Living With the Bush

A point you make in relation to the siting and how siting and building surrounds can influence wind exposure in a building is that the presence of trees is a protective feature in this instance.

Can you explain that?

Justin Leonard ---That's right. A barren landscape allows winds to move across the landscape but be very aggressive at and near ground level. But, as you increase the roughness or the texture of that landscape by putting trees and built environment and elements or even perturbations in the terrain, that tends to lessen the wind speed experience that close to ground level.

You may have a high wind speed well above the terrain but you have a thicker, we call it a boundary layer or region, where the flame speed isn't aggressive. So putting vegetation around your house or nestling your house in an environment where there are other buildings actually gives you some degree of protection from that exposure.

I probably should have asked this earlier, but is it also the case that having trees around a house may shield it from ember attack to an extent?

Justin Leonard ---That's right. The trees can also be effective in trapping embers. They are a complex structure. So, if an ember contacts another tree it can be stopped or entrapped or stalled and it may just fall to the ground or extinguish. Hence they are an effective way of attenuating or reducing the ember attack that may then pass on to a structure.

But of course if the tree catches fire, it then becomes an independent source of exposure?

Justin Leonard ---That's right. That's true of many elements. Many elements can be barriers and they also can become sources throughout the fire exposure. ..............

As you say in your report, there are many event paths that can lead to the ignition of a building?

Justin Leonard ---That's right, a very broad extensive range, not that even detract from the way that various elements adjacent to a house may actually ignite and then represent a local ember, radiation or flame attack on the structure itself, and that could be a very significant event path that wouldn't automatically be obvious.

So, for example, a woodpile right next to a house?

Justin Leonard---That's right. A fence, a car, even a rubbish bin or stored material are all very common elements found adjacent to houses that may not be readily identified as a key event path leading to structural loss.

Bushfire Royal Commission testimony of JUSTIN LEONARD CSIRO on building standards and bushfires