Short fire runs: Assessing bush fire risk from small areas of vegetation

The size and shape of small areas of vegetation influences the behaviour of bush fires and the associated risk to the built environment. Small or narrow parcels of vegetation have less opportunity to support fully developed bush fires because of their limited size. These areas are referred to as a short fire run (SFR). In 2001 the NSW RFS introduced legislation to mitigate the impact of bush fire on the bushland interface. Those restrictions were revised in 2006, published as Planning for Bush Fire Protection 2006 (PBP). Under this legislation there is no formal method to assess the reduced risk to assets from SFRs relative to fully developed fires. As a result, SFRs have been assessed as if they are large scale hazards often resulting in unrealistic and onerous bush fire protection requirements. Historically, SFRs have been assessed based on expert judgement by fire managers which often resulted in an inconsistent approach and outcomes. The NSW Rural Fire Service has developed a new methodology for assessing SFRs by calculating the associated levels of radiant heat for purposes of determining construction requirements. Assessment is undertaken by determining the reduced head fire width and flame height of the SFR, as appropriate, then calculating the amount of radiant heat impacting an asset. From this calculation more appropriate bush fire protection measures or risk treatments can be determined. The SFR method is based on the same AS3959-2009 Method 2 calculations used for PBP but introduces modifications to calculate the reduced fire head width and flame height attributed to developing bush fires on smaller parcels of vegetation. The head fire width of the SFR is determined using a description of the size and slope of the site to calculate the length/breath ratio of the fire from the predicted forward rate of spread (ROS) of the fire and wind speed. The shape and growth of the 'design fire' can then be determined mathematically and presented as an ellipse. To determined flame height, equations from CSIRO Project Vesta model for forest and woodland vegetation formations using surface, near surface and elevated fuels only is used. There is no allowance for bark and canopy fuels as the SFR is not expected to support a crown fire. Heath formations continue to use the AS3959 Method 2 process to calculate flame length. Fire width and flame vertical extent are then used to calculate the flame view factor and radiant heat load on the building from the design fire. Standard PBP rules for determining radiant heat thresholds are then applied.