Wildland-Urban Fire—A Different Approach

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