What are bush and forest fires?
The Australian bush can, very generally, be described as shrubs, bushes and small trees from ankle or knee height right up to the tallest of trees. The term we use for bush here on the East coast of Australia is, “Sclerophyll forest”, and it covers much of the coastal and mountainous areas in and around NSW and the ACT.
The most common species in these forests are eucalypt trees and as such, fire in a sclerophyll forest is what most Australians probably think of when they hear the words “bushfire”. The leaves of eucalyptus contain flammable oils that burn extremely well in a fire. But it is also important to remember that fire is a natural part of the environment and not only are many of the species fire resistant, but some actually depend on fire to trigger new growth and to germinate seeds.
The ACT is surrounded by bush or forest, particularly in the Brindabella mountains to the west and north, where our greatest fire threat comes from. However, there are large tracks of what we call Lowland or open woodland that surround the rural and suburban areas of Canberra where both grassland and scattered shrubs and trees grow.
Bush or forest fires are quite different to grassfire because:
- there is typically more fuel (leaf and bark litter on the ground, shrubs, grasses, trees etc.) for the fire to consume in the bush;
- the height of the fire is much greater because the flames can reach well into the tree canopy above the tree tops; and
- As a result of the above two characteristics the heat is far greater.
As a result, bush or forest fires can be difficult for fire fighters to control and they are the most destructive of any fire and usually consume all in its path.
As a general rule, flame height is between three (3) to five (5) times the height of the fuel that is burning. So, if one (1) metre grass is burning then the height of the flame could possibly be between 3 to 5 metres high, but if you had a 10 metre tall tree burning the height of the flames could be 30 metres or greater.
Another point of difference between grassfire and bush or forest fire is that trees and shrubs drop leaves, bark and twigs (fine fuels) down to the ground underneath them. Over time this fine fuel gets thicker and deeper. Fire needs fine fuel to start and burn well, so the more that accumulates underneath vegetation the hotter and more dangerous a fire can become. If a fire was to start or an ember was to land in this fine fuel it would burn very well and would help the fire to climb up the bark and branches of a tree into the tree tops or canopy.
When a fire gets up into the canopy it is very hot and dangerous and is called a crown fire. This type of fire was experienced in the Canberra 2003 bushfires and is beyond the direct control of our fire fighters.
Bush and forest fires also create larger and longer lasting embers than from a grassfire. These embers can travel over many kilometres before landing and starting another, new spot fire well ahead of the main fire front. Falling material from a crown fire can also start new surface fires below.
In a severe fire, the heavier fuels such as branches, logs and tree trunks can become involved in fire, particularly if they are already dead and dried out.