The wildland–urban interface fire problem – current approaches and research needs

Many individuals at Building and Fire Research Laboratory (BFRL)-NIST were helpful in conducting the experiments described in this article. The authors acknowledge the NIST Large Fire Laboratory staff, in particular Mr Laurean DeLauter, Mr Edward Hnetovsky and Mr Jack Lee (deceased). Mr John R. Shields of BFRL-NIST participated in the firebrand experiments and, along with Dr Tom Cleary, Dr Seul-Hyun Park, developed the rapid-response instrumentation. The assistance of Mr Jimmy Zurenko in the post-fire field studies is gratefully acknowledged. Partially funding for this work was received from the Joint Fire Sciences Program under Project JFSP 07-1-5-08.

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The Federal Register (2001) defines interface WUI communities as having a clear demarcation between wildland fuels and the community development area; structure density is at least 7.5 HU ha^–1 (3 HU acre^–1) or, alternatively, population density is at least 96 people km^–2 (250 people mile^–2), where HU, housing unit. Intermix WUI communities have no clear line of demarcation, structures are scattered, and wildland fuels are continuous throughout the developed area. Structure density ranges from structures very close together to 1 HU per 16 ha (1 HU per 40 acres) or, alternatively, a population density between 11 and 96 people km^–2 (28–250 people mile^–2). Occluded WUI communities have structures surrounding an area of wildland fuels usually less than 400 ha (1000 acres) in size. Structure density is similar to WUI interface communities.

Theobald and Romme (2007) found that the measure of at least 96 people km^–2, as opposed to at least 7.5 HU ha^–1, is more representative of WUI interface communities of relevance to fire managers. By assuming that there are 2 people HU^–1 (D. M. Theobald, pers. comm., 2007), they determined that the interface housing density corresponding to at least 96 people km^–2 is at least 1 HU per 2 ha. Another requirement is that the interface area is at least 10 ha in extent. Intermix areas have housing densities from 1 HU per 16 ha to 1 HU per 2 ha. Occluded WUI communities were not considered.

Stewart et al. (2003) define the WUI interface and intermix communities as both having at least 1 HU per 16 ha. Interface areas are defined to have less than 50% vegetation and are within 2.4 km of an area that is both over 500 ha in extent and more than 75% vegetated. Intermix areas are defined to have more that 50% vegetation. As presented in table 1 of Stewart et al. (2003), both the interface and intermix can be characterised by the density of housing units. Three levels of housing density were considered: high (>7.5 HU ha^–1); medium (1 HU per 2 ha to 7.5 HU ha^–1); and low (1 HU per 2 ha to 1 HU per 16 ha). Occluded WUI communities were not considered.

The Federal Register (2001) also specifies that at-risk interface communities are in the vicinity of untreated wildlands, but does not quantify what is meant by ‘vicinity’. Stewart et al. (2003) use a distance to untreated wildlands of 2.4 km (1.5 mile) to identify at-risk interface WUI areas. This is based on the observation that firebrands from wildfires can be lofted 2.4 km downwind and cause spot fires. Theobald and Romme (2007) use three different distances based on typical fuel treatment buffer zones: 0.8, 1.6 and 3.2 km (0.5, 1 and 2 miles respectively).

It is important to note that, with the exception of using the distance to untreated wildland fuels to identify WUI at-risk interface communities, the definitions of the WUI listed above are not based on any fire behaviour or fire risk considerations. By fire risk, we mean a measure of how easily a fuel, under given fire assault conditions (i.e. radiative or convective heat flux or firebrand attack), can ignite and undergo a transition from ignition to sustained flaming.