Appendix D - Bushfire Risk Assessment and Management Plan

Table 1 Relative Hazard Ratings of Fire Run Distances
                                                Relative Hazard Rating
 Fire Run Distance (m)                            Medium growth
                             Fast growth fire                            Slow growth fire
                                                  fire
 <=50                        0.42                 0.25                   0.18
 50-100                      0.75                 0.59                   0.47
 100-200                     0.84                 0.70                   0.59
 200-500                     0.91                 0.81                   0.72
 500-1000                    0.95                 0.89                   0.82


The relative hazard ratings shown in Table 1 can be used to quantify the effect of fire
run distance on overall hazard level by factoring the overall hazard index/score by a
corresponding relative rating of a give fire run distance. By doing so, the potential over
- prediction of hazard level for a block of bushfire prone land with a small fire run
distance can be avoided and therefore more cost-effective mitigation measures can be
chosen and implemented.
4. Implication for Prescribing APZs
The objective of APZs is to mitigate the impact of bushfire by providing a buffer zone
between a development and surrounding bushfire hazard. Apparently the wider the
APZ prescribed for a site, the less impact the site will be subject to. However, if an
APZ is over prescribed, then developments on the subject site may become more
costly. In view of this, it is necessary to accurately quantify the level of attack from the
bushfire hazard for a given site. To do this, NSW RFS has developed a systematic
methodology which comprises a set of equations (Tan et al. 2005). The principle of the
methodology is to predict the potential bushfire behaviour and the resultant radiant heat
flux to which a site at a given distance from fire front may be exposed. With the NSW
RFS methodology, the required minium width of APZ can be determined by finding the
distance at which the maximum radiant heat flux to which the subject site may be
exposed is equal to the predefined radiant heat flux threshold.
The rate of fire spread equation used in the methodology predicts the quasi-steady rate
of spread and therefore the APZs prescribed by using this methodology might be more
than adequate in the cases of bushfire prone lands with small fire run distances. In
order to prescribe cost-effective APZs, it is necessary to take the effect of fire run
distance on bushfire behaviour and the resultant impact into account. This can be
done by incorporating Equation (1) and Equation (2) into the current methodology. For
instance, the APZ for forest under FDI = 80 and slope = 0 degree is determined to be
28 m. when taking fire run distance into account, the corresponding APZs under the
same conditions are determined and shown in Table 2.
As shown in Table 2, the modelled widths of APZs for three different fire scenarios vary
from fire run distance and will tend to be a constant when fire run distances become
large enough. This means that a smaller APZ needs to be prescribed if the maximum
fire run distance is less than a threshold distance. For instance, if the maximum fire
run distance of a block of land covered with forest is 100m, then the corresponding
APZs for fast, medium and slow growth fires will be 24m, 21m and 19m compared with
a APZ of 28m for a steady state fire.