The rapid escalation of a small fire due to fire channelling can result in a catastrophic decay in both firefighter and community safety that is counterintuitive.
That is how the authors of a paper wrapped up their findings about a weather phenomenon that can cause a wildfire to spread in unexpected directions. “Fire channelling” can force a fire on the lee side of a ridge to spread 90-degrees from the general wind direction. For example, if a west wind pushes a fire across a north-south ridge, on the lee or east side of the ridge the fire could spread to both the north and the south, counterintuitively.
Generally a strong wind has the most effect on the direction of spread of a fire — more so than topography or fuel. If a fire is spreading with a strong west wind, the rate of spread on the flanks, the north and south sides, will be much less than the head of the fire on the east side. Unless — fire channelling is occurring.
Firefighters usually face less risk when they attack a fire on the heel or flanks of a fire. In most cases it can be impossible to safely attack the head of a fast-moving fire in heavy fuels. But this fire channelling phenomenon has the potential to present firefighters with unexpected fire behavior, putting them in a dangerous situation on what they expected to be the flanks of a fire that suddenly converted to heads of the fire.
The authors of the paper, which is titled Wind–terrain effects on the propagation of wildfires in rugged terrain: fire channelling, considered several causes of fire channelling, including thermally induced winds, pressure-driven channelling, forced channelling, and downward momentum transport, but they settled on wind–terrain–fire interactions as the most likely mechanism driving the atypical spread. Here is an excerpt providing some details about wind–terrain–fire interactions:
…If a fire happened to spread into a region affected by a separation eddy, then the hot gas from the fire could be entrained within the eddy, with the strong wind shear at the top of the eddy impeding mixing between the synoptic and separated flows. Hence, supposing a fire enters a region of separated flow at the north end of a slope or valley, and treating the air within the eddy as a quasi-isolated system (i.e. a system that involves only limited mixing with the surrounding environment; cf. Byron-Scott 1990), the air within the northern part of the eddy will be at a higher temperature and pressure than the air within the southern part of the eddy. As a consequence, the air within the eddy will tend to move towards the south in response to the thermally induced pressure gradient or simply owing to thermal expansion of the air within the eddy. Based on the available evidence, such an interaction constitutes the most likely mechanism driving the atypical spread.
Access to the research
If you want to read the paper you will have to pay CSIRO Publishing $25, in spite of the fact that the authors appear to be funded by the Australian government. It was written by Jason J. Sharples, Richard H. D. McRae, and Stephen R. Wilkes who are associated with three organizations in Australia, the University of New South Wales at the Australian Defence Force Academy, the Bushfire Cooperative Research Centre, and the ACT Emergency Services Agency.
This is another example of government funded research that taxpayers have to pay for twice. Once when the government-paid employees conduct the research and write the paper, and a second time if a person wants to read it. We have written about this lack of Open Access numerous times before. However, this example is a little murky, in that the government sponsored research was published by CSIRO, a governmental body. But many U.S. wildfire researchers who are government employees publish their papers in the same CSIRO publication, the International Journal of Wildland Fire, behind a pay wall.
Sign the petition
At the U.S. White House web site you can sign a petition to make government funded research available at no additional charge to the public. Let President Obama know that you oppose HR3699, the Research Works Act, which is an attempt to put federally funded scientific information behind pay-walls and confer the ownership of the information to a private entity. You will need to register at the site, giving them a name and a real email address.
Their is more than one place to locate the full-text of the research (http://www.tathrafirebrigade.org.au/pdf/wind_terrain.pdf), it is an intensely interesting paper. I suspect the recent fire which spread excesively contrary to wind direction and speed (in the Gippsland region of Victoria) may have had a channeling event. Nonethless, it is bloody interesting and should be required reading.