Fire behaviour and fuel reduction burning: Bemm River wildfire, October 1988

Summary

On 14 October 1988 a major wildfire occurred in the Bemm River area, 33 km east of Orbost in the East Gippsland region of Victoria. The weather was dominated by wind speeds of gale force strength. The fire burnt under “very high” to “extreme” fire danger conditions with a maximum McArthur fire danger index of 82, a maximum mean forward rate of spread between 3.5 and 4.0 km/h and a maximum mean fireline intensity in the range 31,200 to 35,500 kW/m. The final area of the wildfire was 5,670 ha and the perimeter was 80 km. A highly significant influence on fire behaviour was the layer of aerated, highly flammable shrub fuel. This was combined in the coastal forests of the Bemm River area with high quantities of litter and the high spotting potential of the stringybark eucalypts. Total fine fuel loads averaged about 20 t/ha.

The rate of spread predicted by the McArthur Mk 5 Forest Fire Danger Meter during the initial two hour period of the wildfire was 2.0 km/h and the observed mean rate of spread was between 2.0 and 2.6 km/h. During the second two hour period, the fire danger index decreased and the predicted rate of spread was 0.8 km/h. However, the observed mean rate of spread increased to between 3.5 and 4.0 km/h.

The contribution of a previous wildfire and of fuel reduction burns, of various ages and coverage, to fire protection within the coastal forest fuel type burnt by this wildfire is assessed. A minimum burnt area of greater than 50% is recommended to achieve effective protection. Dramatic protection was demonstrated under “very high” fire danger conditions in fuels burnt one-and-a-half years previously. Similar protection was achieved under “high” fire danger conditions in fuels burnt six months and two-and-a-half years previously, and a measurable effect was apparent in fuels burnt seven years previously.