Off the charts: Bushfires may be 20 times more intense than the largest fires humans can control

Ten times the fuel means 100 times the intensity

Hardly anyone is talking about these numbers yet they show just how far beyond our control the pyroconvective firestorms are and why we need to be so much smarter at preventing them. They also show how irrelevant temperatures onsite are, compared to fuel load and wind speed.

Controllable fires are 3MW per meter, but we now have loads of 70MW/m

Not only are these fires obscenely, catastrophically intense, it doesn’t matter how much fire fighting equipment we buy, how many dams we empty, they are a man-made disaster, and we’ve known for years how to prevent them. (Some would say, thousands of years). The message in here is that cool controllable burns are tiny, less damaging, and far less intense. The pyroconvective monsters are totally different creatures.

Andrew Bolt interviewed fire expert David Packham in November:

Top fire expert David Packham says forget global warming. It’s the reckless failure to burn-off fuel loads that have turned parts of Australia into death traps. Near Melbourne “we’re looking down the barrel in these areas at 1000 deaths”.

His key point is that if we increase the fuel by ten, the fire intensity is 100 times greater. Today we are dealing with fire intensities with figures like 70MW/m. The maximum we can extinguish — with helicopters, bulldozers, tankers, is just 3MW/m.

“We now have fuel loads ten times greater than … what the indigenous people had.”

”Fuel load …is not behind a lot of it, it’s behind all of it.”

“The amount of fuel determines the amount of energy that is released”.

“The amount of fuel determines how fast the fire moves”

“The most dangerous place in the world is just north of the Yarra and the north facing slopes fo the Dandenongs.”

” The big threats are not just life, but the environmental damage, the threats to the water supply, and our electricity network.”

h/t to Skeptical Sam. Thanks to Roger Underwood for assistance.

The fireline intensity of the Australian 2020 fires is “off the chart”

The fireline intensity tells us how much damage a fire will do, how long it takes life to recover, or even whether it will recover. It also helps us figure out what we should try to do to minimize the damage. Bear in mind Packham was speaking of potential fire damage long before the huge New Year’s Eve fires.

The 1983 Ash Wednesday Bushfires in Victoria were estimated to be 60MW/m:

The freakish conditions spawned unique effects: a car was forced 90m along a road with its handbrake on, burning mattresses were seen hurtling through the air, … road surfaces bubbled and caught fire and sand liquefied to glass. CSIRO experts later reported that, from evidence of melted metal, the heat of the fires after the change rose to 2000 °C, exceeding that recorded during the Allied bombing of Dresden in World War II. In fact, the Ash Wednesday fires were measured at around 60,000 kilowatts of heat energy per metre, leading to similarities with the atomic bomb dropped on Hiroshima. — Wikipedia (Baxter 1984)

The intensity of fire is hard to measure, especially live. We can get some approximation “after the fact” by looking at the damage. Be aware, there are many variations of both units and estimates, but the same message keeps coming. The intensity of the wildfires that “make their own weather” — the true firestorms are a magnitude far beyond what we can control.

Packham talks about the Byram Equation (1959) of Fireline Intensity. Defined “as the rate of heat output per length of fireline (I), expressed as kilowatts per metre of fire edge, as shown in Equation 1.”.

He argues there is one more factor — which is the dryness of the fuel (moisture content). But notice what else is missing — temperature. It’s not even worthy of being a recognised variable. The R, or forward rate of spread would be a result of the wind speeds and slope, with some effect from temperature.

Note the scale: the experts says hazard reduction burns should be kept below 0.3MW/m. A low intensity fire is defined as less than 0.5MWh/m. So everything above that is “high intensity”. Australian trees need “high intensity fires” to germinate seeds, but these are only 0.5MW/m and above, not 70MW/m firestorms. So half of the chart below describes low intensity cool fires at a hazard reduction level. With intensities above 1MW the 70MW firestorms would barely register across the top corner in this logarithmic graph.

The rate of spread is determined partly by the amount of fuel, so high fuel levels rapidly turn into intense uncontrollable fires even at lower wind conditions (or lower rates of spread). There are four types of numbers on the graph below. The fuel load below is the x-axis. The side axis is the rate of spread. The intensity is the diagonal line result in the graph. The numbers on the lines like “6.00m” are the flame lengths. So in this case six metre long flames are generated in a 14MW/m fire.

Next — a similar graph in an alternate form . Byram’s fireline intensity is the energy release rate per unit length of fire line – so kW/m. The graph above uses calories per cm2. And below (grimace!) in the form of BTU/ft2.

In Figure 1 the 2000 BTU per foot squared is equivalent to a 7MW fire, 70 MW/m is “off the charts”.

