Understanding Fire Response to Spatial Variations in Vegetation Distribution and Wind Flow
Dr. Bret Butler | US Forest Service, Rocky Mountain Research Station
One challenge facing Department of Defense (DoD) managers is managing fire spread over a landscape that has significant fuel discontinuities (natural and introduced). Fire spread can be characterized as a continuous sequence of ignitions. Ignition is a local phenomenon, governed by complex interactions between temporal and spatial variations in fuel and environment. Seemingly insignificant changes in vegetation orientation or spacing can significantly affect the ignition process and result in fire either bridging a gap in fuels or extinguishing at the gap boundary. This study seeks to improve understanding of ignition by exploring the physics underlying fire’s response to gaps in vegetation under the influence of wind and microtopography.
The underlying hypothesis for this work is that ignition is a dose/response mechanism. The dose is total energy absorbed by fuel particles and the response is ignition or extinction. This study seeks to identify the minimum energy dose to affect ignition through study of fire’s interaction to gaps in the fuel array (essentially increase gap size until fire extinguishes at the gap boundary). Light detection and ranging will be used to characterize the three dimensional distribution of fuels. Local microscale (<1 m) winds will be measured at the gaps using sonic and mechanical anemometers combined with fine scale wind simulations. Local energy transport within and above fuel gaps will be characterized using established sensors and methods. Infrared and visible imagery of flames and fuels from both horizontal and nadir orientations will characterize flame presence and geometry and kinetic and radiometric fuel surface temperature at the gaps. From these data a statistical dose/response relation will be developed and analyzed to identify critical spatial scales in fuels that affect ignition and subsequent fire spread. Lab- and field-scale data will be used to evaluate a new heat transfer based coupled fire/atmosphere model.
The findings from this effort will identify key physical processes driving ignition as a function of fuel spatial distribution under a range of environmental conditions. Knowledge of the physical processes controlling gap bridging will substantially enhance the ability to predict fire ignition and rate of spread in discontinuous fuels under a wide range of fuel and environmental conditions. Findings will be formatted into guidelines that will assist fire managers in determining when they should “pay attention” to variability in wind, microtopography and vegetation distribution. New knowledge regarding the physical processes controlling ignition will facilitate development of operational models that can more realistically predict fire behavior and rate of spread in discontinuous fuels more accurately than currently possible. Findings from this work will improve understanding of fire and how it affects fuels and soils. Critical research needs and paths that promise improved operational fire model performance will be identified.