Appendix D - Bushfire Risk Assessment and Management Plan

EFFECT OF FIRE RUN DISTANCE ON BUSHFIRE
BEHAVIOUR AND ITS IMPLICATIONS FOR BUSHFIRE
HAZARD RATING AND PRESCRIPTION OF ASSET
PROTECTION ZONES

                                       By Dr Zhenxiang Tan
1. Introduction

In Australia, the bushfire behaviour prediction system used by Australian land
management/fire agencies is based on McArthur Fire Danger Meter (McArthur 1973)
developed on the basis of over 5000 documented wildfires and 500 prescribed fires
(Chandler et al. 1983). Potential fire behaviour is predicted from fire danger ratings
derived from meteorological data (McArthur 1966, 1967, Noble et al. 1980, Luke and
McArthur 1978). The predicted behaviour by the meter is only for quasi-steady or
established fires with no provision for initiating fire behaviour. Therefore the potential
bushfire behaviour based on the meter may be over-predicted in the case of an
unestablished bush fire. The hazard ratings and the prescribed widths of asset
protection zones based on the over predicted bushfire behaviour will then be over
estimated in such a case. In order to rate bushfire hazard or prescribe APZs properly,
it needs to determine whether a block of bushfire prone land will allow for a fire to
develop to a quasi-steady state or not and take the effect of an unsteady fire on hazard
rating or APZ prescription into account accordingly.


In theory, a bushfire will eventually develop to a quasi-steady state if the distance of fire
run is long enough. This means that there must exist a relationship between bushfire
belabour and fire run distance. This paper is aimed to establish such a relationship and
analyse the implications for rating bushfire hazard and prescribing APZs. To do this, a
theoretical model developed by Cheney and Barry (1969) for predicting the rate of fire
spread at its early stage has been used to derive the corresponding equation of fire run
distance. Based on these equations, an approach for predicting the effect of fire run
distance on rate of fire spread has been established and the resultant implications for
rating bushfire hazard and prescribing APZs have also been studied.
2. Mathematical Model

It has been identified that bush fires from a point source ignition increase their spread
rates as they grow and eventually develop to the so-called quasi-steady state. The
equation predicting fire growth in forest fires given by Cheney and Barry (1969) was:
        R = Rss e-a/t                                                      ……(1)
Where
        R = rate of spread at time t
        Rss= equilibrium spread rate
        t = elapsed time since ignition
        a is a constant.
The integrated form of Equation (1) is:
              t                                t
        S=    R
             0
                   SS   e -a/t dt   =Rss      0
                                                   e -a/t dt                ……(2)

Where
        S = fire run distance