One of Premier Brumby’s key reactions to yesterday’s Bushfire Royal commission interim report has been to call for the burning and clearing of roadside vegetation from entry and exit roads to towns in bushfire areas. It sounds logical, but this may in fact be a counterproductive approach that will put people at risk.

However detailed research from the Forestry Commission suggests that effectively managed road reserves can in fact provide protection rather than fuel.

After the Ash Wednesday fires the then Forestry Commission republished a booklet called Trees, Farms and Fires by K.J.Simpfendorfer. It was first published in April 1978 and republished in August 1984. In the foreword to the republished booklet the then Chairman of the Forestry Commission R.J.Goss wrote:

Trees can protect us from fire — and they can increase the danger. While some trees are slow to burn, act as windbreaks and block out flying embers, others ignite easily and contribute to the burning debris that flies through the air towards property and people.

Trees, Farms and Fires goes on to deal in detail with the affect of forested windbreaks. For example, on page 15 under the heading “Windbreaks and building”:

Provided a well-formed windbreak is kept clean of flammable material internally and externally, it is one of the best means of protection from fire for homes and buildings. It functions in three ways:

  1. reducing wind velocity
  2. filtering out airborne debris
  3. checking the ground fire

1) Effects of windbreaks

A windbreak functions by providing a physical barrier to the wind currents, diverting them over its top to descend at some distance to the leeward. The area protected is that between the windbreak and where the wind currents have descended to ground level and assumed their original open ground speed. The effectiveness of windbreaks varies with their density or permeability — dense windbreaks give a high level of protection over a relatively small area whereas a more open one gives a lower level of protection but this extends over a greater area.

However the ultimate “total” protection is about the same; it becomes a question of selecting the right windbreak type for the uses in mind. Usually a dense windbreak is preferred around buildings and facilities for the greater protection over a smaller area that it provides whereas a more open type is more appropriate for paddocks and crops, giving less decrease in wind speed but extending it over a greater area.

And then on page 16:

The area protected or affected by a windbreak depends largely on its height … most protection is usually required around homes and buildings and the relevant points area:

  1. the greatest reduction in wind velocity is obtained with a dense windbreak
  2. with a dense windbreak:(a) the area with the greatest protection is immediately behind the windbreak
    (b) the area extends back to about 4 — 5 H (where H is the height of the trees that form the wind break)
    (c) wind increases rapidly from about 4 — 5 H reaching open velocity at about 12 — 15 H

There is an emphasis in Trees, Farms and Fires on “cleaning” such windbreaks of dead, more-flammable material such as loose bark and dead vegetation. It is clear however that much of the vegetation that exists along roadsides — based on the research — can in fact provide protection from fires.

Trees, Farms and Fires points out that such wind breaks catch embers and protection from radiant heat and goes on to provide a comprehensive list of tree and shrub species with details of where and how they grow and vital information such as the volatile oils they contain.

It is vital that the vegetation that remains on roadside reserves throughout the state is comprehensively and systematically evaluated. It needs to be managed around fire seasons and less flammable understorey species developed into dense tree wind breaks. Dead twiggy vegetation and bark need to be removed prior to the fire seasons. Careful development of local low flammability understorey to a canopy of eucalypts will provide the maximum protection from radiant heat and flying embers for people using roads, homes, other buildings, crops and farms.

To simply clear these areas and risk subjecting people using them to the full force of the radiant heat of fire and ember attack would be an unfortunate knee jerk reaction.

Peter Fray

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