Appendix D - Bushfire Risk Assessment and Management Plan

                                     1.0

                                     0.9

                                     0.8
   Relative Rate of Spread (R/Rss)




                                     0.7

                                     0.6                       Fast
                                                               Medium
                                     0.5
                                                               Slow
                                     0.4

                                     0.3

                                     0.2

                                     0.1

                                     0.0



                                             Fire Run Distance (m)


             Figure 3. Relative Rate of Spread and Fire Run Distance
3. Implication for Rating Bushfire Hazard
Fire run distance influences the rate of fire spread and therefore fire hazard. The
hazard associated with a given vegetation type is usually measured by fire intensity
which is expressed by (Byram 1959):
                              I = H W R/36                              ……(10)
Where
  I = fire intensity (kW/m)
  H= heat yield of the fuel, fuel related constant
  W = Available fuel load (tonnes/hectare)
  R = Rate of spread (km/h)

Based on Equation (10), the fire intensities of steady fires and non-steady fires can be
determined by:
             I = H W R/36                                                ……(11)
             Iss = H W Rss/36                                            ……(12)
Divide Equation (11) by Equation (12), we have
                      I/ Iss =R/Rss                                      ……(13)
Equation (13) suggests that the relative rate of spread is equivalent to the relative
hazard rating for a given fuel type. The relative rate of spread for a given fire run
distance can be predicted with Equation (1) and consequently its implication for hazard
rating. If fire run distance is grouped into the categories of <=50m, 50-100m, 100-
200m, 200-500 and 500-1000m, then the relative hazard rating for a given vegetation
type can be determined for the three different fire scenarios ( See Table 1).