To achieve this a core fire is started. This generates its convection column after a few minutes. Any fires lit around this on the perimeter of the block then get sucked towards the core, giving a safe burn.
There are some factors to watch out for:
The indrafts only affect about 100 m radius - any fires lit beyond that radius will behave independently.
Any bulk overhead winds can get entrained in the indraft and quickly "appear" on the ground, producing surprises for those who lit the fire.
The corners of a block are always difficult to keep within the indraft.
Any perimeter fire that gets hotter than the core will take over as the core, dragging the rest of the burn towards it.
A good indraft will override the effect of slope on fire spread, but only as long as it remains the dominant effect - if the core cools off the indraft will abate, and the fire will run upslope.
For safety purposes the fire should only reach the perimeter as a backing fire. The flames are so large that they can easily cross a fire trail if they are allowed to lean over such a break.
Slash burns do not burn according to the McArthur Indices.
These indices refer to the flammability of fine fuels only. These fuels change their moisture contents readily and wildly in response to rainfall history, drought levels, and the daily cycles of temperature and relative humidity. Larger fuel particles (such as logs) have much smaller surface area to volume ratios - making it harder to get moisture in or out.
If you are in extreme drought the large fuels are bone dry and it will take more than a small amount of rain to wet them.
To get a slash fire going in the conditions at Boboyan requires igniting the fine fuels first, which was sometimes a bit tricky. Once the large fuels ignite it is a matter of "stand back and grab the marshmallows".
Block 1 was burnt the next day, but it was rather borderline. The theoretical equations for the Byram Keetch Drought Index and the Drought Factor do not allow for factors such as heavy morning fogs or frost which alter the drying rates. It was intended as a safety zone should things get out of hand on any of the slash burns - a burnt-out eucalypt block makes a useable refuge in such a situation.
Getting the right burn was difficult. Under the extreme drought levels present at the time, a drought factor of 10 was the normal state, and this is too hot to burn. We had to wait for rain, and then wait 2 days to get the right drought factor. 1 day too soon and the fire would not burn, 1 day too late and the fire would be too hot.
It may seem that in late May it should be more of a problem getting things dry enough to burn - not the other way around. When in a severe drought (the BKDI was 135 mm) things tend to go counter-intuitive like this.
After burning Block 1 we tried to burn Block 2 - the first slash block. We expected none of the difficulties from Block 1 but were somewhat surprised to get 12 m flame heights!
Block 2 was a small triangular block - all corner and no core. We had a lot of Parks Brigade units and lit it according to the rules. We got little convection column formation in the core, but good flames around the perimeters. On retrospect each patch around the perimeter was forming its own convection, with little interaction. The steep slope allowed the lower edge ignitions to run upslope, but as the top margins were already alight, there were no dramas.
So we had Day 1 out of the way and simply had to wait a week for the pig trapping program in Namadgi to conclude and we could go ahead with consecutive days 2 and 3. Rain was no problem, as we were not burning fine fuels.
So along came Day 2 and eighty-odd pairs of yellow (and orange) overalls appeared at the Yankee Hat Car Park. It was an impressive sight. Though the most startling thing was the appearance of some 60 odd ANU forestry students in a convoy of mini-buses - a sea of brightly coloured hard hats.
The first target was block 3. The block was relatively level with moderate fuel loads (around 50 t/ha only). Ignition went by the book. A track through the middle broke it into 2 burning blocks of ideal size.
A nice feature of this burn was that after the gee-whiz flames had died down, leaving the smouldering process to clean up the logs, the heat production became great enough to produce swarms of fire devils.
These are "tornadic" vortices created when a rotating convection column contracts due to increased heat input, but must conserve angular momentum, making it tighten-up. The result is a rapidly spinning, and noisy, vortex which forms a well outlined core of half a metre diameter and heights up to one or two hundred metres.
These awesome features can carry embers outside a fireline. We saw some over the Mt Taylor fire just before last Christmas, and giant ones were photographed on Ash Wednesday and the recent Pilliga fire.
Block 4 was bigger and much steeper and the air of expectation among the massed troops was remarkable - everyone wanted to feel the power of Boboyan. The plan was to use the central track (which followed the creek line through the middle of the block) as access to the drip torch crews.
They were working in pairs - one to light up and one to use the radio for safety purposes. They kept in touch with each other, and no one else did anything until all crews had reported that they were out. Once the core was lit they were to light up through the middle as they exited.
