Modeling the spatial distribution of forest crown biomass and effects on fire behavior with FUEL3D and WFDS

Abstract

Crown fire poses challenges to fire managers and can endanger fire fighters. Understanding of how fire interacts with tree crowns is essential to informed decisions about crown fire. Current operational crown fire predictions in the United States assume homogeneous crown fuels. While a new class of research fire models, which model fire behavior with computational fluid dynamics (CFD), can be used to model fire in heterogeneous crown fuels, the potential impacts of variability in crown fuels have not yet been explored. In this study we introduce a new model, FUEL3D, which incorporates allometric theory and a simple 3D plant architecture approach to model the distribution of fuel within individual tree crowns. We then test the effects of crown fuel heterogeneity on fire behavior with a CFD fire model, WFDS, by simulating fire under a homogeneous tree crown and a heterogeneous tree crown modeled with FUEL3D, with two different levels of surface fire intensity; model output are used to estimate the probability of tree mortality, linking fire behavior and fire effects at the scale of an individual tree. Variability within a tree crown altered the timing, magnitude and dynamics of how fire burned through the crown; effects varied with surface fire intensity. In the lower fire intensity case, the heterogeneous tree crown barely ignited and would likely survive, while the homogeneous tree had nearly 80% fuel consumption and an order of magnitude difference in net radiative heat flux. In the higher intensity case, both cases burned readily. Differences for the homogeneous tree between the two surface fire intensity cases were minimal in nature but were dramatic for the heterogeneous tree. These results suggest that heterogeneity within the crown causes more conditional, threshold-like interactions with fire; both crown fire initiation and propagation may be more sensitive to crown fuel architecture than currently considered.

Citation

Parsons, Russell A.; Mell, William; McCauley, Peter. 2010. Modeling the spatial distribution of forest crown biomass and effects on fire behavior with FUEL3D and WFDS. In: Viegas, D. X., ed. Proceedings of the VI International Conference on Forest Fire Research; 15-18 November 2010; Coimbra, Portugal. Coimbra, Portugal: University of Coimbra. 15 p.