Fuel shape is a significant factor in the problem of

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Fuel shape is a significant factor in the problem of spotting. Figure 6 on page 8 illustrates some fuels that are likely candidates as aerial firebrands. Each of these has been found to have traveled distances of 10 miles or more downwind from a large, raging forest fire. In these cases, their flatness and greater surface-area-to-volume ratios have increased the aerodynamic qualities of the particles, thus making it easier for convection columns to lift them to greater altitudes. The shape of fuels is also important to spotting downslope by rolling fire-brands. Pine cones, round logs, and round yucca plants are particularly troublesome in their respective areas.
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We've been using the terms large fuels versus small fuels in a relative sense. To be more specific for fuels analysis purposes, we normally break dead fuels into four size classes. Under item B, please note the following diameter sizes: Grass, litter, duff--less than 1/4-inch diameter; twigs and small stems--1/4-inch to 1-inch diameter; branches--1-inch to 3-inches diameter; large stems and branches--more than 3-inches diameter. We will give these fuel-size classes more significance later. Please turn to page 9. The next principal fuel characteristic we need to discuss is compactness. Compactness can be simply defined as the spacing between fuel particles. This affects the rate of combustion. Figure 7 illustrates how the closeness and physical arrangement of the fuel particles affects both ignition and combustion. Those that are closely compacted have less surface area exposed and less air circulation between particles, thus requiring more heat or time for ignition. It's time now for another question. Mark your choice or choices in question 4, then return to the text In question 4, you should have marked all four choices. These are all good reasons why loosely compacted fuels usually burn faster. On page 10, let us look at horizontal continuity as a principal fuels characteristic. Horizontal continuity is the extent of horizontal distribution of fuels at various levels or planes. This characteristic influences where a fire will spread, how fast it will spread, and whether the fire travels through surface fuels, aerial fuels, or both. Figure 8 shows an area of continuous fuels and an area of patchy or discontinuous fuels. If the open areas in the right-hand illustration are barren and void of any fuels, it will obviously be difficult for fire to travel from one fuel island to another. It would probably require a strong wind with spotting for fire to travel through such patchy fuels. Such fire situations do occur, and what might appear to be natural firebreaks or barriers may not stop a fire's spread. Now do question 5. Mark your choice or choices, then return to the text. You should have marked statements 2 and 3 as being true. Number l is false, since horizontal continuity applies to all levels of the fuels complex. In number 4, the continuity of fine fuels is especially important to the spread of surface fires, since fire intensity can be much less in this fuel level. Before
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  • Spring '04
  • MIchealJenkins
  • Combustion, fuel, Wildfire

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