Wildland-Urban fire spread is commonly sustained by the burning community, principally structures as well as vegetation and other materials. Previous WU fire disaster examinations (Cohen 2000; Cohen and Stratton 2003; Cohen and Stratton 2008; Graham et al. 2012; Calkin et al. 2014) determined that fire spread within a community, particularly communities with overlapping HIZs, occurred hours after the wildfire ceased significant burning near the community. Hence, disastrous community fire destruction during extreme wildfires is a structure ignition problem not a wildfire control problem.
Key Finding (5): Structure ignitions during extreme wildfires are principally determined by the local ignition conditions of the home ignition zone (HIZ); that is, a structure and its flammable attachment’s ignition vulnerabilities to burning embers in relation to burning materials within 30 m. An extreme wildfire provides ignition hazards from flames and burning embers; HIZ ignition vulnerabilities determine structure ignitions and community fire disasters. Hence, WU fire disasters are a structure ignition problem.
Examples illustrating how HIZs determine ignitions
Figure 17: Post-fire Lytton Village scene between 1 and 3 Streets on July 12. The yellow X identifies the surviving house identified by a yellow arrow in Figure 14 (First Nations Emergency Services Society of BC).
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