.0
1.0
0.39
4.5
1.2
0.45
5.0
1.4
0.50
5.5
1.7
0.56
6.0
1.9
10 R(km/hr)
H(m)
S(km)
0.06
1.0
-
0.12
2.0
-
0.18
3.0
0.2
0.23
4.0
0.4
0.29
5.0
0.6
0.34
5.5
0.8
0.45
7.0
1.2
0.56
8.5
1.7
0.67
10.0
2.1
0.78
11.0
2.5
0.89
12.0
3.0
1.0
13.0
3.4
1.11
14.0
3.8
15 R(km/hr)
H(m)
S(km)
0.09
2.0
-
0.18
3.5
0.2
0.26
5.0
0.6
0.35
7.0
0.9
0.43
8.0
1.2
0.51
9.5
1.5
0.68
12.0
2.2
0.85
14.0
2.8
1.02
**
3.4
1.18
**
4.1
1.35
**
4.8
1.52
**
5.4
1.68
**
6.0
20 R(km/hr)
H(m)
S(km)
0.12
2.5
0.1
0.24
5.0
0.5
0.36
7.0
0.9
0.48
9.0
1.3
0.60
11.0
1.7
0.72
13.0
2.2
0.96
**
3.0
1.2
**
3.8
1.44
**
4.7
1.68
**
5.6
1.82
**
6.4
2.16
**
7.2
2.39
**
8.1
25 R(km/hr)
H(m)
S(km)
0.14
3.0
0.1
0.30
7.0
0.6
0.45
10.0
1.1
0.60
12.0
1.6
0.75
14.0
2.1
0.90
**
2.6
1.2
**
3.6
1.50
**
4.6
1.8
**
5.6
2.10
**
6.6
2.40
**
7.6
2.70
**
8.6
3.00
**
9.6

** CROWN FIRE

R = Rate of Spread (km/hr)
H = Flame height (meters)
S = Average spotting Distance (km)

Fuel Quantity is expressed in tonnes per hectare of combustible material less than 6mm in diameter.


THE MEASUREMENT OF METEOROLOGICAL ELEMENTS

(1) Temperature: The screen temperature at the time the fire danger is determined.
(2) Relative Humidity and Dew Point: The calculated values corresponding to the screen temperature.
(3) Wind speed: The average wind speed estimated or recorded over a period of at least five minutes in an open flat locality. The measurement should be made at a height of 10m above ground level or above tree top level in restricted forest openings.
(4) Rainfall: The amount of rain measured at 9 a.m. from a standard rain gauge. The afternoon of a day on which rain is recorded at 9 a.m. is taken as being one day after rain. If rain falls after 9 a.m. use the zero setting. If rain is recorded on a number of successive days the 9 a.m. totals should be accumulated and treated as a single fall.
(5) Drought Index: This is used as a measure of seasonal severity and fuel availability. It is derived from daily records of maximum temperature and rainfall.
(6) Drought Factor: This is a broad measure of fuel availability as determined by seasonal severity and recent rain effects. Where the effect of one rain period is superimposed on another, use the lowest drought factor.

Drought Factor Calculator See the article by Rick McRae on DROUGHT FACTOR MODELLING for more info.



Acknowledgements:
A.G. McArthur, Forest Fire Danger Meter Mk V 1973.
Bush Fire Council of N.S.W. Officer Training Module CL/4 - Fire Behaviour Second Edition