Topography

Topography and slope influence the speed that a fire will travel, its intensity and the height of flames.

Fire burns faster uphill. As the slope increases so does the speed of the fire and its intensity. When burning up a slope, flames and radiant heat preheat the vegetation ahead of the fire, drying it out and making it easier to burn.

As a rule, for every 10˚ slope the fire will double its speed. For example, if a fire is travelling at 5 km per hour along flat ground and hits a 10˚ slope, it will double in speed and travel up the hill at 10 km per hour.

Fires tend to move more slowly as the slope decreases. The flames reach less fuel, and less radiant heat preheats the vegetation in front of the fire. For every 10˚ of downhill slope the fire will typically halve its speed.

In areas of rugged terrain where topography includes steep slopes and long areas of continuous vegetation, more extreme fire behaviour is likely as it is driven by the landscape. Other topographical features such as gullies in more mountainous terrain can channel fire and increase its intensity and speed.

Other features such as rivers or other natural water bodies may affect how a fire traverses across a landscape. This may depend on their size and aspect, or the direction a fire approaches from.

Key consideration for planning decision making:

Topography is detailed in both the bushfire hazard landscape assessment and bushfire hazard site assessment.

Where the topography is in large hazard areas, the bushfire hazard landscape assessment will need show where extreme bushfire behaviour may arise.

In the bushfire hazard site assessment, topography is used to assess the slope under vegetation within 150m of development. This informs the separation distance required between the bushfire hazard and new development.