Spring 2018 Public Meeting

From: 
Thursday, 27 September 2018 - 7:00pm to 8:00pm
Where: 

(parking off Hedland Circuit)

Dynamic bushfire behaviour on Canberra's western edge

Once again we are fortunate to have Assoc Prof Jason Sharples as the guest speaker for our next public forum. He will give an update of his work on extreme fire risks, focussing on Ginninderry. 

* Note the 2018 AGM will be held before this presentation starting at 6:30.*

Jason Sharples is Associate Professor of Applied Mathematics at the University of New South Wales, where he works on various aspects of extreme and dynamic bushfire propagation, the development of large conflagrations and bushfire risk management. He is also a volunteer firefighter.

On 18 January 2003, four lives were lost, 490 people were injured and 500 houses were destroyed by bushfire in the Canberra suburbs of Duffy and Chapman.  Others suffered PTSD for years after.  At the time, a number of unusual fires were observed in which bushfire spread sideways across the wind direction and was accompanied by rapid and intense downwind fire spread caused by embers.  Since these fires, much research has been conducted to understand why some fires turn into firestorms with vastly different behaviour from ordinary grassfires.  Firestorm events consistently cause the greatest wildfire damage and pose an increasing challenge worldwide. 

A firestorm results when the plume generated by the heat of a large fire develops characteristics similar to a thunderstorm.  The technical term for a firestorm is ‘pyrocumulonimbus’, which literally translates as ‘fire thunderstorm’.  Firestorms involve significant coupling with the atmosphere, which can modify local weather patterns.  For example, firestorm events can result in more erratic and unpredictable winds, mixing down of drier air from the upper levels of the atmosphere, increased potential for firebrand generation and transport, and dry lightning.

Slopes over 20 degrees, as occur in the gorges along the Murrumbidgee and Molonglo Rivers and the lower reach of Ginninderra Creek, are known to be associated with firestorm behaviour.  The Ginninderry/Parkwood peninsula is surrounded by these steep slopes and exposed to the prevailing north-westerly wind direction.  To the west, is the Brindabella Range where lightning strikes frequently start fires which are driven towards the ACT.

Fires have affected Canberra, invariably, on the western side with major fires occurring in 1903, 1926, 1927 and 1939, three times in 1952, and in 2001 and 2003.  Most of the fires were started by lightning strikes during dry seasons that followed a warm, dry winter and spring and most were accompanied by very strong winds.  The 2003 fires, therefore, were not a one-in-100-year event.

Subsequently, researchers have been working to understand what made this, and other fires, behave so differently from the usual bushfire.  A/Prof. Sharples will outline the current knowledge of how, where and why such extreme fires occur.


See summary prepared by GFA.

File attachments: 

Comments

There are 15-20 researchers at UNSW involved in working out the dynamics of bushfire behaviour. The traditional opinion has been that of a quasi steady state with independent foci. There are many examples where this assumption is violated.
Junction Fires - vortex point allows joining of fires to rapidly advance.
Eruptive Fires - with a plume attachment. Where the slope is greater than 25 degrees there is an extra dynamic where there's an interaction between terrain and atmosphere and the plume instead of rising vertically lies over the ground to cause acceleration and this can be irrespective of the prevailing wind.
Vorticity driven lateral spread (VLS) Fires - fires normally travel around 4-5 km/hour whereas it can now travel up a Lee slope greater than 20 degrees across the wind of up to 40% change, at 7 km/hour. These are like a whirlwind and as they reach the crest the fire generated can act like corkscrews going left and right. They are developed where there is a heavy fuel load like in forest, not in grasslands (ie unlikely to develop in Hawker). Part of this complex is Deep Flaming, where spot fires merge leading to violent pyroconvection and firestorm development that the reach higher into the atmosphere with more erratic and unpredictable wind mixing with dry upper atmosphere air and creating down draughts and hence increased ember transport.
The Australian Standard 3959 on bush fires is fundamentally flawed as it doesn't encompass these realities and is being rewritten. It is predicated on radiation causing loss, not encompassing ember attack and pyroconvection.
Although not the province of our speaker, one way of protecting one's house could be to install sprinklers on the roof with independent access to power and water.

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