The Flaming Quartet: Four Types of Fire

Panarchistic Architecture :: Chapter #4 [4.2]

Citation: Sterry, M. L., (2018) Panarchistic Architecture: Building Wildland-Urban Interface Resilience to Wildfire through Design Thinking, Practice and Building Codes Modelled on Ecological Systems Theory. PhD Thesis, Advanced Virtual and Technological Architecture Research [AVATAR] group, University of Greenwich, London.

4.2.5 The Flaming Quartet: Four Types of Fire

Reverting to the words of “fire laureate” (Omi, 2005, p.84) Stephen Pyne, fire’s “character” is formed by its “context”, and no less so than in regard of the biomass stratum through which it is burning. While the National Fire Danger Rating System recognises four types of fire, fire ecologists generally describe three (Scott and Reinhardt, 2001): Ground, Surface, and Crown, with Spot the lesser referenced of the quartet. In turn, these are attributed to ground, surface, or aerial fuels that are grouped into four size classes: Grass, Litter, and Duff of less than 1⁄4 inch [0.64cm] diameter; Twigs and small stems of 1⁄4 - 1 inch [0.64 - 2.54cm] diameter; Branches of 1-3 inches [2.54 – 7.62cm] diameter; and large stems and branches greater than 3 inches [7.62cm] diameter (Jenkins, 2004). More generally, fuels are referred to as being light or heavy, the former attributed to fuels with a diameter of 1⁄2 inch [1.27cm] or less, thus accommodating grass, litter, duff and smaller twigs and stems, the latter referring to larger twigs, stems, branches, logs, and tree trunks (Schneider and Breedlove, n/d). Light fuels dry out faster than heavy fuels, which in combination with their higher surface area to volume ratio [74], thus higher volume to oxygen ratio means they also ignite several times faster, as do flat fuels (Ibidem). Timelag refers to the time it takes for “a dead fuel particle to lose or gain 63% of the difference between its initial moisture content and its equilibrium moisture content at a constant temperature and relative humidity” (Knapp, 2007). Each of the four fuel size classes has an affiliated timelag: less than 1⁄4 inch [0.64cm] diameter = 1 hour timelag fuels; 1⁄4 to 1 inch [0.64 – 2.54cm] diameter = 10 hour timelag fuels; 1-3 inch [2.54 – 7.62cm] diameter = 100 hour timelag fuels; and greater than 3 inch [7.62cm] diameter = 1000 hour timelag fuels (Jenkins, 2004). Furthermore, size and shape underpin how and why fuel stuffs may move within a wildfire. For example, the flatness and surface area to volume ratio of pine needles endows them with aerodynamic properties that increase the probability that, upon ignition, they will be carried, as firebrands, within a wildfire plume, to thereon fall several miles from the main fire (TDA, 2002). In contrast, though too heavy to travel any great distance, pinecones are perfectly shaped to spread fire upon igniting, falling and tumbling down a slope (Ibidem).

In toto, there are seven fuel characteristics that are indicative of how a fire will behave upon ignition: as above, fuel size and shape, and moisture content, together with loading, compactness, chemical properties, horizontal continuity, and vertical arrangement, of which the first five are the foremost influential (Ibidem). Fuel loading refers to the dry weight of combustible fuels per unit area [i.e. kg/m 2 or t/ha] (CIFFC, 2003), and is usually restricted to surface fuels of less than 3 inches [7.62cm] diameter (Jenkins, 2004). Fuel loadings vary greatly across landscapes: grassland areas typically produce 1-5 tons per acre [t/a]; shrublands 20-40 t/a; logging slash 30- 200 t/a; and timber 100-600 t/a (Ibidem). However, the highly heterogeneous nature of biotic assemblages manifests significant variations in the quantities thereof. Biochemically, the molecular make-up of plants varies by species, and in some instances, by season. At dry weight plants are principally comprised of carbon. Nonetheless, outside of severe drought conditions, water usually accounts for 80-90% of a plant’s weight (Boundless, 2016). Conversely, stripped of its former capacity for homeostasis, though levels rarely drop below 3-4%, dead biota is subject to significant fluctuations of moisture content (Jenkins, 2004). Thus, within ecosystems, and especially those that are located in the mid to high latitudes where seasonality is greatest (NASA, 2017), biota’s ratio of CO2 to H2O is one of relative spatiotemporal flux. Furthermore, while, at dry weight, all terrestrial vegetation largely constitutes the organic [carbon-based] compound cellulose [C6 H10 O5], structural composition, therein density, varies across species. For example, at dry weight Spruce [Picea glauca] is comprised 39.5% cellulose, Eucalyptus [Eucalyptus camaldulensis] 45%, Jack Pine [Pinus banksiana] 45.2%, and Quaking Aspen [Populus tremuloides] 50.2% (GIT, n.d). Hence, while cellulose has a melting point of 467°C (Dauenhauer, Krumm, and Pfaendtner, 2016), content variance across taxonomic groups manifests relative divergence in the combustibility of woody species.

