Overview

The potential for wildfire-caused damage to structures is a growing problem as wildland fuels accumulate, the climate becomes more erratic, and more people choose to build structures in areas with a significant potential for wildfire. Wildfire Risk was created in response to the growing need for scientific, accurate, consistent, and up-to-date wildland-urban interface (WUI) risk assessment throughout the United States.

Wildfire Risk classifies the landscape based on the relative amount of built environment (structures, roads, and other infrastructure) and wildland fuels. This distinction recognizes that wildland fires behave differently when burning in pure wildland fuels than fuels interrupted by structures and roads6. Similarly, suppression of wildland fires is conducted differently, and with varying degrees of success, in remote areas versus densely populated areas. These variables are captured in Wildfire Risk by recognizing three separate wildfire risk types, each modeled with its own individual set of inputs and associated risk assessment methodology. The three risk-type modules are:

• Wildland
• Intermix
• Interface

Wildfire Risk Weighted Components

Wildland

Wildland areas are those with relatively continuous fuel and limited presence of structures, roads, and other human-caused disturbances. Relatively few people live in these areas, limiting the anthropogenic ignition source, but any structures present in these areas are surrounded by fuel. Depending on weather and topography (both accounted for in Wildfire Risk), this can make suppression difficult or impossible.

Potential mitigation measures are typically focused on the treatment of vegetation immediately surrounding a structure and hardening the structure itself.

Fires burning in wildland areas will typically burn uninterrupted until conditions are no longer favorable or the fire moves to a less volatile fuel.

Intermix

Intermix areas are characterized by a higher density of structures – denser than wildland areas but less so than interface areas – roads and other infrastructure that break up the continuity of natural fuel on the landscape. Threats to structures in these areas focus on fuels and the complexity of suppression in this environment. Higher road densities allow better access for suppression resources, but they also introduce an element of potential confusion for access and egress.

Suppression strategies in intermix areas must account for groups of houses instead of single structures, which might be encountered in wildland areas. Along with suppression complexities, the presence of greater numbers of people in an intermix area can also mean a higher risk of ignitions due to barbecues, fireworks, matches, etc. Wildfire Risk addresses the complexity of intermix areas by using input datasets not used in wildland and interface areas.

Interface – The Ember Zone

Interface areas are different because they are the only place in Wildfire Risk where firesheds, a spatial aggregation unit in Wildfire Risk, are not utilized. Firesheds are delineated primarily by topography and generally represent areas with the same fire behavior. In interface areas, only the fringes of large towns and cities are threatened by wildfire. Beyond the distance where large numbers of embers could commonly threaten homes, wildfire risk becomes much lower. Wildfire Risk refers to this as "Minimal Likely Exposure."

Instead of firesheds, interface areas use ember zones and structure distance to segregate the landscape. The purple areas shown in Figure 5 are where the threat of flame impingement and ember cast is the greatest. The distance this so-called "Adjacent Zone" penetrates the interface is based on maximum spotting distance as calculated by fire behavior modeling. The adjacent zone is subdivided based on the average distance between structures and topography. Smoke is the primary threat to structures in the "Peripheral Zone," which extends even farther from major fuel beds.

'Fire' service is available for the US only. It contains the following methods: