Bushfire backgrounder

Bushfires are a part of high summer in Australia. In winter each year, Australians carry out control burns, small fires aimed at reducing the amount of standing fuel. These may help to contain the fires or stop them, but given the wrong weather, no amount of control burning can stop fires happening somewhere. The science is against any other outcome.

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.

First, let us consider the biology of bushfire in Australia. Fire is a natural part of the bush cycle, so the natural environment should survive fairly well, just so long as there is no heavy rain, too soon afterwards. That is why the fire fighters will concentrate on saving property and lives.

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.

Next, let us consider the geology and geography of urban Sydney bushfires. When the first Europeans reached Australia in 1788, they settled in what is now Sydney, either on flat land near the sea or on the ridges.

Sydney sits on a bed of sandstone, two to three hundred metres thick, with joints running north-south and east-west. It was laid down in a Triassic delta, rather like Bangladesh today, with a huge river braiding back and forth, washing out the finest minerals, the clay and other mineral-rich sediments, and leaving just the quartz grains behind. The grains were rounded, and had probably been in an earlier sandstone somewhere else, but they settled where Sydney is now, almost 200 million years ago, waiting to play their part in shaping modern Sydney.

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.

Now let us look at the question of weather and bushfires. The weather is the last factor in the bushfire equation. At the moment, we have hot dry nor-westers, gusting at up to 50 knots, pushing the fires downhill as well as up. Usually, a fire front can be beaten as it crests a ridge.

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.

In this style of fire-fighting, the major fire limps over the ridge, only to find that most of the fuel in its path has already been burned. Starved, it falters like a wounded beast, and puny men and women rush in to attack it with sprays and hoses. But with high winds, this ploy is too dangerous to attempt, as the fire lighters in its path could easily be over-run, as it leaps over the fire break they have just made.

Within hours of the fire, the seeds will be dropping from the woody fruits of the she-oaks, Hakeas and Banksias, and the trunks and underground stems of other plants will already be starting to shoot. In three weeks, there will be green all over the bush. In time, the bush will recover, and so will the animals. The homes can be rebuilt, and lives, so long as they have not been lost, will go on. It is all part of the natural cycle. The animals will take longer, but some will survive, and others will move in from unburnt areas, but recovery is a slow natural cycle.