Understanding Fire Spread in Wildland-Urban Interface Communities
Szasdi Bardales, Fernando
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More than 70,000 U.S. communities, containing 46 million homes, are at risk of wildfires, yet only about 7,000 communities have adopted protection measures. The wildland urban interface (WUI) is defined as a geographic area where human developments and flammable vegetation merge in a wildfire-prone environment. The losses due to wildfire in WUI have been rising in the past decade; which could be attributed to changes in climate, resulting changes in vegetation growth and fuel availability, and increased land developments in the WUI. The majority of the wildfires ranked among the most destructive in California have taken place in recent years. While the number of households at risk is rising and the population living in the WUI increases, there is no indication that this problem will recede in the near future. The layout of communities and the characteristics of building construction play an important role on how the fire spreads within a community. However, existing research on wildfires has focused on fire progression within the wildland, with no well-established model to study fire spread within a WUI community. The existing fire spread models at the community level have been developed for post-earthquake fire scenarios for an urban environment involving equally spaced and equally-sized square buildings in a dense urban area, which is not necessarily the urban environment in a WUI community. Accurate physics-based models are thus needed to investigate the process of fire spread in a WUI community. Once physics-based models are established, vulnerabilities in a community can be identified, effective defensive actions can be recommended, and more efficient firefighting response strategies can be planned during a fire event. This research studies the process of fire spread inside WUI communities affected by wildfires in North America. The suitability of existing urban fire spread models to predict fire progression inside a WUI community is assessed. As part of this process, first, the currently available data sources that could be used to determine the fire spread rate inside communities affected by wildfires are identified and described. Then, fire spread rate is determined for nine wildfire events, which are ranked among the most destructive wildfires in the history of North America. The results are compared with historical 20th century fires documented in existing literature. The same nine fires are also analyzed using an existing simplified fire spread model in order to assess the validity of the model to predict the temporal and spatial distribution of new fire ignitions within the affected community. The results show that the simplified model underestimates the fire spread rate for recent wildfire cases. Finally, a semi-physics-based fire spread model is proposed. It considers radiation and fire branding mechanism as modes of fire spread inside a community. The model is validated using the case of 2007 Witch and Guejito fires in the Trails community in California. The proposed model can be used to quantify fire spread rate and the number of affected structures inside a community during a wildfire event.