[LINK] Bush Fire Speeds (was Re: home emergencies)

[REYNOLDS,Graham]

Jan,

Fires typically burn uphill more rapidly than on the flat or downhill because the fire pre-heats the fuel load in front of it. Convection of hot air (and flammable gases) upwards drys the uphill unburnt fuel, and the distance between the fire front and unburnt fuel is also shorter so that radiant heat has an increased effect. These combine to raise the temperature of combustible fuel to close to its flashpoint and reduces the fuel moisture content.

The rule of thumb for the effect of slope on rate of spread (ROS) is for every 10 degrees of up-slope, double the ROS(compared to on the flat). So 20 degrees of upslope gives 4xROS. You also get more intense fires burning uphill and higher flame heights. Conversely for every 10 degrees of downslope, half the ROS. 

There are many other factors involved that will have a significant impact, including fuel moisture content, the fuel load, the mix of fine and heavy fuels (fine fuels burn first and fast), the arrangement of the fuels (compacted vs loose), topology, aspect (eg. NW vs SE slopes)(NW slopes tend to be dryer and have less fuel load), atmospheric conditions, etc. In the end you can end up with a wild fire that is 'unpredictable'. High wind and wind eddies occuring over the top of a ridge can drive a fire rapidly downhill for example.

Regards,
Graham

________________________________

Dr Graham Reynolds
Interoperability & Digital Architecture, Digital Education Group
;: Graham.Reynolds at deewr.gov.au  ): 02 6240 9312  2: 02 6123 7573

-----Original Message-----
From: link-bounces at mailman1.anu.edu.au [mailto:link-bounces at mailman1.anu.edu.au] On Behalf Of Jan Whitaker
Sent: Sunday, 22 February 2009 19:01
To: link at anu.edu.au
Subject: Re: [LINK] Bush Fire Speeds (was Re: home emergencies)

At 03:49 PM 22/02/2009, Antony Barry wrote:

>Then there would be hot spots where the fire front was curved,
>running round a depression for instance. After the Canberra fires
>there was a depression near the Tuggeranong Parkway. Outside the
>depression tree trunks survived. Inside there was nothing. The heat
>was focussed into the area.

huh
I was just out on a drive in our area and couldn't believe what I was 
seeing. Probably within 5km of my house! I had this vision of a 
somewhat contained, single paddock or a little park area. No way. 
There were hectares of burned paddocks and trees. I understand there 
were 3 houses lost, but we couldn't find where they'd been. We did 
see several houses where the fire came very close.

Back to burning patterns, I made a comment that fire burns up hill 
faster than down. To me it made sense because the fire would be 
closer physically to the hill upslope than to the downslope. Does 
that hold, assuming of course there wasn't wind driving down a hill 
for some reason?

Jan



Melbourne, Victoria, Australia
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