Fires are also mosaics, with a patchwork of different intensities side-by-side.

As Pyne et al. (1996) demonstrated in their Figure 2.18 (reproduced in Figure 1 below), fires of equal intensity may, in fact, be produced in quite different types of fuel and with different forward rates of spread. Average fire intensity around the perimeter of a fire also varies by a factor of up to 10 (Catchpole et al. 1992)

There are several ways to get “awful fires”.



Flame length

It would be nice if we could just use the height of the flames to estimate the intensity, but it’s only so useful. In large fires the flames lean forward diagonally with the wind, so the length is a lot longer than the height. But even if we could measure the length accurately, it maxes out long before we get to pyroconvective fire levels. As Phil Cheney explains, it’s only useful in fires of 10MW/m or less:

After 10 000 KW/m (a crown fire in dry forests) there is little difference in the height of the flames or the look of the defoliated forest. The main effect thereafter is in the influence on the atmosphere but this is also strongly affected by the temperature and moisture structure in the atmosphere.

It’s useful though to know just how much of our understanding and research on fire is based on studying small, low intensity fires.

Intense fires cause much more damage

Now down to the business end of the charts. At what level do fires become truly awful catastrophes:

“McArthur (1962) noted that fire damage was closely related to fire intensity. Thus a reduction in the destructiveness of wildfires can generally be achieved by broadscale prescribed burning—where the primary objective is to reduce the accumulation of fuel over a wide area. Such broadscale reduction in fuel should result in significantly decreased rates of spread and intensities of a wildfire, which should in turn assist suppression forces in controlling the fire (McArthur 1962).

Not surprisingly, it’s difficult to measure “live” fire intensity:

Despite fire intensity being considered a good indicator of fire behaviour in general, fire intensity is difficult to measure accurately (Burrows 1995), especially over short periods of time. Often it is only estimated post-fire.

Fires are patchy things producing a mosaic pattern that encourages diversity of species (though, who knows, possibly a broadscale firestorm reaches 100% consumption of fuel that makes it less patchy; this report does not say that.)

Thus, fire creates environmental diversity at several scales. At the broad scale (i.e. across thousands of hectares) there is a mosaic of areas burned one, two, three, up to 50 or so years previously. At the local scale (i.e. across single hectares) spatial variation in fire intensities produces a patchiness in the resultant effects.

It may take up to 40 years for the mammals to recover if they lose shelter, food and breeding sites — even animals that survive the fire are likely to die in the aftermath:

Suckling and Macfarlane (1984) commented that the rate of survival (during a fire) of mammals is a function of fire intensity. However, longer-term recovery also depends on the recovery of habitat (i.e. shelter, food, breeding sites), in both composition and structure, which may be quite rapid or may take 20 to 40 years for complete recovery (ibid.). Suckling and Macfarlane (1984) also found that predation and starvation caused a high rate of mortality in fauna after high intensity fires.

Table 2: A low intensity fire is Class 1. High Intensity with seed regeneration is Class 2. A full crown fire is Class 4 with intensities of 70MW/m.

Seeds only need fires greater than 0.5MW to be activated. A high intensity fire is only a “class 2″ level fire. More intense fires will change the structure of the forest — reducing canopy cover and increasing the scrubby understorey:

In respect to the flora, fire intensity can have a marked effect on the extent of post-fire recovery and on the relative abundance of plant species regenerating from seeds or vegetatively (Christensen et al. 1981 ; Lutze & Terrell 1998). Low-intensity fires (less than 500 kW m-1) will result in a low overall death rate of trees because the amount of bark removed from and damage to the tree boles and of crown scorch are minimal. On the other hand, greater damage occurs to the boles and crowns of trees and more bark is consumed in high-intensity fires (greater than 500 kW m-1) and higher plant mortality is likely. High intensity fires are also more likely to affect the canopy cover of the overstorey, which may enable the better establishment of understorey species—Chesterfield (1984) noted that bracken increased in dominance under a eucalypt overstorey after a number of ‘hot fires’ decreased the canopy cover.

Eucalypts are remarkably well adapted to low to moderate intensity fires:

McArthur (1962) noted that the genus Eucalyptus is remarkably resistant (in terms of overall survival of individual trees) to fires of low to moderate intensity. He also noted, however, that their resistance to fire depends on the intensity of the fire and the seasonal dryness as indicated by the Keetch-Byram Drought Index. Many adaptations in the genus also make them resistant to damage from high-intensity crown fires, as only a few species are likely to be killed outright in such fires.

