Specific Property Risk and Bushfire Risk Analysis Part 5

Ken Walker

In the past few articles, we have looked at specific things a property owner can do to protect themselves against an approaching bushfire. Let’s now look at analysing our specific risks and what equipment and processes will work to mitigate or reduce that risk.

How long are bushfire seasons anyway?

About two decades ago we could have said the occurrence of bushfire was pretty much consistent and the really big ones came around every 15 to 20 years. There have always been bushfires in Australia but mostly with the rapid suppression efforts of firefighters, they have generally been able to suppress them quite quickly.

Something has changed over the last decade and fires occur more regularly and with greater ferocity. As a firefighter in the 80’s and nineties. Large bushfires in Queensland and New South Wales seemed few and far between, with Victoria and South Australia seemingly bearing the brunt of the devastation. That is certainly not true in 2020.

In 2019 we saw quite devastating New South Wales fires in June (the winter). There may have been a drought but the season came earlier than normal. Then out of the blue Queensland also had the bushfire season occur earlier and more devastating than normal.

Amongst firefighting circles in California (USA) it has been said that the fire season seemed to be all year round. If this is the case, and there seems to be evidence supporting such claims. It is a worrying trend and communities throughout Australia and the world are going to have to adjust their thinking. If they are going to survive, in terms of preparedness, response and recovery from bushfire.

In Australia, our preparations are now being tested regularly and the general top-down structure of command and control may be becoming obsolete. Thus, community resilience in the future may see communities becoming more involved in bushfire preparedness, response and recovery. Discussions around this are for another article.

Even so, it is becoming more and more evident that the fire seasons are longer and we must make changes, as landholders, to overcome such challenges.

fire danger rating sign

Topography and ground slope.

Firstly, let’s look at the ground around our property. The amount of slope will have some impact on how quickly a bushfire approaches. Especially if there is an increasing slope leading right up to your property on the northern and south-west sides.

There is a general rule of thumb for determining how a given slope will increase the bushfire speed on top of other factors like wind. The fire speed will double its speed due to the slope for every 10-degree increase in slope. I.e. if the speed of fire was 5kph on flat ground the impact of 10 degrees would be 5 kph x 2 = 10 kph.

And again if the slope increased by another 10 to 20 degrees then the speed of the approaching bushfire would be 20 kph ( 10 x 2 degrees = 20 kph).
The same is correct for a decreasing slope. If a downslope is -10 degrees. A 5 kph rate of spread would be halved to 2.5 kph. This would seem great if you live in a valley, but other wind factors must be analysed in conjunction with slope risk factors. I.e. Wind can be channelled up a valley at far greater speeds than flat open paddocks.

Finally, in terms of topography (slope) if your property is situated on the relative flat open country then the effects of the slope will be minimal. Those living at the top of slopes need to make the defendable space two metres longer for every one degree of slope.

I.e. if you live on flat ground then the managed defendable space would be 30 metres. If the slope to the north or west is 10 degrees, then the managed defendable space should be at least 50 metres.

When determining the size of fire systems and amount of fire-resistant plants. You will need to think about the number of resources required to protect the increased area.

How much vegetation would be ok to have?

Vegetation management is one of the most important factors when it comes to protecting your home from an approaching bushfire.

As mentioned in other articles, all areas within 10m of the property you are trying to protect, should either have a continuing high moisture content and/or have other fire-resistant capabilities. This vegetation should be no greater than 1200mm to 1600mm high.

The amount of dead material within the plants must be minimal and/or well managed during the fire season. Such vegetation must not interfere with impact and other types of sprinklers. Remember the more shrubs and plants within this are the more likely fire hoses could be stuck and hard to handle. Just try and think about how you will fight a fire when locating plants.

The outer zone – 10 to 30 metres

To the north and western areas, we need to create a continuous barrier. This doesn’t mean we have to build a hedge out of fire-resistant trees. Locate the trees strategically so embers can’t get past without hitting one of the trees or shrubs. Remember shrubs shouldn’t be located directly under the trees as they may provide a ladder for the fire to reach the canopy of the trees. Known as a crown fire, which can be very devastating.

You need to be able to get under the trees anyway so you can remove all fine fuels. Fuels with a diameter less than 6mm.

The eastern and southern sides of the building can have breaks for views but still ensure the vegetation within the defendable space is of a fire-resistant nature.

the devastating results of a bushfire in australia

The history of bushfire in your area

Where humans congregate and set up home there is increased potential for fire. Whether accidental or deliberate. Do you live in a populated area or is there a populated area to your north. When analysing risk we look at how many and how recently fires have occurred in your area.

If fire regularly occurs (yearly) within 50 km of your home then the risk is quite high. Even if you may have never had a bushfire pass your property, for example. Your neighbour, 10km up the road, had their house burnt down 10 years ago. Therefore analysis should take into account all areas within 50km. Unless the area to the south is the ocean!

Roads in and out of your property.

If there is only one road in and one road out of your property then protection needs to be very good. You may decide that the risks are too great and you need to leave early on high-risk days.

Consider how you may leave if indicated that you should evacuate and staying and defending just doesn’t seem like something you want to do. Until a person is confronted by a fire they won’t really know what their reaction may be. Could you provide leadership if your wife and children were screaming and yelling and totally terrified? Be aware of all egress and access routes around your property.

A brief word on insurance.

As climate change continues to kick in. Ensuring your assets is only going to get more expensive. Currently, a number of property owners check their insurance is up to date every fire season and leave early when the fire danger is severe, extreme or code red. They can always rebuild.

The Federal Emergency Management Agency (FEMA) in the USA have indicated that generally governments only provide between 6% to 15% of the resources to rebuild a community. So insurance is very important to cover the gap. We can’t expect the taxpayer to cover rebuilding costs. Also, those that have insurance often recover from emergencies much quicker, but recovery can still take many months or years.

bushfire out of control

In conclusion.

When determining the resources required to protect your property it must include all factors. It would be simple to just place a sprinkler system on your house and think that will protect you and your family. I think we have established that this is not the case and we must account for everything from roads in and out to the type of vegetation we plant.

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