Slope

Slope plays an important role in how a bushfire behaves

This page includes a step-by-step tutorial for determining the Site slope and Effective slope of your property. These values can be used to calculate your Bushfire Attack Level (BAL).

Slope is the average rise and fall of the lands surface. A steep slope is one that rises (or falls) suddenly, while a shallow slope is one that rises (or falls) slowly. Slope plays a very important role in How bushfires behave. For example, fire travels much faster when moving uphill (up-slope) compared to downhill (down-slope), while a steep slope is generally more dangerous than flat terrain.

There are two slopes to consider when determining the level of bushfire hazard to your home or the site of your new build: namely, the Site slope and the Effective slope. Site slope is the average slope of the land surrounding your home or the site of your proposed build – specifically, the area between the building and the nearest Classified vegetation. The Effective slope is the average slope under the Classified vegetation.

Drawing illustrating the difference between Site slope and Effective slope.

Site and Effective slope affect the Bushfire Attack Level experienced at the house

  • Slope can be calculated in a variety of ways. It is possible to calculate slope on your own using a free online tool or mobile phone app. Alternatively, a professional land surveyor can be contracted to do the work for you. The information below should help you in making a decision.

    • If you know the elevation of the start and end points of the slope, then the average Site slope can be calculated as the change in elevation (rise) divided by the distance (run) between the two points.
    • You could also use a handheld device called an inclinometer to calculate the slope. An inclinometer (also called a clinometer) is a device for measuring angles of slope. These instruments vary in cost and complexity.
    • There are also mobile phone Apps that can be used to calculate slope. Try searching your mobile device’s App store to find one that works for you.
    • A map with topographic contours can also be used to estimate slope. MapShare is a good resource for people living in Victoria.
    • Finally, you could contract a contour or site survey through a qualified land surveyor. This is the most accurate method of determining slope, although this option can be prohibitively expensive if you are working with a limited budget.

  • The Site slope influences how much of the fire can be seen from your house – in other words, its level of exposure to radiant heat emanating from the burning of classified vegetation (see Bushfire risks for more information). You will need to determine the Site slope to calculate your sites Bushfire Attack Level.

    Using one of the methods from Step 1:

    • Calculate the slope of the ground between the site (of the proposed or established building) and the nearest area (or areas) of classified vegetation. When you calculate BAL, the Site slope is capped at a maximum of 20 degrees. For example, slopes steeper than 20 degrees should be set as 20 degrees in the BAL calculator.

  • A fires behaviour and intensity is strongly influenced by Effective slope. You will need to determine the Effective slope to calculate your sites Bushfire Attack Level.

    Using one of the methods from Step 1:

    • Calculate the slope of the ground underneath the classified vegetation closest to the site of your proposed or established building. Like the Site slope, the Effective slope is capped to a maximum value – in this case, 30 degrees. For example, Effective slopes steeper than 30 degrees should be set as 30 degrees in the BAL calculator.