Report - Fire Dynamics In Mallee-Heath

FIRE DYNAMICS IN MALLEE-HEATH
- Fuel, Weather and Fire Behaviour Prediction in South Australian Semi-Arid Shrublands

Mallee-heath vegetation occurring in semiarid and Mediterranean climates develops a vertically nonuniform and spatially discontinuous fuel complex. The heterogeneity of the fuel layers sustaining fire propagation leads to fire behaviour characterised by nonlinear dynamics where small changes in the drivers of fire spread lead to large changes in observed fire behaviour. Within this fuel complex fire behaviour is not just determined by the effect of fuels and weather, but to a large extent determined by the interactions between those variables and the structure of the flame front. The aim of this research is to understand these nonlinear processes and ultimately fire behaviour in this fuel type through a series of experimental fires conducted in the Ngarkat Conservation Park, South Australia from 2006 to 2008.

Fuel complexes in the experimental burning program comprised mallee and heath vegetation with ages (time since fire) ranging from 7 to 50 years old. Dominant overstorey mallee vegetation comprised Eucalyptus calycogona, E. diversifolia, E. incrassate and E. leptophylla. Fuel complex structure was assessed through destructive sampling and visual hazard assessment methods. Vertical wind profiles (from 10-m above ground to ground height) were characterised for each fuel complex. A total of 67 fires were completed. The range of fire environment conditions within the experimental fire dataset were: air temperature 15 to 39°C; relative humidity 7 to 80%; mean 10-m open wind speed 3.6 to 31.5 km/h; Forest Fire Danger Index 1.7 to 46. Fire behaviour measurements included rate of spread, flame geometry, residence time and fuel consumption. Total fuel load ranged from 3.8 t/ha in young (7-year old) mallee to 10 t/ha in mature stands. Measured rate of spread ranged between 50 and 3310 m/h with fireline intensity between 144 and 11,000 kW/m.

The dataset provided an insight into the mechanisms that allow the development of a coherent flame front necessary to overcome the fine scale fuel discontinuities that characterise the semi-arid mallee-heath fuel types and allow self-sustained fire propagation. The dataset was also used to develop models to predict the likelihood of fire propagation (go/no-go threshold), surface fire rate of spread, crown fire spread regime and rate of spread. The models will be used in planning and conducting prescribed fire operations and supporting wildfire suppression decision-making in mallee-heath fires.