This section will look into the issues of an increase in land area that is exposed to bushfire risk including questions of siting and town planning.
Much of the information included here comes from fact sheets provided by both the South Australian Country Fire Service and the Victorian Country Fire Authority. Links to these documents can be found within.
One of the most effective ways to reduce bushfire risk is the appropriate location of a house within a property. This means an understanding of the site must first be made to take in any contributing factors to fire spread. These may include;
Features of the topography - steep slopes, ridge lines, gullies, flat points
Existing vegetation - both within or close to the property
Existing water sources on site
Aspect - prevailing winds and solar radiation
Access to the site - both for escape and to allow for defense
The bushfire risk of the surrounding area - where is fire likely to come from, where is safest to escape to
Once the risks and opportunities of your site are understood, they can be avoided or worked in to your siting and design. Basic principles are;
Avoid siting the building on steep slopes, particularly at the top of narrow ridges. Fire burns faster uphill. As the slope increases so does the speed of the fire and its intensity. Flames and radiant heat preheat the vegetation ahead of the fire, drying it out and making it easier to burn.
The building should be located away from unmanaged vegetation and debris
Use natural gradients and existing water sources to direct water to landscaping closest to the building
Create wind breaks from prevailing winds and shade from the most intense solar radiation
Locate the building close to public roads and access ways, avoiding steep driveways if possible
In directions where fire is likely to approach from, limit window and door openings and increase landscape defence strategies
Defendable space is an area of land around a building where vegetation is modified and managed to reduce the effects of flame contact and radiant heat associated with bushfire. It breaks up continuity and reduces the amount of fuel available to a bushfire. It is one of the most important aspects of preparing properties for bushfire. This is because defendable space separates the bushfire hazard and the house. The greater the separation from the bushfire hazard, the lower the risk. Defendable space can prevent direct flame contact and minimise the effects of radiant heat on the house. This reduces the risk of house loss during a bushfire, regardless of active defence.
Defendable space is comprised of;
The Inner Zone
The area immediately around the house. It provides separation from fuel sources, reduces radiant heat, eliminates direct flame contact and reduces ember attack. Vegetation needs significant and intensive management. Fuel is managed to a minimum level in this zone.
The Outer Zone
This area sits between the inner zone and unmanaged vegetation. Vegetation is managed to a more moderate level to substantially decrease the ground fuel and restrict the fuels available to an approaching bushfire.
Landscape planting is an integral component within your overall bushfire protection plan. No plant is completely fire-resistant, however some are much more flammable than others. Counterintuitively, the inclusion of some planting has the potential to reduce fire risk, however it must be a part of a well designed and selected landscape.
The role that vegetation can play in preventing the spread of bushfire includes;
Trees reducing wind speed if correctly positioned and maintained
Fire retardant trees trapping embers and sparks from spreading
Fire retardant ground covers can slow fire spread through leaf litter
Fire retardant shrubs can protect trees from fire within the leaf litter
Fire retardant plants with high moisture content can act as thermal mass, absorbing radiant heat
What makes a plant more or less fire retardant is influenced by many characteristics. Broadly these are the readiness for the material to burn and how its structure will influence burning. Things to look for include;
Moisture content - Leaves with a high moisture content take longer to dry out and therefore longer
to catch fire.
Branching pattern - Trees with low or hanging branches allow for fire to spread easily from the ground up.
Density - Open airy canopies will ignite easier, however very dense plants are often more flammable as there is a higher fuel load readily available to burn.
Leaves - Wide, flat, soft and thicker leaves have a higher moisture content relative to their surface area and burn less readily than fine, hard leaves.
Bark Type - Generally stringy, fibrous or loose bark that sheds easily will ignite easier than smooth bark. Limiting this danger can decrease the spread of fire from the ground to upper branches.
Oils, waxes and resins - Some chemicals that are found naturally in plants will increase their flammability
Retention of dead material - Dead leaves, twigs, bark and branches that are retained on the plant or accumulate on the ground or in shrubs can increase the flammability
It is important to remember though that the arrangement of vegetation within a garden, rather than the flammability of individual plants, has a greater impact on how a bushfire will spread.
Provides a detailed guide to building siting and garden design
Provides further detail on plant selection and a list of recommended fire retardant plants