Fire in the Wildland-Urban Interface: Understanding Fire Behavior

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Fire in the Wildland-Urban Interface: Understanding Fire Behavior 4called a fire danger index. Many southern states postcurrent fire danger indices on their Web sites duringthe wildfire season (see Appendix for links). TheKeetch-Byram drought index is a commonly useddrought index associated with wildland fire dangerthat provides an estimate of dryness in the soil.Topography: Topography significantly affectsfire behavior. Fires move faster uphill than downhill.Slope orientation also influences fire behavior.Forests on southern or southwestern slopes generallyhave lower humidity and higher temperatures thanthose on north or northeast slopes because of the pathof the sun. Consequently, fire hazard is often higheron south and southwest facing hills.Of the three factors affecting fire behavior, fuelsare the easiest to manage by homeowners and landmanagers to lower fire hazard.Strategies to Reduce the Risk of HighIntensity FiresHomeowners: Homeowners in high fire hazardareas (see fact sheet Fire in the WIldland-UrbanInterface: Considering Fire in Florida's Ecosystems)can reduce the risk of high intensity fires affectingtheir property. An important concept for homeownersliving in hazardous areas is that of defensible space.Defensible space is defined as an area of modifiedvegetation between wildland fuels and homes thatallows firefighters to protect the home or, in absenceof firefighters, allows the home to better survive onits own. The most extensive modification ofvegetation should occur within an area extending atleast 30 feet outward from the house. Beyond thisarea, additional modification of wildland vegetationcreates a larger buffer from an approaching wildfireand further decreases the risk of damage.1. Create and maintain a defensible space byremoving the lower branches up to 10 feet fromthe ground on large trees, separating beds oflandscape plants, and removing flammablevegetation from within 30 feet from your house.Homeowners living on a steep slope (or hillside)should maintain a larger primary zone ofdefensible space by clearing flammablevegetation from an area extending up to 100 feetoutward from the house. Consult your localforester or extension agent to determine theappropriate area of defensible space for yourneighborhood.2. Landscape your yard with fire-safe plants withthe following characteristics: slow growth withlittle accumulation of dead vegetation, highmoisture content and low concentrations of oilsor other flammable chemicals in the leaves andbranches, and open and loose branching pattern.3. Maintain healthy landscape plants by wateringperiodically (as needed) and removing deadbranches (pruning).4. If natural areas exist on your lot, reduce theintensity of an approaching fire by removinghighly flammable shrubs/trees (contact yourlocal forestry agency for information on plantswith high fire hazard), reducing overall treedensity by removing small trees (calledthinning), and reducing the continuity of ladderfuels. This area can serve as a buffer betweenunaltered natural areas and a modified area ofdefensible space.Land Managers: Individuals who managelarger tracts of woodlands surrounding developedareas can also reduce the risk of high intensity fires.This is particularly important if the lands have a highfire hazard, such as many southern pine forests. Fuelreduction on these lands should focus on, but not berestricted to, creating 30-foot wide buffers at theedges of the property next to residential orcommercial developments. When prescribed fire isused to manage these buffers, a zone extending from50 to 100 feet into the forest or natural area is usuallyburned.Fuel reduction at the interface between naturallands and developments can reduce the intensity ofthe fires before they enter populated areas. Landmanagers have several options to reduce fuels:prescribed fire, mechanical reduction (using mowersor heavy equipment), herbicides, and grazinganimals. Land managers need to check localordinances for restrictions on the use of thesetreatments.
Fire in the Wildland-Urban Interface: Understanding Fire Behavior 5Applying Fire Behavior Knowledge toAssess Fire HazardWith a basic understanding of fire behavior, wecan rapidly assess the relative fire hazard of differentlandscapes. Consider the following two examples:Landscape A, a natural forest composed ofmature pine trees and a dense understory of hardwoodshrubs and small trees, is left unmanaged within 15feet of a woodland home (Figure 6). Fire has beenexcluded from the forest for more than 10 years.Landscape B also includes a natural pine forestlocated adjacent to a housing development (Figure7). However, the owner of the pine forest hasimplemented a fuel reduction program that involvesthe removal of all ladder fuels within a 100-footbuffer of the development and the use of prescribedfire every 3 to 5 years to reduce surface fuels, such asdead pine needles, downed branches, grasses, andshrubs. Notice the vertical and horizontal separationof fuels in Landscape B. In addition, the homeownercreated a 30-foot area of defensible space.vents, (2) radiant heat igniting blinds through thewindows or raising the temperature of the housesexterior to the point of ignition, or (3) flames carrieddirectly from the forest to the side of the housethrough burning fuels (shrubs, pine needles, drygrasses, or dead leaves).In Landscape B, a low intensity surface firecould develop in the surrounding forest. With theabsence of ladder fuels and a lower density of pinetrees, the likelihood of a crown fire developing issignificantly less than in Landscape A. If a crownfire should reach this forest, it may be reduced to asurface fire due the lack of sufficient understory fuelsto sustain it. Assuming that only a surface firedevelops in Landscape B, how do the same threepotential home ignition sources fare? (1) The risk ofignition from firebrands would be lower than withLandscape A because there are fewer plants toproduce firebrands. (2) The 30 feet of defensiblespace reduces the amount of radiant heat to which thehouse is exposed and consequently, reduces thepreheating or ignition of interior items and exteriorfeatures of the house. (3) Finally, the defensiblespace reduces the chance of any flames reaching theexterior walls of the house.ConclusionFigure 6. Landscape A with an unmanaged surroundingforest and insufficient defensible space. Credits:Figure 7. Landscape B with a managed pine forest andappropriate area of defensible space. Credits:Predicted Fire Behavior: In Landscape A, thepresence of ladder fuels could facilitate thedevelopment of a high intensity crown fire in thesurrounding forest. Even without a crown fire, thefire could exhibit a rapid rate of spread through theunderstory and high radiation (hot temperatures).The potential for firebrands would also be great.Possible sources of ignition for the home would be:(1) firebrands landing on the roof or entering theWhen wildfires occur near homes, homeownersoften feel powerless to protect their property. Anapproaching fire may seem unpredictable anddifficult for trained firefighters to control. However,with an understanding of the concepts of firebehavior outlined in this paper, you can anticipate theintensity and movement of a fire by consideringseveral key conditions: fuel structure, local weather,and topography. Furthermore, by altering theconditions around your house, such as thearrangement of landscape vegetation, type ofvegetation, and distance of vegetation from thehouse, you can influence the movement and intensityof fires, and consequently, improve the survivabilityof your home.ReferencesCalifornia Department of Forestry and FireProtection. 2000. Structural Fire Prevention FieldGuide: For Mitigation of Wildland Fires. California
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