Bushfire Report

                                    1.00

                                    0.90
  Relative Rate of Spread (R/Rss)

                                    0.80

                                    0.70

                                    0.60

                                    0.50                                       Fas t
                                                                               Medium Fas t
                                    0.40
                                                                               Slow
                                    0.30

                                    0.20

                                    0.10

                                    0.00
                                           0    5    10    15      20     25    30     35     40   45   50   55   60

                                                                        Elapsed Time (min)


                                           Figure 1. Graphic Formats of Equations (3), (5) and (7)
To predict the actual rate of fire spread for a given time or the fire run distance up to
this point of time, the quasi-steady rate of fire spread R ss needs to be determined first.
For forest and woodlands, the rate of spread equation derived from McArthur meter
can be used for this purpose and its mathematical expression is (Nobel et al. 1980):
                                      Rss= 0.0012 FFDI w e0.069θ                                        ……(9)
Where
                                     Rss = quasi-steady rate of fire spread (km/h)
                                     FFDI = Forest Fire Danger Index
                                     w= fine fuel load (t/ha)
                                     θ = slope (degrees)

For FFDI =80, w=25 t/ha and θ =0, R ss will take a value of 2.4 km/h which is equivalent
to 40 m/min. Therefore, the actual rates of spread will be 38, 35 and 30 m/min at time
=30 minutes after a point ignition for the above three fire scenarios. The fire run
distances can also be determined by doing numerical integrations of the fire run
distance equations or using the curves shown in Figure 2.