As in all blocks the water units were deployed evenly around the perimeter to patrol for problems and to provide a drip torch crew for the perimeter ignitions.
As it turned out, the core of Block 4 failed to produce a vigorous convection column. As you may have by now guessed, this allowed the fire to proceed upslope. Just when the core was lit, a fairly nippy easterly wind kicked in, which was thus blowing upslope.
What happened then was that sheets of flame started to travel upslope - they were 15 m tall and over 50 m deep. From photos I have measured their rate of spread at just on 1 km/hr. Given a maximum run of 200 m, this gave a travel time of twelve minutes to reach the upper margin of the block.
I would like you now to consider the predicament of the crew of Rivers 20 (the old Mt Stromlo village protection unit). They were between dense slash fuel and fuel reduced eucalypt, on a twenty degree cross-slope. Once the fire was lit the smoke plume followed the ground and obscured their view of proceedings. At some point it would have been clear that they should seek advice as to their safety.
This request would have been evaluated against the expectations of the fire's behaviour and what was being seen in reality. Options were considered. At this point the fire passed over their unit, partially melting it. The crew did everything right and did not panic.
What we have here is a wonderful case study of what can go wrong at a REAL WILDFIRE. In a real fire, things happen much faster and the problems can have far worse consequences. Incident Controllers are struggling to keep up with events, and with the constipated flow of information. It is vital here for crew leaders to realize that at all times they must ensure that they are able to safely "exit-stage-left". This has to be balanced against the expectation of others in their sector that they will stay and hold the line!
I am not criticizing anyone here, just asking you to ask yourself how you would have reacted.
Block 4 also produced some large vortices and the first of the truly impressive convection columns, that broke through the inversion and started to build their own clouds overhead.
Over on the other side of the valley, out of sight of most, crews were lighting up block 7. This block had steep slopes, patchy fuel and a lot of fuel-heavy eucalypt forest immediately upslope with no ready control lines. The priority here was to light up in such a way as to avoid spotting into the eucalypts. While this was largely achieved, it took all day to do so. Some short distance spotting early on forced a rethink - no mass infernos like in the other Blocks.
Day 3 started with drizzle, and threatened drizzle on and off through the day. Block 5 was a large flat block with scattered fuel and damp fine fuel.
We knew that we would not get another inspiring fire here. The core was slow to build up, and the edges ignited rather patchily in places. Just when it looked like a slow morning, the fire started generating an easterly ground wind of 35 km/hr gusting 50 km/hr. This had the effect of pushing the fire to the western edge along windrows, with dense smoke laying hard to the ground. Crews along the western margin had a hard time.
Fortunately no spot fires occurred - a good thing as the drought had killed over 10% of the trees on the hills downwind, which must surely have increased flammability!
Block 6 was upwind of the continuing-to-smoulder Block 5. It was undulating terrain with a general uphill run (with the wind) of almost 1 kilometre. It was later in the day, so the fine fuels were drying out (despite the drizzle), there was lots of slash and the crews were keen for something special.
They got it.
Three core fires drew the edge in well, sometimes with an upslope run to the core. 12 to 15 m flames quickly developed, sending kangaroos, rabbits and birds fleeing. The flames continued to grow. The large area of burning logs produced the most spectacular convection column of the event. Seen from the other side of the valley it was producing a stream of large cumulus clouds, which were being pushed downwind by a fresh wind aloft.
An interesting event was noted when a badly charred kangaroo came hopping out of the block. A dozen firefighters betrayed their urban backgrounds when they just watched and wondered what to do about the unfortunate beast. A radio call summonsed a ranger who was helpless when the beast re-entered the fireground.
In the end, a couple of ‘dyed-in-the-wool’ bushies were able to corner it inside the fireground and dispatch it - the bush way and the best for the poor animal.
Are you, dear reader, of an urban mindset (i.e. "What can I do now?") or a rural mindset (i.e. "S**t, lets just do it!")?
Many people experienced extreme intensities for the first time at these burns. It is one thing to be told in the training lectures on a winter night that these produce 30,000 kilowatts per metre of fire perimeter (30,000 single bar electric radiators per metre!) but to experience it is something memorable.
Should any more such opportunities arise, I implore you to get there by any means possible!
Rick McRae.