Significance Recent wildfire events throughout the world have highlighted the consequences of residential development in the wildland-urban interface (WUI) including hundreds to thousands of homes burned during a single wildfire to, more tragically, firefighter and homeowner fatalities. Despite substantial investments in modifying wildland fuels near populated areas, losses appear to be increasing. In this article, we examine the conditions under which WUI wildfire disasters occur and introduce a wildfire risk assessment framework. By using this framework, we examine how prefire mitigation activities failed to prevent significant structure loss during the Fourmile Canyon fire outside Boulder, CO. In light of these results, we suggest the need to reevaluate and restructure wildfire mitigation programs aimed at reducing residential losses from wildfire.
The National Fire-Danger Rating System (NFDRS), implemented in 1972, was revised during a 3-year project (1975 to 1978) and reissued as the 1978 NFDRS. This report describes the developmental history of the NFDRS and its technical foundation. Detailed information is provided on modeling forest fuels and fuel moisture, and on development of the NFDRS components and indexes. The report presents equations used in the 1978 NFDRS and an extensive bibliography.
Large wildfires of increasing frequency and severity threaten local populations and natural resources and contribute carbon emissions into the earth-climate system. Although wildfires have been researched and modeled for decades, no verifiable physical theory of spread is available to form the basis for the precise predictions needed to manage fires more effectively and reduce their environmental, economic, ecological, and climate impacts. Here, we report new experiments conducted at multiple scales that appear to reveal how wildfire spread derives from the tight coupling between flame dynamics induced by buoyancy and fine-particle response to convection. Convective cooling of the fine-sized fuel particles in wildland vegetation is observed to efficiently offset heating by thermal radiation until convective heating by contact with flames and hot gasses occurs. The structure and intermittency of flames that ignite fuel particles were found to correlate with instabilities induced by the strong buoyancy of the flame zone itself. Discovery that ignition in wildfires is critically dependent on nonsteady flame convection governed by buoyant and inertial interaction advances both theory and the physical basis for practical modeling.wildfires | buoyant instability | flame spread | convective heating
The work reported herein was done while we were assigned to the Intermountain Forest and Range Experiment Station's Northern Forest Fire Laboratory at Missoula, Montana. We were aided materially by Robert E. Burgan. Other researchers who contributed to the updating of the National Fire-Danger Rating System and their contributions were from the Intermountain Station, Missoula, Montana-Richard C. Rothermel, Frank A. Albini, and Patricia L. Andrews, who assisted in adapting the current fire modeling technology, including the addition of 1000-hour fuels and herbaceous to 1-hour transfer; Hal E. Anderson and James K. Brown, who worked on fuels and fuel models; Donald F Fuquay, and Donald J. Latham, who worked on the lightning-caused fire occurrence index; North Central Forest Experiment Station, East Lansing, Michigan-Von J. Johnson, William A. Main, and Craig A. Johnson, who worked on human-caused fire occurrence index; and the Rocky Mountain Forest and Range Experiment Station, Fort Collins, Colorado-Michael A. Fosberg, who worked on fuel moisture models; and R. William Furman and Glen F. Brink, who collected fire weather data needed to develop the system. Cover: Four climate classes in the United States are associated with different vegetation moisture contents, which affect fire spread and fire danger rating.
We explore the basis of understanding wildland fire behaviour with the intention of stimulating curiosity and promoting fundamental investigations of fire spread problems that persist even in the presence of tremendous modelling advances. Internationally, many fire models have been developed based on a variety of assumptions and expressions for the fundamental heat transfer and combustion processes. The diversity of these assumptions raises the question as to whether the absence of a sound and coherent fire spread theory is partly responsible. We explore the thesis that, without a common understanding of what processes occur and how they occur, model reliability cannot be confirmed. A theory is defined as a collection of logically connected hypotheses that provide a coherent explanation of some aspect of reality. Models implement theory for a particular purpose, including hypotheses of phenomena and practical uses, such as prediction. We emphasise the need for theory and demonstrate the difference between theory and modelling. Increasingly sophisticated fire management requires modelling capabilities well beyond the fundamental basis of current models. These capabilities can only be met with fundamental fire behaviour research. Furthermore, possibilities as well as limitations for modelling may not be known or knowable without first having the theory.
Wildland-urban fire destruction depends on homes igniting and thus requires an examination of the ignition requirements. A physical-theoretical model, based on severe case conditions and ideal heat transfer characteristics, estimated wood wall ignition occurrence from flame radiation heating and piloted ignition requirements. Crown fire experiments provided an opportunity for assessing model reliability. The crown fire experiments were specifically instrumented with wood wall sections and heat flux sensors to investigate direct flame heating leading to home ignition during wildland fires. The experimental results indicated that the flame radiation model overestimated the structure-to-flame distance that would result in wood wall ignition. Wall sections that ignited during the experimental crown fires did not sustain flaming after crown fire burnout. The experiments also revealed that the forest canopy attenuated the flame radiation as the crown fire spread within the forest plot. Ignition modeling and the associated crown fire experiments described the flame-to-structure distance scale associated with flame heating related to wall ignition. Résumé :Les pertes causées par un feu de forêt en milieu urbain viennent des maisons qui s'enflamment et exigent par conséquent un examen des conditions requises pour qu'elles prennent feu. Un modèle à la fois physique et théo-rique, basé sur les conditions de cas sévères et les caractéristiques idéales de transfert de chaleur, a servi à estimer si un mur de bois s'enflammerait en étant soumis à la chaleur irradiante des flammes et à des conditions d'ignition contrôlées. Des feux de cime expérimentaux ont fourni une occasion d'évaluer la fiabilité du modèle. Les expériences avec des feux de cime ont été conçues avec des sections de mur de bois et des détecteurs de flux de chaleur pour étudier la chaleur directe des flammes qui met le feu aux maisons lors d'un feu de forêt. Les résultats expérimentaux montrent que le modèle de chaleur irradiante des flammes surestime la distance requise entre la structure et les flammes pour qu'un mur de bois s'enflamme. Les sections de mur qui se sont enflammées à la suite des feux de cime expérimentaux n'ont pas continué à produire de flammes après l'extinction des feux de cime. Ces expériences ont également montré que le couvert forestier a atténué l'irradiation des flammes pendant que les feux de cime se propageaient dans la parcelle de forêt. La modélisation de l'ignition et les feux de cime expérimentaux qui y sont associés donnent une idée de la distance entre les flammes et la structure qui est nécessaire pour que la chaleur des flammes mette le feu à un mur.[Traduit par la Rédaction] Cohen 1626
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