Canopy Fuel Treatment Standards for the Wildland-Urban Interface

This paper describes a canopy fuel treatment standard based on models of crown fire flame size, initiation, spread rate, and firefighter safety. Site-specific prescriptions can be developed with NEXUS or nomograms. A general prescription designed to be effective at 20-ft windspeeds up to 25 mph during drought summer fine dead fuel moisture conditions (1-hr = 4%, 10-hr = 5%, 100-hr = 7%, live = 78%) calls for a crown-fire-free zone (CFFZ) 380 ft wide with a maximum canopy bulk density of 0.10 kg/m 3 . Minimum canopy base height ranges from 2 to 18 ft depending on surface fuel conditions; for fuel model 10 (timber litter and under- story), minimum canopy base height is 13 ft. ouses and other structures can be ignited during a wildland fire by direct flame contact, radiation, or burning embers. The probability of structure ignition can be greatly reduced, but not eliminated, by surface fuel modification immediately adjacent to structures and by adherence to design and construction standards for the structure itself. However, except for an exceptionally well-designed structure, firefighter intervention is needed dur- ing the passage of a wildland fire to suppress incipient ignitions. Therefore, when designing fuel treatments for structure protection in the wildland-urban interface we should plan for the presence of firefighters at a structure during fire front passage. Firefighters need a zone around the structure in which to lay hose, raise ladders to the roof, inspect the home exterior for ignitions, and suppress ex- ternal structure ignitions. This immediate area around the structure should not allow a spreading surface fire. Surface fuels around this fire-free zone must be treated so that flame lengths allow firefighters to work safely. Even in full protective wildland clothing, firefighters are more prone to burn injury from flames than a structure is prone to ignition by radiation (Cohen and Butler 1998). In other words, radiation from flames will injure a firefighter or homeowner before untreated wood siding would ignite. There- fore, fuel treatments around structures should be designed to protect firefighters, not structures. Fuels should be treated such that the structure is within a firefighter safety zone. This is the basis for defensible space—the area around a structure where firefighters can safely work (California State Board of Forestry 1996). Many surface fuel treatment standards for creating defen- sible space exist (for example: International Fire Code Institute 1997, Moore 1981). This paper presents a method for determining the size and characteris- tics of a coniferous forest canopy treatment. Firefighter safety zone size, and thus the required area of canopy fuel modi- fication around a structure, is a function of expected flame height. A physical