Fire Management – 101. Fire Behaviour – FFDI.

Fire behaviour is usually described in terms of fire intensity (heat output – kw/m), flame heights, rates of spread and spotting distance.

It is influenced by a complex interaction of many factors including forest type, fuel quantity and arrangement, fuel dryness, temperature, atmospheric stability, wind speed and direction, Relative Humidity (RH), topography, aspect and even slope.

The Forest Fire Danger Index (FFDI) was originally developed by the legendary pioneer of Australian bushfire science, Alan Grant McArthur, during the 1950s and ‘60s.

Alan studied forest science at the University of Sydney in 1945, and later the Australian Forestry School in Canberra.

In 1953, Alan transferred to the Commonwealth Forestry and Timber Bureau in Canberra as a fire researcher. Five years later he was appointed Principal Research Officer in the newly created Division of Forest Research within the Commonwealth Scientific and Industrial Research Organization (CSIRO).

Alan was a very hands-on forester and fire researcher. To gather data, he deliberately lit over 450 experimental fires between 1956 and 1961 under a range of low to moderate weather conditions in the Kowen Forest and Bulls Head Creek area around Canberra. There were other test fires near Traralgon and the Wombat Forest near Daylesford. The experimental data was reinforced from studies of a few well-documented bushfires.

For obvious reasons, he couldn’t light test fires under extreme weather conditions, so his subsequent fire danger equations needed to be extrapolated.

Much of his raw field data was collected by students from the Australian Forestry School and later the Australian National University where he lectured. Alan used his fit and hardy crew, armed with time-stamped rocks, to mark the progress of the test fires and collect the data as well as map fire progression for his meter, a hugely useful improvement in fire prediction at the time.

Alan made thousands of detailed observations of things like wind speed, RH, temperature, cloud cover, rainfall, fuel moisture content, flame height, fire intensity, spotting distance, rate of spread and fuel quantity. He also made subjective assessments of fire suppression difficulty.

Alan published his landmark paper, “Controlled burning in eucalypt forests” in 1962. Leaflet No. 80, as it was known, proved a turning point for forest and fire managers across Australia.

More importantly, Alan was a very practical forester and wanted his work to be useful to people in the field, so after several iterations he came up with the now familiar circular slide rule called the Forest Fire Danger Index meter (FFDI).  The Mk 4 version first appeared in operational use in 1967.

There is also a grasslands fire danger meter.

The FFDI meter uses measurements of dryness, based on rainfall and evaporation together with the Keetch-Bryram Drought Index to calculate a Drought Factor (DF) ranging from 1 to 10.

The Drought Factor is then combined with wind speed, temperature and relative humidity to calculate a FFDI in a range from 0 to 100.

By assessing fuel load (tonnes/ha) and slope, the fire behaviour characteristics such as Rate of Spread (ROS), flame heights and spotting distance can be estimated under a range of fire danger indices.

The rate of perimeter growth is generally three to four times the rate of forward spread. But long-distance spotting can increase these figures.

Fires travel uphill and with the prevailing wing much faster than on flat ground. A five-degree slope increases the spread by 33% and 25 degrees by as much as a factor of four. These rates reduce by the corresponding amount going downslope.

The McArthur FFDI meter was designed for a fuel load of 12.5 tonnes per hectare. Increasing fuel increases fire intensity, rates of spread, perimeter growth and flame heights.

Most successful firefighting, and indeed fuel reduction burning, occurs when the FFDI is in the “Moderate” range between 5-12. The FFDI rises to “High” between 12-24 and “Very High” between 24 and 50. A day with an index exceeding 50 is considered “Extreme”.

Alan used the weather and fuel conditions of the 1939 Black Friday fires as his example to set the upper limits of FFDI at 100.

However, the FFDI went “off the scale” on both Ash Wednesday in 1983 and Black Saturday in 2009. Under these extreme or catastrophic bushfire conditions, the weather rather than fuel load or arrangement, becomes the dominant factor influencing fire behaviour.

But for anyone who has been involved in bushfires they will know that the FFDI has its shortcomings.

• The original system was only designed for use in forests and grasslands.  But Australia has lots of different types of vegetation such as Mallee heath, woodlands and open savanna, and the FFDI system does not forecast those well.
• The FFDI meter does not consider all the conditions which have an impact upon fire behaviour such as wind changes and atmospheric stability.
• The meter tends to overestimate fire danger and rates of spread on cloudy days with stable atmosphere and also in the early morning.
• Due to the time lag in moisture uptake by fine fuels the meter tends to underestimate fire danger in the late afternoon and early evening
• The meter begins to break down at the extreme end of the scale and small changes to temperature, humidity and wind speed can have a huge influence on the fire danger index.

But no matter what the limitations of the FFDI meter, Alan’s scientific legacy is unquestionably huge and has served forest firefighters very well over the decades. New research will undoubtedly refine and develop even better models.

McArthur, A. G. (1962). Controlled burning in eucalypt forests. Leaflet No. 80.

https://drive.google.com/file/d/1HfE6isPBiGQ7Yzvg_ETJZTR3CZWXq6Vl/view