HIGHFIRE RISK PROJECT

REGISTER OF CAUSES OF DEEP FLAMING

"Normal" fire behaviour reflects terrain, fire weather, and fuel. Knowledge of those allows a good prediction of fire behaviour when it has entered a quasi-steady-state of spread.
When deep flaming occurs the strength of the convection column causes a coupling with the atmosphere above. This forms dynamic fire behaviour, which reflects terrain, the atmospheric profile and the fire itself (a feedback loop). Often the feedback is due to the mixing down of drier air aloft to replace what is going up in the plume.
Deep flaming and the resulting violent pyro-convection may reflect full flammability of large dead-and-down fuels. This requires limited rain over the preceding month, and is often reflected in low or dry river flows (which can be used for predictions).
The coupling allows formation of a Blow-Up Fire event (BUFE), which typically burns for 2 to 3 hours and takes out an area of 7 km by 12 km (sometimes much more). A fire with one or more BUFEs is termed an Extreme Wildfire. Normal Incident Action Plan objectives during a BUFE, for affected sectors, need to be replaced by a focus on saving lives.

Why is this needed?

The risks that arise due to deep flaming greatly exceed those from normal wildfire. In Canberra, over 80 years we lost no houses to normal wildfires. In one afternoon deep flaming caused the loss of 512 houses (with a further 1000 saved). Normal 10 year fire risk to houses on the urban edge: likelihood = likely, consequences = minor (due to effective mitigation), risk = moderate. Extreme wildfire 10 year risk to houses on urban edge: likelihood = moderate, consequences = catastrophic, risk = high.

IMPORTANT CAVEAT:

IMT and Operations Staff should only apply these concepts if they are covered by Standard Operating Procedures that allow it. This ensures protection in the event of a Coronial Enquiry or similar.


BUFEs require one of a number of known causes of deep flaming to come into play. Currently there are seven such known causes:

1. STRONG WIND

A line fire takes a set period of time to burn out. This reflects the fuel state. If the wind is strong (over, say, 40 km/hr) the flaming front travels a long way during the burn-out time. In south-west WA fires have been observed by satellite travelling over 90km in an hour (17 November, 2015). This is effectively deep flaming. There is a tendency for this to occur under a stable atmosphere.

Operations goal: Incident Action Plan needs to keep people out of its forecast path.
Requires:
- Strong winds (over 40 km/hr)
- Elevated fire danger
Further information:
- Established fire service doctrine.

2. WIND CHANGE

Whenever the winds driving a running wildfire back or veer by more than, say, 40 degrees, over an hour, parts of a flank become a new headfire. This is often wider than the original headfire. For the first hour or so the cumulative fire perimeters can function as deep flaming. PyroCbs have occured in unstable conditions (for example pyroCb 2006d, 29 November, 2006 - see the Register).
Southern hemiphere: mainly to the left; Northen hemisphere: mainly to the right.

Operations goal: Incident Action Plan needs to keep people out of its forecast path.
Requires:
- Strong winds (over 40 km/hr)
- Significant wind backing or veering.
- Elevated fire danger
Further information:
- Established fire service doctrine.

3. VORTICITY-DRIVEN LATERAL SPREAD (VLS)

As discovered from the 2003 Canberra fires (18 January, 2003), fires on a ridgetop (or other major changes in slope) aligned at right-angles to the wind can interact with lee slope eddy winds. These winds require a prevailing wind speed above, say, 25 km/hr. While creating dense spotting for kilometres downwind, the fire also travels sideways at up to 5 km/hr, taking the spotting with it. This requires fine fuel moisture content below, say, 5%. This is a very effective way of creating deep flaming, and is a key risk driver anywhere with forest fuels in rugged terrain. VLS has caused serious burn-overs.

Operations goal: IMT needs to plan to either prevent fire entering VLS generator when conditions for VLS are in place, or mitigate risk in VLS footprint downwind. Consider lookouts for field crews. Sector Leaders need to factor this potential into Safety Plans.
Requires:
- Winds over 25 km/hr
- Dry fine fuels (below 5%)
- Noticable change in slope in direction of prevailing winds (see Terrain maps).
Further information:
- Scientific literature on VLS.

4. ERUPTIVE SPREAD

Fire in a steep trench shaped feature can continue to accelerate, effectively without limit. This was discovered from the Kings Cross Railway Station Fire in London (18 November, 1987). As seen in the Elephant Hill Fire in British Columbia (August 2017) at the top of a trench there can be an ember storm that spays out of the top of the trench for c. half a kilometre. In that case parallel trenches created overlapping ember storms and deep flaming. Has caused burn-overs, even on apparently mild days.

Operations goal: Watch for fire aproaching a trench-like terrain feature, especially downslope of field crews. Sector Leaders need to factor this potential into Safety Plans.
Requires:
- Trench-like landform feature, with a slope above 26 degrees.
Further information:
- Scientific literature on eruptive spread.

5. USE OF ACCELERANTS

There are two ways that fire crews operating under the direction of their approved Incident Action Plan can create deep flaming.
Firstly, in lighting up an area in a short time period they can create deep flaming.

Operations goal: Plan burn-out tactics to avoid this.
Requires:
- Fuel flammability to support a burn-out strategy.
- Burn-out pattern able to mimic deep flaming.
Secondly, ignition upwind of or inside the lee-eddy of a VLS Generator.

Operations goal: Field crews should be briefed if operating near VLS generators, even if that terrain is not in the fire area.
Requires:
- See VLS, above.
Further information:
- Scientific literature on isoperimetric ratio.

6. DENSE SPOTTING

A dense field of spotfires will inevitably merge. As their edges come together fire intensity increases. The distribution and intensity equates to deep flaming. Dense spotting is a common result of VLS, and of burn-out strategies, but other causes are known.

Operations goal: Field crews need to be aware of the potential for rapid loss of situational awareness. Lookouts are to be aware of potential. Sector Leaders need to factor this potential into Safety Plans.
Requires:
- See VLS and Use of Accelerants, above.
Further information:
- Scientific literature on fire coalescence, VLS or isoperimetric ratio.

7. INTERIOR IGNITION

In areas with complex vegetation patterns across the landscape, flammability can vary in a complex manner. This is especially true in the Boreal Forests, and is made more complex by embedded land-use patterns. Fire can burn through an area on a day, but some vegetation types may become flammable on subsequent days. The cumulative burn pattern can resemble deep flaming.

Operations goal: FBANs to assess multi-day potential.
Requires:
- Complex fuel pattern with variable days of peak flammability.
Further information:
- North American fire danger tables.

FURTHER INFORMATION

For further information on these processes, contact: This is the first draft, published 10 March 2025.