NORMATIVAS APLICABLES AL PROYECTO

There are five tools available that translate the complex science of spotting into an operational context. Like any model or tool based on so many interacting processes, these require many assumptions and simplifications. Frank Albini of the USDA Forest Service developed three of the tools, and each addresses a specific type of ember source. The assumptions, simplifications, and limitations of each model are clearly stated and acknowledged in each case. All three provide the same output, an estimate of the maximum spotting distance. One of the other two tools deals with the probability spotting will span a fire break on a grass fire, and the last tool provides an estimate of the minimum spot- ting distance at which fires can become established before they are overrun.

The first tool is for embers and spotting from single torching trees or groups of up to 30 trees (Albini 1979.) The

calculations estimate the maximum likely spotting distance for such embers, under very specific conditions. The model does not apply to “running crown fires, fires in heavy slash or chaparral under extreme winds, or fires in which fire whirls loft” the embers. It assumes the ember comes from near the top of the torching tree(s). It assumes a specific vertical profile for the wind, one with constant direction and speed increasing from canopy top to a constant value several hundred feet above. Furthermore, it cannot predict the effects of wind eddies caused by terrain, such as lee waves or rotors, or flow in canyons. It assumes the ember is a cylinder of wood, or similar shape. (Laboratory studies by Tarifa et al. (1967) showed that shape is not a major factor most of the time. Cylinders, spheres, and plates of a given density travel roughly the same distance.) The model does not predict the probability of ignition when an ember lands, nor does it predict the number of embers reaching the esti- mated maximum distance. The model allows lofting up to 305 m (1,000 ft) above ground and predicts distances up to 3.2 km (2 mi) over flat terrain with uniform fuels. Multipli- ers for noflat terrain can increase spot distance to as much as 5.6 km (3.5 mi), if the source is atop a 1200-m (4,000-ft) ridge and the distance to the valley bottom exceeds 3.2 km (2 mi).

Albini (1981) modified the torching tool so that it could be used for isolated, more sustained sources such as slash piles or fuel “jackpots.” Although the earlier model assumes a short-lived heat source and a brief surge in the fire’s plume, the newer model allows a more sustained heat source and plume lofting the ember. This model also allows estimation of maximum spotting distance when there is not uniform-height forest along the ember’s flight path. The estimate assumes a neutral or stable atmosphere and still assumes constant wind direction. The strongest plumes and most intense fires, however, tend to develop under conditions with unstable air near the ground: how much this would affect spot distance estimates is not clear.

The third tool/model is from Albini (1983) and extends the earlier models for use with wind-driven surface fires without timber cover. The model assumes the fire is linear,

perpendicular to the wind, and much longer than the front- to-back depth of the flaming front. It also assumes that any embers lofted by a surface fire rise in short surges in fire intensity. This aspect of the model relies on a theoretical model of the duration and strength of the surges, and Albini

said outright “this hypothesis, crucial in the model’s devel- opment, is unlikely to ever be tested directly.” The results of this extension only predict the lofting height of embers, which then go into the Albini (1979) model to provide the estimated maximum spotting distance. Albini (1983) did not state whether the surface-fire model still assumes, cylindri- cal wood embers; if so, it may not be appropriate for grassy fuels without any woody component.

The next tool is a pair of graphs (fig. 6-2) from Wilson (1988) that indicate the probability that a grass fire will spot across a firebreak of given width. Note that this is based on, and only strictly appropriate for, grass fires with very few trees. The only input required is an estimate of the fire’s intensity and whether or not there are trees in the fire perimeter within 20 m of the fire break. The figures (and the equation from which they are derived) indicate the probabil- ity the fire will spot across a break of given width.

The last tool is also a graph, and it is for use during active crown fires. Specifically, it is calibrated for use on crown fires in open canopy coniferous fuel types (Forestry Canada Fire Danger Group 1992). Alexander and Cruz

Figure 6-2—Probabilities of breaching firebreaks of specified width as a function of fireline intensity. Based on equations in Wilson 1988.

Figure 6-3—Mimimum distance at which a spot fire will not be overrun by the main fire. Distance depends on ignition delay and is shown for delays of 0.2 minutes (solid line); 1.5 minutes (dotted line), 5 minutes (dashed line), and 10 minutes (dash-dot line). Following Alexander and Cruz 2006.

(2006) provided an estimate of the minimum distance at which a newly ignited spot fire will not be overrun by the main fire front. This is essentially the depth of the overrun zone mentioned previously. The graph (fig. 6-3) indicates the distance/depth based on rate of spread and ignition delay (ID). It assumes that once an ember ignites a fire, it will accelerate and achieve 90 percent of its steady-state spread rate in 20 min. Fires that require longer to reach a steady- state rate of spread, including those in a more closed canopy environment, will result in increased separation distances or overrun zone depths.