Recovery from fires depends on the season they occur — dry seasons make it harder for seeds and tubers to recover:

Abbott and Christensen (1994) noted that fire intensity depends largely on the time of the year (i.e. the dryness of the litter and the stage in the life cycle), prevailing weather conditions and the period since the last fire (i.e. the amount of fuel available). The recovery of vegetation also depends on the weather conditions following the fire (Chesterfield 1984) and the legacy left in the soil in the form of seeds, bulbs, corms, tubers and lignotubers (Abbott & Christensen 1994). The effects of a fire will also depend on the season, the species involved, the residence time of the fire, fire frequency and the dryness of the vegetation and soil. Therefore, indicators of fire severity may indeed be a more effective measure of the ecological effect of the fire than fire intensity indicators alone.

Direct control and suppression of fires only works at 2- 3MW of fire intensity, above that only “indirect methods” are left.

Fire intensity directly influences the cost of suppression, as the method of suppression depends on fire behaviour or, more generally, fire intensity. For low-intensity fires (i.e. less than 2000–3000 kW m-1), direct attack methods may be used. For more intense fires, however, indirect attack methods are generally required, especially on the fire front.

Fire intensity is the most important factor in the survival of houses. As well as ember attack, firestorms create winds that damage homes:

Wilson (1984) stated that fire intensity is the most important determinant in whether a house survives a nearby wildfire, as compared to its construction material, the presence of flammable objects near the house and the presence of plants less than 5 m tall within 40 m of the house. Wilson and Ferguson (1984) studied houses that were affected (either destroyed or partially burned) in the fire at Mount Macedon on 16 February 1983. Fuel loads (including elevated and surface fine fuels, but not including the crown fuels) in the adjoining forest were up to 21 t ha-1, forward rates of spread in the town were of the order of 3–4 km h1 and the houses were exposed to fire intensities ranging between 500 and 60 000 kW m-1. Of the total of 450 houses surveyed (of which 234 were destroyed), about 10% were exposed to crown fire in the adjacent forest canopy (crowning was infrequent once the fire entered the township) and 50% were exposed to surface fire intense enough to fully scorch surrounding trees. Almost 40% of houses were exposed to a less intense surface fire, but one that was nevertheless accompanied by strong winds and airborne embers. Wilson and Ferguson (1984) noted that houses exposed to high-intensity fires are subjected to severe thermal stresses, and sometimes the strong winds associated with high intensity fires can cause structural damage.

There is more detail on the effects on the forest from this chart from Burrows 1984. The message is the same, minor permanent damage to trees starts with 1 – 2MW/m fires, and by 5MW/m the fully grown pine trees are destroyed, and even the rootstock of gums may be damaged to the point where it will not resprout and recover. Trees that are hundreds of years old can be wiped out.

Prescribed burning should be done with fire intensities of only 0.3MW.

When fuel is allowed to accumulate there would be very few days each year when it would be safe to burn. A lack of hazard reduction makes it harder to do hazard reduction.

McArthur (1962) described, for prescribed fires, characteristic fire behaviours based on a series of fire intensity ranges. He implied that, to ensure that no unacceptable damage occurred in commercial (native) forests, all prescribed burning should be carried out with fire intensities less than 340 kW m-1 (or 100 BTU ft -1s-1). Gill et al. (1987) described the optimum prescribed burning days as those when fire intensities were between 60 and 250 kW m-1. Luke and McArthur (1978) suggested that the upper limit for prescribed fire (for the purpose of fuel reduction) is 4000 kW m-1. However both Cheney (1981) and Christensen et al. (1981) suggest that it would be extremely optimistic to expect a result at this intensity with little or no damage

These next graphs come from an EUreport (Forest Fire Fighting Terms Handbook), the Burrows report from Western Australia (1984) and a 2004 Victorian Govt report on the economic and environmental damage caused by fires.

Why do the Greens hate forests and Koalas so much that they put their own political agenda ahead of them?

REFERENCES

Baxter, John (1984). Who Burned Australia?: The Ash Wednesday Fires. Kent: New English Library. ISBN 0-450-05749-6.

Burrows, N.D. (1984) Describing Forest Fires in Western Australia, A Guide for Fire Managers, Forests Department Western Australia (WA). Technical Paper No. 9.

Karen Chatto and Kevin G. Tolhurst (2004) A review of the relationship between fireline intensity and the ecological and economic effects of fire, Dept of Environment and Sustainability, Research Report 67, Victoria.

Forest Fire Fighting Terms Handbook (2009) Multilingual handbook for fire terms across European borders during forest fire fighting,F.I.R.E. 4 Project Co-financed by the European Commission DG Enviroment Civil Protection Unit

Photo by Christopher Burns on Unsplash

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