Compactness refers to the spacing of fuel particles [i.e. forest litter], of which the closer the proximity, the lower their surface area exposure to air flow, thus the slower the heat spread, as there is less oxygen to sustain the combustion process (Barr and Eversole, 2003). In consequence, fire usually spreads more slowly when burning through a bed of pine needles than in open grassland (TDA, 2002). The fifth fuel facet that significantly influences wildfire behaviour is referred to as chemical properties, but specifically relates to whether vegetation contains volatile substances, such as oils, resins, wax and pitch, that act as natural fire accelerants, or contra, constitute a high mineral content, which subdues fire spread and intensity, thus act as fire retardants (Jenkins, 2004). Examples of plants that contain highly flammable chemicals include the aptly named Burning Nettle [Urtica Urens], California Sagebrush [Artemisia californica], Indian Tabacco [Nicotiana bigelovvi], Cypress [Cupressus sp], and Juniper [Juniperus sp]. Thus, metaphorically speaking, when it comes to assessing the combustibility of forests [and other fire-prone habitats] it is imperative to see the [molecular-level composition of] the wood for ‘the trees’.

In addressing the final two fuel characteristics of consequence in anticipating a wildfire’s behaviour we need move from the micro to the macro level: to the level of the landscape. Horizontal continuity refers to the distribution of fuel across the horizontal plane and encompasses all levels of the fuel complex. The characteristic is indicative of where a fire will spread, the rate thereof, and whether it will burn through surface fuels and/or aerial fuels (Ibidem). Whereupon fuel is distributed continuously fire may spread uninterrupted for miles on end. Per contra, landscapes that are strewn with patches of bare mineral soil, rock outcroppings, and other natural, and anthropogenic barriers tend not sustain fire for great distances, with the exception thereto being the advent of strong winds that facilitate fire spotting (TDA, 2002). Horizontal continuity is of particular significance in forest fires, because the distribution of aerial fuel in the canopy impacts upon moisture content in surface fuel and on near-surface wind speeds, therein fire’s rate of spread (Ibidem). Fuel’s perpendicular continuity [vertical arrangement] influences the biomass strata to which a fire ascends: when vertically continuous, fuel forms a ladder that enables fire to rise up to the canopy, but, in the absence thereof, a fire will remain at the surface. In the instance that wildfire meets uninterrupted fuels its forward rate of spread can reach speeds of up to 10.8km/p/h [6.7 mph] in forested habitat, and 22km/p/h [14 mph] in grasslands (Knowling, 2016). Though theoretically, anywhere between 0-100% of fuel may burn, fire tends consume 5-25% of standing trees in a forest fire, between 5- 95% of vegetation in a shrubland fire, and, if cured, approaching 100% in a grassland fire (Jenkins, 2004).

Ergo, biota’s biochemical composition and distribution defines the probability of a wildfire’s ignition, likely spread and intensity thereafter, together with the likelihood of spotting, torching, and crowning. Each of the 7 fuel characteristics influences between two and four of these behaviours: ignition and intensity is impacted by moisture content, loading, compactness, chemical content, and size and shape; spread by all the above, and horizontal continuity; spotting by moisture content, size and shape and horizontal continuity; and torching and crowning by vertical arrangement and moisture content.

In order of elevation, the four fire types are described below:

Ground

‘Slowly, but surely’ is the mantra of ground fire, which gradually works its way through the densest of the biomass stratum: organic soils, deep duff [75], roots, rotten buried logs, etc (Jenkins, 2004; Scott and Reinhardt, 2001). While slow, ground fire persists, often smouldering or creeping, through comparatively high moisture loads (Jenkins, 2004; Frandsen, 1987, 1991), altering soil structure and hydrology, while causing injury to above ground biomass (Scott and Reinhardt, 2001). Usually ignited by a surface fire, once started, they are difficult to assess, thus control, as they tend have little, if any flame and smoke (Schneider and Breedlove, n.d).

Surface

Burning through grass, duff, understory shrubs, downed and lower branches, and tree stumps (Schneider and Breedlove, n.d), surface fires are the most common of the four fire types (Jenkins, 2004). Their rate of spread and intensity depends upon the biomass composition, both in structure and species. However, the fuels that feed a surface fire are less compacted than that of a ground fire, which is one of several factors that underpin a differential in the rate of spread between the two. In the absence of aerial fuels (i.e. forest canopy), the behaviour of a surface fire is subject to elevated drying by solar radiation and heating, and to stronger wind speeds (Ibidem). Unlike ground fires, surface fires are easy to spot, but their controllability is dependent on the specifics of the site and meteorological conditions (i.e. wind speeds and direction, humidity and fuel moisture content).

