Posted 17 July 2011, by Glenn Minuth (The Herald Review), Sierra Vista Herald, svherald.com
Glenn Minuth is a geomorphologist/geographer/cartographer by training and a part-time instructor in the Center for Life Long Learning, Cochise College who offers fire ecology, geology, ecology, climate and meteorological lectures and field trips in the local mountains as well as for regional nature conferences.
EDITOR’S NOTE: This is the first of a two-part series, the second article to be published Sunday, July 24, offering insight into the factors that contributed to the development and growth of The Monument Fire. The series was written by Glenn Minuth, an instructor at the Center for Life Long Learning at Cochise College.
During the Monument Fire the Type I, Northern Incident Management Team coined a new fire term “blow-outs.”
Textbooks on wildland fire have long recognized the term “blow-ups,” but nowhere in these books will you find reference to “blow-outs.” They are not identical fire situations and apparently, blow-outs are reported for the first time during the Monument Fire. Both Cochise County residents and firefighters had an opportunity to witness an apparently unprecedented occurrence that consistently modeled itself over three separate days giving new insight into how wildland fire can behave. One can assume this new term evolved spontaneously on the fire scene by derivation from the existing wild land fire lexicon and as a variant of the term “blow-up.” The term blow-out will now accommodate what was observed and experienced on these three different days during the Monument Fire in Ash, Stump and Miller Canyons in June 2011. Both types of fire situations have several apparent similarities or traits in common. On these occasions, the blow-outs appeared to evolve from what began as blow-up fires. Either fire term can function in the lexicon as a noun but is derivative of several verb senses found in the dictionary: cause to burst with a violent release of energy; make large; burst and release energy as through a violent chemical or physical reaction; and to swell or cause to enlarge. And all cases here seem to apply in this dramatic sense from what was observed from those close-by and afar, be it in the air or on the ground.
Many residents and professionals have also confused the recent fire behavior as characteristic of a volcanic feature known as a pyroclastic flow (glowing avalanche or nuee aredent). Although it could be said the two have a few similar characteristics, they are vastly different in genesis and physical composition. Pyroclastic flows operate in the temperature regime approaching 2,000°F while wild land fires can reach 1,800°F in only extreme cases like the combusting of Buffel Grass which burns at twice the temperature compared to most species of grass. It was curious to note that an anecdotal (unverified) report from the Monument Fire front in at least one of these canyons was logged at 1,400°F. While pyroclastic flows can travel in excess of 100 mph, these blow-out fires traveled much slower. Pyroclastic flows carry with them much kinetic force and mass, almost always leaving total destruction in their paths compared to blow-outs that carry mainly hot combusting gases and the force of the accompanying descending thermal wind manifested as a wave front. Such a thermal shock wave was anecdotally reported by a resident in Ash Canyon whose house did not burn, but experienced a vibration (possibly the thermal shock of the fire front) strong enough to knock pictures of the wall and crack some windows (or could this have occurred as a concussion from a nearby exploding propane tank — we don’t know yet). Regardless, the resulting devastation from either occurrence appears eerily similar by the time the ash or smoke clouds finally clear so long as complete combustion has occurred in the case of the blow-out fire. But more characteristically in the blow-out fires, some houses will still be left standing unlike in a pyroclastic flow.
This report combines fire observations from the Monument Fire with known pyrochemical principles, meteorological observations, and geological processes. By blending more concepts from several other disciplines including explosion dynamics and physics, the author suggests a causal mechanism for the intriguing blow-out phenomena experienced during the sequential fires occurring in three different canyons of the Huachuca Mountains in June 2011.
