GRASSLANDS FDI's - Fire Behaviour Relationships

With an index of 1 or 2 fires will either not burn or burn so slowly that control presents little difficulty. At an index of 100 they will burn so hot and fast that control is almost impossible.

The intensity of a fire and its difficulty of control is also affected by the quantity of grass in the pasture. Heavy pastures burn faster and with a greater intensity than light pastures. In addition the finer the grass the faster a fire will travel.

The rates of spread are average values for fires in annual and perennial pastures carrying a continuous body of fuel and occurring on level to undulating ground. Spread rates will be less than indicated in sparse, discontinuous pastures and will also vary according to topography.

½ hr 1 hr 2hr 4hr Sparse Pasture Average Pasture Heavy Pasture
2 0.3 Low.
Headfire stopped by road and tracks
3 20 80 320 3 0.3 1.0 3.0
5 0.6 Moderate.
Head attack easy with water.
6 40 160 640 16 0.6 2.0 3.5
10 1.3 High.
Head attack generally successful with water
15 90 360 1440 65 1.0 3.0 5.5
20 2.6 Very High.
Head attack will generally succeed at this Index
35 210 840 3360 450 2.0 3.5 7.0
40 5.2 Very High.
Head attack may fail except in favourable circumstances and close back burning to the head may be necessary
80 480 2000 8000 2400 2.5 5.0 9.0
50 6.4 Extreme.
Direct attack will generally fail. Backburn from a secure good line with adequate manpower and equipment. Flanks must be held at all costs.
105 630 2500 10000 4000   5.5 10.0
70 9.0 170 1000 4000 16000 10000   6.0 11.0
100 12.8 300 1800 7000 28000 32000   7.0 13.0
**Note: This assumes that the head fire burns unchecked. suppression action which is only partially successful wil reduce these areas.

Degree of Curing:

The amount of greenness in the pasture or degree of curing is estimated visually for large areas. 100% cured is when all pastures are fully cured and there is no green material at all. When using the meter for estimating fire behaviour over broad areas, an aerial inspection is often the best way to determine a general degree of curing for the area. Some grass species such as wild oats or barley grass will be fully cured while other species will only be partially cured. Always ensure that the degree of curing is appropriate to the area under consideration. For small areas, curing can be estimated by taking a handful of grass and expressing the amount of dead stalks as a percentage of the total.

The Effect of Slope:

The rates of spread given by the meter apply to level or gently undulating ground. Over short distances the effect of slope is very pronounced. The rate of forward spread will double up a 10 degree slope and will be four times greater up a 20 degree slope. The rate of spread will be correspondingly reduced on a downslope, except that massive fire whirlwinds are likely to develop under severe burning conditions.

Wind Speed:

Is measured at a height of 10 m in the open and refers to the average wind speed (over 10 minutes).

When the wind is gusty, fire behaviour will be erratic, particularly in fine fuels which respond rapidly to 'changes in wind speed.

Perimeter Increase:

For all practical purposes, the perimeter of a grassfire can be taken as 2.5 times the forward spread, e.g. if the forward spread is 10 km/h, the perimeter spread will be 25 km/h.

Area Increase:

The area of a fire increases as the square of the burning time, e.g. the area at 4 hours from start will be 16 times the area at 1 hour. This indicates the need for very fast initial attack and quick control.

Warning Signs:

Abundant, fully cured grasslands occurring after a rainfall deficient period of 4-6 weeks; increasing temperatures, falling humidities and rising winds immediately preceding a cool change. Always remember wind changes associated with a cool change.

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