A note first about terms: in Australian English, ‘bush’ is what others might call forest, heath or scrub. The term was brought to Australia by early settlers who had previously lived and worked in North America, so this quintessentially Australian term is in fact an early American import! A ‘bushfire’ is a fire running wild in the bush.
Many botanists in the past have been forced to change their research to ‘bushfire regeneration’ after their plots were burned out, and the cost of fires has meant that there has been a great deal of research on the topic.
They will fight fire with fire, knowing that what they burn deliberately will grow back again, refreshed by the flames. Australia’s bush, after all, lived with fire for many millions of years, long before humans came here. The bush will grow back after the fires have done their worst.
Some of the sandstone beds are better bonded than the others within this ‘Hawkesbury sandstone’, but they are otherwise pretty much the same, right through the deposit. (Hawkesbury, in case you are wondering, was a minor 18th century English politician who had a local river named after him. The stone was later named after the river.)
In the last Ice Age, the sea level around Australia was much lower, due to all the water tied up in the northern glaciers. Then, today’s Sydney Harbour was a river valley, shaped by the jointing pattern in the sandstone. Joints, planes of weakness in the stone, were eroded into crevices which became valleys, with the more resistant sandstone forming ridges. Later, the sea level rose, creating a ‘drowned river valley’ with a characteristic fern leaf shape, the modern Sydney Harbour. A few of the higher ridges have a shale capping which offered rather better soil than the sand which derives from sandstone.
The first whites settled on the coast, then headed (a) for the flat land of the ridges, where roads were easier to build, and (b) for the richer soil on the shale-capped ridges. First, they built small farms and market gardens, then roads were built to service these, and soon the residences followed, as a young city grew. Down in the valleys, close to the sea, the bush was left alone. It was too hard to build roads down to there, and so people left it alone. Even today, much of the valley bush is preserved, with homes sitting on the ridges above: a sure recipe for trouble, because heat and flames rise.
Fuel builds up in the bush over a period of years. Gum trees shed their bark, branches and leaves, smaller shrubs in the under-storey die and are replaced by others, and after a few years of recovery, the lowest three metres or so is a closely packed mass of dead and drying twigs. Until they break and fall, these pieces of finely divided wood rot very little in the dry bush, and even on the forest floor, rotting is a slow business, for the sandy soil drains fast after rain. Heath regenerates fast.
Some of them can be ready to burn again, just six months after a major fire. Other areas can take ten to twenty years to be ready for a major burn. As a general rule, after 40 or 50 years, any area at all will be ready to sustain a ‘blow-up fire’.
Now for the physics of bushfires in Australia. When any fire starts, it begins very slowly. It takes time to develop from a maker of smoky wisps into a maker of misery. The dangerous fire is one that roars and gusts through the tree tops, the crowns of the trees, a firestorm traveling at 50 kilometres an hour or more, leaping ahead of itself, and destroying all in its path.
Crowning fires can cross 400 metres of open water, as the sparks and burning rubbish fly up in the roaring flames, and then tumble down on the other side. Any footage you see on your local TV will be of these crowning wildfires.
You will see flames gouting 30 metres or more into the air, searing the upper branches of gum trees, leaping across the fire breaks, and almost impossible to control until the weather improves.
Fires go fast uphill and slow downhill, but they do run downhill. On the forward side of any advancing fire, you will find a wind blowing towards the flames at the front of the fire. If you can set small fires on the far side of a ridge, they will gather strength and rush up, sucked in by the fire wind from the blaze on the other side, until the small fires meet the major fire coming the other way.