Concepts for Future Large Fire Modeling

A small number of fires escape initial attack suppression efforts and become large, but their effects are significant and disproportionate. In 1983, of 200,000 wildland fires in the United States, only 4,000 exceeded 100 acres. However, these escaped fires accounted for roughly 95 percent of wildfire-related costs and damages (Pyne, 1984). Thus, future research efforts logically will focus on the difficult and complicated task of modeling large fires. Large fires often demonstrate higher fireline intensities, different modes of propagation and different distribution patterns through time than do steady-state fires. It is essential, although difficult, to simulate, model, and predict large fire management considerations. In the future, wildland fire modeling efforts are likely to focus on the concepts and peculiarities that large fires present in terms of behavior, suppression and control attributes. This paper reviews the problems that large fires pose to fire analysts. Presented at the Symposium on Wildland Fire 2000, April 27-30, 1987, South Lake Tahoe, California. Graduate Research Assistant and Professor, Department of Forestry and Resource Management, University of California. Berkeley. CONSIDERATIONS FOR MODELING LARGE FIRES No known computer model is currently available that can reliably simulate the behavior of large fires. The reasons are several: 1. Heterogeneous conditions of weather, fuel, and topography are encountered during a large fire because of the lengthy burning times and the large size that the fire finally attains. Hence, every model's inherent assumptions of fuel and weather continuity and uniformity are violated by large fires. 2. Large fires spread by means other than a flaming front through surface fuels. Crowning and spotting are different modes of propagation that often occur during a conflagration. 3. Fire-related phenomena--such as firewhirls, horizontal roll vortices, and convection columns--not common during small fires, often are present during large fires. Those phenomena magnify the spread and heat output rates and make the suppression efforts more difficult. 4. Large fires often demonstrate sudden, exponential increases in fireline intensity ("blow-up" phenomenon) accompanied by violent convection, which is sufficient to preclude direct control or to upset existing suppression plans. Fires seem most likely to blow up when high loads of heavy and dry fuels, unstable atmosphere, and windspeeds greater than 20 miles per hour exist simultaneously (Byram, 1959).