Tree canopy separation - flawed or misused science or both?

I'm still working to get to the bottom of the requirement to remove 500 trees to achieve the CFA's initial one-size-fits-all requirement for canopy separation to protect a new dwelling, as mentioned in my 15 November posting "Bushfire Protection, VCAT, Fallibility and Fairness", as this arbitrary requirement affects some of my clients, too.

From my experience a rare instance of candor by the CFA representative — rare instance, in that questioning some in the CFA dealing with the Bushfire Management Overlay on their position usually receives the response to take it to VCAT if you're not happy, and that can be a flawed and terribly unfair process — when questioned why blanket two metres canopy separation was required the response was that the fire modelling program Phoenix RapidFire indicated that "crown fire" was likely on the land in question.

For those unfamiliar with Phoenix RapidFire this overview may help you gain some understanding of the 'tool' and its application.

For those interested a paper available on the web "Wildland–urban interface (WUI) fire modelling using PHOENIX RapidFire: A case study in Cavaillon, France" provides some information. This paper provides further confirmation that wind-borne embers or firebrands are the major source of fire spread, which has been well known by the fire management community in Victoria for many years, or it should be.

Under the heading "2.1 Fire case study" in the first paragraph:

Observations of the fire indicated that wind-carried firebrands were an important mechanism for fire spread during the initial stages of the fire, with embers launched from ridges initiating new fires in adjacent gullies.

In my 13 July 2016 blog posting "The nature of bushfire Part 3 ... how fire moves across the land" I provided some information on how fire entered Wye River-Separation Creek during Christmas Day 2015, together supporting aerial photographs.

Extinguished while still small, property loss can be prevented or minimised significantly. But this would require a change of mindset by the Victorian government i.e. departing its "leave early and live" policy.

But for now, CFA's one-size-fits-all tree canopy separation requirements and the CFA representative's reference to Phoenix RapidFire.

As I recall our discussion, the assertion was that the canopy of medium height predominately Stringybark eucalypts would be exposed to flames reaching a height of 29 metres and fire would burn up the stringbark trunks causing "crown fire" and that "two metres separation" of tree canopies was necessary to prevent crown fire occurring. And, while I'm on it, what is "crown fire?

The Australasian Fire and Emergency Service Authorities Council Bushfire Glossary, January 2012, provides the following definitions:

Crown fire – A fire that advances from top to top of trees or shrubs.

Crowning – A fire ascending into the crowns of trees and spreading from crown to crown.

Crowning potential – A probability that a crown fire may start, calculated from inputs of foliage moisture content and height of the lowest part of the tree crowns above the surface.

Crown scorch – Browning of the needles or leaves in the crown of a tree or shrub caused by heat from a fire.

From the definitions, some important issues:

1. Note, a fire that advances from top to top of trees or a fire spreading from crown to crown, not just the odd tree or shrub foliage or crown burning in relative isolation!

2. Note, crowning potential, amongst other factors, takes into consideration foliage moisture content and height of the lowest part of the tree crowns above the surface!

First, on "crowning" and "crown fire", photographs included in postings on 4 October 2016, 12 January 2015 and 22 June 2014 that show tree canopies exposed to fires burning under total fire ban weather conditions that only suffered "crown scorch". Why only scorching, is it due to the moisture content being sufficiently high enough to prevent ignition before the heat from the fire begins to drop after the fire front has gone past?

Looking around for a reasonably simple explanation, the theory explained in the following extract from An Introduction to Fire Dynamics, Second Edition, Dougal Drysdale, Wiley, 1998, seems to suit my purpose:

Following is a photograph taken on 2 July 2014 following a fire that occurred on the western side of the Geelong–Bacchus Marsh Road, at approximately midday, on 11 March 2014, a day of total fire ban. The photograph shows the transition from leaf scorch to all or parts of leaves that have actually been ignited. Ignited after the moisture had been expelled.

The following two photographs also taken on 2 July 2014 show that part of the roadside plantation near where the fire entered it heading south from the grass before wind from the northeast at approximately 24 kph gusting to 28 kph, recorded at Point Wilson. Note the scorched vegetation alongside where the fire burned south through the plantation, significant to my argument that moisture needs to be dispelled before they would actually burn.

The following two photographs are of the surface fine fuel, mainly dead and dry Casuarina tree 'needles', the average depth of which is indicated by the marker pen in the second photograph, and near surface fine fuel unaffected by the fire.

The marker pen provides some perspective on how aerated, hence conducive to complete combustion, was the distribution of the surface fine fuel.

The following two photographs illustrate how complete was the combustion of the surface fine fuel, yet the combustion of the living vegetation on some of the trees above what would have been a hot fire was incomplete.

The upper red arrows indicate unburnt leaves and the lower arrows indicate fallen leaves that fell from the scorch zone well after the fire was out.

The red arrows indicate scorched Casuarina 'needles' that despite their close proximity to the ground were not completely incinerated.

The following three Google Earth photos are of the area covered in my 2 July 2014 photos taken prior to the fire.

So far I’ve only covered the need for moisture to be removed from the fuel to enable ignition. In my next posting I’ll deal with flame i.e. its structure and temperature variation and the height of the fuel canopy being subjected to the drying process.

I’ve earlier pointed to postings showing examples of where only scorching occurred under extreme fire conditions, which supports my argument that true crown fire is a relatively rare occurrence.

Causes me to wonder if Phoenix RapidFire is flawed or being misused by the CFA to justify its one-size-fits-all approach to tree canopy separation or both. And, if Phoenix RapidFire is being used to support the government’s bushfire emergency warnings, is it reliable or does it under or overestimate fire spread potential?

And, on the suitability-to-role of people who won't or can't consider each case on its merits, that's something we should all be considering.

Finally, always open to respectful questioning or challenging in the comments.