To begin to understand the new occurrence of blow-outs, one must comprehend the evolution of blow-up fires first. The fire environment responds to the conditions in the fire that can increase its rate of spread and intensity and fire is capable of actually accelerating with bursts. Of great significance in the initial fire acceleration, are the critical burnout time and presence of surface winds and slopes. When combined in nature, wind and slope have the consequence of bending flame fronts and focusing the heat from fire upward in a canyon. Thus, it is imperative, in order to sustain combustion that heat accumulate in surplus and then be adequately focused. It is well known that: if the heat liberated by flaming combustion equates to that required for pyrolysis (trans: “heat-divided” — the burning of gases released by heating of a fuel source), then fire will stabilize and burn essentially in a steady state — meaning with a constant rate of energy release and at a constant rate of spread. Alternatively when conditions feature liberation of excess heat energy that is governed by fuel characteristics, as well as an air mass suitably structured to support a robust convective column, the fire will likely undergo acceleration. In this case, the emission of smoke and convective heat no longer expresses as widely disseminated and unorganized smoke, but now forms a tightly compact plume capable of modifying, by ventilation, the fire’s intensity beneath.
Conventional fire ecology wisdom recognizes fire as exhibiting three principle types of spread in the wild: ground fire, which spreads mostly by creep and is buttressed by glowing combustion; surface fire, that expands via ground fuels consistent with a flaming front; and the infamous crown fire that, although sustained by surface fire, expands into the forest canopy fuels with ferocity and in its classic style with discontinuous surges. These are not to be understood as sequential or evolutionary stages of fire growth and it must be recognized that the change (transition), from steady state to accelerating fire concerns more than what crowning might make one imagine. Any fire transition mandates first, an accumulation of immense heat. This translates to a flaming front ratio that is thicker in depth relative to its flame length, functioning no longer as a point or linear source for energy release. Rather, because of its size, it now functions as an area source of heat generation that is capable of upsetting the surface wind field. These conditions require a heavy fuel load, excluding, for example, such thinner fuels as grass and forest litter.
The next elemental ingredient for successful transition mandates an air mass orientation in which surface winds will not quickly shear off the potential convective column or be stymied by a suppressing air inversion that could degrade its potential for vertical development. The generation of the large fire fed by convective forces, relies chiefly on the energy discharge, expressed chiefly as buoyancy of the smoke pillar as its upward force must be in excess to overcome the local wind field. Lastly, a triggering device, that heavily magnifies the burning intensity is essential and is often evidenced by involvement of heavier fuels or reduction of the capping inversion. Transition can initiate almost instantaneously at which point, the fire is deemed to have blown-up. After passing through this boundary condition, the combustion process and fire spread achieve a state of unorganized discontinuity. Fire spread may now proceed by saltation (a geologic term that means in leaping movements) rather than by steady absorption of heat energy. There is no longer a steady heat transfer, instead, fire brands spew new ignition primers and fire whirls serves as migrating points of ignition. This frenzied state of combustion begins to separate clouds of volatile gases that penetrate the atmosphere while detonating into flame.
Curiously, the fire now propagates no longer across a flaming front, but rather in surges, that spare some areas wholly while ravaging the balance with complete burnout. Fire intensity increases by power factors of three or four, handily explaining the formation of fire whirls on to the lee of the convective plume. Because of intense convection now, the local feeding winds can acquire great speed along with occurrences of vortices and intense turbulence while complicating effective drafting with the column. A linear flame front can evolve but now with multiple heads. This is the breaching point where fire spread can no longer depend on conventional thermodynamic heat transfer mechanisms of conduction, convection, and radiation, but rather by extreme processes. At this stage words like conflagration come into play as the wind field and convective plume dynamically exchange energy. This is governed in part by topographic considerations that exacerbate fire behavior while fragmenting it into a discontinuous physical chemical affair. It is this transition state that mimics to some degree, the crossing of the boundary for a nuclear chain reaction — going from steady state to quickly achieving episodic reaction states. This then, is the transition process that precipitates the quickly expanding blow-up fires that have killed many firefighters, such as the: 1949 Mann Gulch Fire, Helena National Forest, MT (13 died); 1994 South Canyon Fire (Storm King Mountain), CO (14 died); and the 2001 Thirty-Mile Fire, Okanogan Forest, WA (four died).
(Ed Note: Please visit the original site to view the photograph associated with this article)