Crown

If ground fires are akin to a flaming tortoise, crown fires are the hare. Occurring in diverse forest ecoregions of the United States (Agee, 1996), crown fires typically spread several times faster than surface fires (Rothermel, 1983), and yet faster still than ground fires. Known for being unpredictable and volatile (Schneider and Breedlove, n.d.), crown fires burn through the canopy, consuming fuels both live and dead (i.e. foliage, lichen, and fine branchwood) of which the moisture content is higher, but the density lower than in the fuels found at the surface (Scott and Reinhardt, 2001). Comprised three subcategories, crown fires are classified as passive, active, or independent (Ibidem; Van Wagner, 1977):

Passive crown fires exhibit relatively diverse behaviour, which in some instances becomes manifest as a single tree torching out, and in others extends to a group of several (Scott and Reinhardt, 2001). Particularly common in forests with an understory comprised share-tolerant conifers, though, in and of themselves passive crown fires are small in size, the radiation they emit tends accelerate the flame front’s rate of spread, while their embers are prone to igniting spot fires downwind (Ibidem).

Active crown fires engulf both the surface and canopy, to form a continuous wall of flame that rises from the fuel bed upwards (Ibidem). Radiation from burning surface fuels drives crowning, and vice versa, as radiation and spotting from crowning accelerates the rate of spread of fire at the surface (Ibidem).

Independent crown fires burn in the absence of surface fires, occurring rarely (Van Wagner, 1993), they form whereupon high winds lick crowning on steep slopes covered in vegetation with low moisture content (Scott and Reinhardt, 2001).

Passive, active and independent crown fires are not mutually exclusive, as factors including spatial variability in the landscape and temporal variability in winds speeds causes crown fires to metamorphose between the three variants: a fire behaviour which is termed an intermittent crown fire (Ibidem). While, technically, the term canopy fire applies to scenarios in which the entire canopy stratum is alight [active fires], with ‘crown’ applying to passive fires, fire ecologists tend to apply the latter term more or less generically (Ibidem). Within the study region, the Chaparral shrublands of California are particularly associated with crown fires, which are recognised as having played a significant role in the evolution of the region’s biota (USGS, 2015).

Spot

Propagated by flying or rolling sparks or embers, spot fires develop beyond the perimeter of the main fire (NOAA, 2016) and commonly manifest in consequence of unstable meteorological conditions [i.e. wind, fire whirls, and/or convection within wildfire plumes] (Andrews 1996; Schneider and Breedlove, n.d). Several fuel types are prone to spotting, including moss and lichens in the canopy, and needles, leaf litter, cured grasses, rotten and dead snags and foliage, and logging slash at the surface (Jenkins, 2004). Spotting can occur over short, intermediate, or long distances, ranging from several meters to tens of kilometres beyond the main fire (Albini, 1983).

But, no matter the many potentialities in a wildfire’s behaviour, the steadfast fundamentals of thermodynamics are of universal applicability whereupon postulating on possible trajectories: water vaporizes at 100°C, therein whatsoever its previous state, whereupon woody matter is exposed to temperatures thereto it will dry to a fire- ready state; the first phase of combustion, pyrolysis [76] occurs in woody matter at 230°C, releasing flammable gases in the process; at 380°C woody matter heats sufficiently to smoulder, thereon igniting at 590°C; 800°C is the temperature to which, through the process of heat transfer [convection and radiation], a wildfire front can preheat the atmosphere before it (Knowling, 2016). Hence, when bearing witness to these, and to many other physical processes that are bound to particular thermodynamic parameters, we can read a wildfire’s behavior in a not dissimilar way to that of an art historian decrypting the meaning of an Old Masters painting: the picture before us is encoded with signs and symbols that are known to the expert eye.

>Continue to Chapter 4.2.6 here.

Footnotes

[74 /75] The finer the fuel, the higher the surface area to volume ratio, i.e. whereas a blade of grass might have a ratio of 1:3,000, a log might have a ratio of 1:6 (TDA, 2002).

[76] Pyrolysis refers to an irreversible thermochemical decomposition that occurs in carbon-based [organic] matter whereupon exposed to high temperatures in the absence or near absence of oxygen. The process turns organic matter into its gaseous components, leaving a solid residue in the form of carbon, ash, and pyrolytic oil (Boslaugh, 2017).