Wildland-Urban Fire–A Different Approach

– by Jack D. Cohen, Rocky Mountain Research Station, U.S. Forest Service

Wildland-urban fire occurs when a fire burning in wildland vegetation fuels gets close enough with its flames and/or firebrands (lofted burning embers) to potentially create ignitions of the residential fuels (Butler 1974). Residential fire destruction is the principal problem during wildland-urban fires, but homes that do not ignite do not burn. Recognizing the potential for wildland-urban home ignitions and preventing home ignitions is the principal challenge.

Understanding how homes ignite during wildland-urban fires provides the basis for appropriately assessing the potential for home ignition and thereby effectively mitigating wildland-urban fire ignitions. Fires do not spread by flowing over the landscape and high intensity fires do not engulf objects, as do avalanches and tsunamis. All fires spread by meeting the requirements for combustion—that is, a sufficiency of fuel, heat, and oxygen. In the context of severe wildland-urban fires, oxygen is not a limiting factor so this type of fire spreads according to a sufficiency of fuel and heat. Homes are the fuel and the heat comes from the flames and/or firebrands of the surrounding fires. Recent research indicates that the potential for home ignitions during wildfires including those of high intensity principally depends on a home’s fuel characteristics and the heat sources within 100-200 feet adjacent to a home (Cohen 1995; Cohen 2000; Cohen and Butler 1998). This relatively limited area that determines home ignition potential can be called the home ignition zone.

During a wildland-urban fire a home ignites from two possible sources: directly from flames (radiation and convection heating) and/or from firebrands accumulating directly on the home. Even the large flames of high intensity crown fires do not directly ignite homes at distances beyond 200 feet. Given that fires adjacent to a home do not ignite it, firebrands can only ignite a home through contact. Thus, the home ignition zone becomes the focus for activities to reduce potential wildland-urban fire destruction. This has implications for reducing home ignition potential before a wildfire as well as implications for emergency wildland-urban fire response strategy and tactics.

One might argue that preventing the occurrence of wildfires would prevent wildland-urban fire destruction. However, our current understanding indicates that wildland fire is an intrinsic ecological process in nearly all North American ecosystems (Arno and Brown 1989; Wright and Bailey 1982). Wildland fire will always occur in forest and rangeland fire environments and will thus have an impact on people, property and resources. We may have some choice of when and where we have wildland fire, but we do not have the choice of not having wildland fire occurrence. Thus, it is not reasonable to form agency and public expectations for the nonoccurrence of wildland fires, including wildland fires encroaching on communities.

Recognizing the inevitability of wildland fire occurrence coupled with how homes ignite during wildland fires suggests a mitigation approach specific to wildland-urban fire. Given a wildland-urban fire, the home ignition zone principally determines the potential for home ignitions. This suggests a management approach that focuses on preventing home ignitions. That is, we reduce a community’s vulnerability to wildland fire rather than attempting the elimination of wildland fire encroachment. This implies an approach of community compatibility with wildland fire.

Agencies need to recognize that wildland-urban fire strategy and tactics are fundamentally different from their traditional tasks. The principal efforts for reducing ignitions focus on the home ignition zone before the wildfire occurrence. Since homeowners largely own the home ignition zone, agencies must function as partners and facilitators for implementing wildlandurban mitigations. During the wildfire, wildland-urban protection activities continue to focus on the home ignition zone for the prevention of home ignitions. Even with ignition resistant homes, protection effectiveness relies on an understanding of how homes ignite during wildland fires along with recognizing operational and logistical fire suppression limitations. These differences suggest the need for wildland-urban fire specialists both before a wildfire and during a wildfire.

Before the wildfire, the wildland-urban fire specialist uses home ignition expertise to identify vulnerable residential areas and facilitate community efforts to reduce home ignitability. During wildfires, the specialists work with homeowners and multi-agency wildland-urban fire protection teams to identify and implement effective actions for reducing home destruction during wildfires.

The above article is based on technical information that can be found at www.firelab.org

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