We were sent an article recently that was quite intriguing.

It suggested that rather than adding to fire risk, eucalyptus actually fights wind-driven fire up to the point it is overcome. It does this by breaking up the flow of the wind, and trapping flying embers that would otherwise lodge in homes and buildings, setting the dry structures on fire. Essentially, eucalyptus can act as a firebreak.

Burning up to the Eucalyptus

(Photograph and article courtesy W. Lofft, Scripps Central, San Diego CA)

The article, below, based on work by Computational Physicist David Porreca, points out that fires in California (and Australia) are driven by hot dry winds. The way these winds flow, and the dry vegetation they feed on, have a major effect on these “vorticity-driven fires.”  The best way to understand these flows in through a mathematical technology called Computational fluid dynamics – which simulates things that flow.  It’s become one of the best tools for understanding the dynamics of wild fires.

Here’s a long excerpt:

“Gum barks [i.e. eucalypts] are ‘flammable’ (like all other dried vegetation), but they act as wind and fire breaks.  In wind situations such as Santa Ana’s, the tree canopies swing back and forth like a windshield washer in the high velocity winds, [and] actually both protect and assist the gum barks in their survival by retarding the flying embers in flight...  The vivid photos from Scripps Ranch [fire] in 2003 not only show few tree tops singed, they also clearly display unburned trunks in the vicinity of structures otherwise decimated.

“Gum barks are thus now used for both wind and fire protection. Tall and older gum barks of various varieties are considered better than others for actually deflecting embers and retarding the advance of airborne embers. The major culprit in long distance leap-ahead of plume effects and plume creation are the flying embers, as well as the advancing firestorm’s tsunami-like heat torrent.  Flying embers can leap 1-2 kms ahead–and are usually the major cause of home ignition in advance of the firestorm—with embers lodging under eaves or in other wood-joined exterior components of the home.

“On the Big Island of Hawaii, non-indigenous ironwood and gum bark wind and fire breaks are planted all over the dry western part of the island, especially between the lava fields and the populated west coast of the island.  This has been done for years, precisely because these trees break up the turbulent flow dynamics and can reduce the flying embers.”

Read the whole article below.

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Computational Fluid Dynamics and Combustion Engineering

Computational Fluid Dynamics is a mathematical technology that enables the study and simulation of things that flow—like gases, liquids, heat and mass transfer, chemical reactions, and the physics of the firestorm process.  Computational Fluid Dynamics modeling has become one of the best tools for understanding the dynamics of wild fires. It has shattered some pre-conceptions about extraordinary cause and effect—as well as improved understanding of the basic science of fires and fire storms.

The primary fuel for California and Australian fires are native grasses, brush, dead/dried woody/brush-like matter of all types.  The age, distribution pattern and topography of the sage lands along with weather and wind patterns/velocities determine the extent to which foehn winds {dry, down slope}, known as Santa Ana’s in California, become so turbulent as to create the horizontal torrent and plume effect (or a sea of whirls and fire plumes) which by definition are an organized source of angular momentum to produce large-scale effects.  This has been witnessed in the San Diego region over the last decade, principally in Scripps Ranch (2003) and Rancho Bernardo (2007).

These vorticity-driven fires, over a large length of range and hilly canyon-rutted terrain, significantly alter the entrainment and combustion dynamics of fire. New models that focus on angular rotation, temperature variation and dissipation dynamics allow a better understanding of large temperature and density variations, and identify best practices for engaging, and preparing for, these Santa Ana-driven firestorms.

In Australia, clear cutting gum bark (eucalyptus) trees is not considered a best practice for containment in canyon-like regions and finger-like topographical areas–or probably any other—even though it is sometimes done solely for economical reasons.  Gum barks as a species use fire to survive, because fire removes the brush and undergrowth that strangles them, and they generally survive in enough quantity to perpetuate the species.

Gum barks are “flammable” (like all other dried vegetation), but they have unique properties as wind and fire breaks.  In wind situations such as Santa Ana’s, the tree canopies swing back and forth like a windshield washer and the high velocity winds actually both protect and assist the gum barks in their survival by retarding the flying embers in flight!  This is why the Gum barks have survived for ages in Australia.  The vivid photos from Scripps Ranch in 2003 not only show few tree tops singed, they also clearly display unburned trunks in the vicinity of structures otherwise decimated.

Gum barks are thus now used for both wind and fire protection. Tall and older gum barks of various varieties are considered better than others for actually deflecting embers and retarding the advance of airborne embers. The major culprit in long distance leap-ahead of plume effects and plume creation are the flying embers, as well as the advancing firestorm’s tsunami-like heat torrent.  Flying embers can leap 1-2 km’s ahead–and are usually the major cause of home ignition in advance of the firestorm—with embers lodging under eaves or in other wood-joined exterior components of the home.

On the Big Island of Hawaii, non-indigenous ironwood and gum bark wind and fire breaks are planted all over the dry western part of the island, especially between the lava fields and the populated west coast of the island.  This has been done for years, precisely because these trees break up the turbulent flow dynamics and can reduce the flying embers.

Gum barks also retard desertification, which in turn helps with water drought situations.  Gum barks do not require regular water; they capture the run-off that would otherwise mostly evaporate in geological and climatic zones such as San Diego.

All typically agree that consistent ground fuel brush removal is absolutely required for fire safety.  But the rationale behind selective live tree removal—and canopy reduction—is dangerous to fire management and spread prevention.  The clumping of live trees–rather than thinning—insures that less lanes (or vortices) are available to an advancing fire.  More open lanes make a tunneling effect more likely—as acceleration through those lanes occurs as the process seeks new fuel and oxygen.

Even if a grove burned in whole, it would be the perfect place to fight the advance of the fire as its angular momentum would be contained at that location for some time, allowing mobilization of fire/rescue resources.

The myths surrounding gum barks are ill-considered and have no basis in science or empirical evidence.  The actual experience in Australia points to the directly opposite conclusion.

Clear cutting of Gum barks reduces safety from firestorms, both along the Urban Wildland Interface (UWI) as well as internal defensible space areas where they assist with high-risk ground fuel mitigation.  Gum barks can be successfully used to create a defensive flying ember shield along UWI areas in San Diego where topological bowling alley entrances exist, as well as assist with ground fuel brush management plans.

This document is based on research done by Computational Physicist David Porreca.  Porreca provides senior consulting and advisory services based on his extensive professional experience as Chief Scientist, Chief Technologist and President/CEO and Managing Partner of numerous firms, and his resultant national reputation in high technology with focus in computer and cognitive sciences.

Porreca has served as Senior Scientist at RDA, Director of Program Development at SAIC, Chief Scientist at Titan Corp, President/CEO of Cayenta and Expersoft, Founding Partner of Transnational Partners and Beagle Holdings, and Founding Principal of the Zazi Forum, an independent group of Senior Scientists and Analysts focused on the study of key issues facing government agencies and commercial enterprises.  In addition, Porreca created and led the Sempra Energy Center for Excellence initiative.