WUI Fire & Ember Exposure Scale

The destruction of homes and businesses from wildland/urban interface (WUI) fires has been steadily escalating, as have the fire suppression costs associated with them. Since 2000, more than 3,000 homes per year have been lost to WUI fires in the United States. This compares to about 900 homes in the 1990s and 400 homes in the 1970s.

The WUI fire problem affects both existing communities and new construction. In the United States, the problem is most acute in the western and southern states; however, WUI fires have also recently destroyed homes in the Mid-Atlantic states and the Pacific Northwest.

One of the fundamental issues driving the destruction of homes in the WUI is the very limited research concerning the relationship between building codes and standards, and potential fire and ember exposure. The limited ember exposure information currently available does not address the full range of possibilities for home ignition  and offers little context for the design of ignition-resistant landscapes and buildings.

Although fire agencies have been aware of how fire starts and spreads in the WUI for some time now, there’s no current way to determine just how different building constructions respond to different realistic ember exposures. The resulting gap in information between fire/ember exposure and structure ignition has created a lack of tested and implementable hazard mitigation solutions.

Solution: A WUI Scale
To close that informational gap, the fire service and the general public need a WUI fire and ember exposure scale (or a WUI scale) that can consistently predict the expected severity of WUI fires by calculating the expected ember and fire exposure at specific locations during an event. This could be achieved through a combination of post fire studies, laboratory and field experiments, and computer modeling.

The technical foundation of the WUI scale has been developed jointly by NIST and the U.S. Forest Service. Once the scale is established, the information obtained from it can help form the foundation for building codes aimed at providing a level of structure ignition protection commensurate with the expected fire and/or ember exposure.

The concept is based on determining the amount of expected fire and ember exposure throughout a single, existing WUI community. The proposed WUI scale can be used to explicitly identify WUI areas that have a fire problem, as opposed to areas that meet housing density or wildland vegetation requirements. The scale can therefore also be used to determine boundaries where specific land-use and/or building construction regulations would apply. Lastly, the exposure scale can be used for both new and existing WUI communities.

The Approach
In the WUI scale approach, each fire and ember exposure threat is categorized into one of four levels. The intensity of the threat increases from category 1 to 4, and it decreases as the distance from the fire increases. So a community in or near the WUI may include one or more areas or zones at a given exposure level. Any one location in the community will have both a fire and an ember exposure rating. As an example, a location could have no fire exposure, yet have an intermediate ember exposure. Note: During an actual WUI fire, both ember and fire exposure levels need to be measured to capture the total threat to a structure. Additionally, both need to be accounted for when protecting a structure from ignition.

Two Threats
This two-component (fire and ember) exposure scale is necessary because these two threats have different origins, each with a different “reach.” To put it simply, the heat generated by a fire decreases as you move farther away from the flame front, and it is mainly affected by the fuels in the immediate vicinity of the fire. Embers, on the other hand, can travel hundreds of meters or more.

Embers that have traveled some distance may pose a threat to a particular structure, even if the fire creating the embers isn’t exposing the structure to the fire’s heat.

Exposure Sources
Fire and ember exposure can be traced to four primary sources: fire in 1) wildland fuels, 2) ornamental vegetation and 3) structures (including homes, auxiliary buildings, such as sheds and garages) and 4) vehicles. The WUI scale is designed to consider all of these sources, as well as topography and local weather. These combined parameters are referred to as FTLW–fuels, topography and local weather (wind speed, wind direction, temperature and relative humidity). In the current proposed scale, an exposure rating isn’t related to the ignition response of a particular structural element or landscaping attribute.

What’s the Point?
The primary objective of developing a WUI scale is to reduce the ignition risk of structures in the WUI. This will be accomplished by juxtaposing a structure’s ignition resistance to its anticipated exposure level. During a WUI fire, a given structure can be exposed to fire and/or embers, but it can also be hardened for embers, fire or both. Also, a closed metal-frame window could break under direct flame exposure, and combustible insulation may ignite from embers that have traveled inside the attic and away from exterior attic vents.

Once ember generation information and structure ignition information becomes available, the WUI scale will provide fire departments with a tool to assess the impact of a wildland fire on an existing community. Once the initial impact is determined, the scale may be further used to predict the WUI fire spread across the community. Fire department will be able to use the WUI scale to identify high hazard zones and effectively plan their response strategies well before a fire reaches a specific community. GIS may be used to visualize fire and ember exposures in the wildlands and through a community. Lastly, the scale may be used not only to educate homeowners and HOAs, but it may also be used to prioritize retrofit solutions.

Technical Assumptions
The following assumptions were used in the development of the WUI scale:

  • The fire and ember exposure conditions at a given location can originate from fire both in wildland fuels and in fuels within the WUI community.
  • The fire/ember exposure that each area or zone experiences is the result of both externally and internally generated exposures. In other words, structures within a zone will experience a significant ember assault from their proximity to wildland fuels and from any burning fuels within the zone itself.
  • During a WUI fire, both the fire exposure and ember assault at a given location will change with time. The fire and ember scales are intended to capture both the peak intensity and maximum duration of the exposure/assault.

A Framework for Safety
To prevent further catastrophe and property loss to WUI fires, fire agencies need a reliable resource that they can use to determine the fire risk of certain structures or subdivisions within the WUI. The WUI scale and zone concept offers a framework for evaluating the fire and ember exposure of proposed and existing WUI communities. Although there’s a lot of work to be done before we can fully implement the proposed scale, once it is implemented, it will provide a data-driven, cost-effective way to reduce losses from future WUI fires–and that will increase both civilian and firefighter safety.

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