Flame spread and combustion dynamics in pine litter under unsteady wind conditions

Understanding flame spread and combustion dynamics in surface fuels is important for both planning and evaluating the impact of prescribed fires. However, prescribed fires tend to be conducted under light and unsteady wind conditions as compared with those which have been well studied for wildfire scenarios. To improve this understanding and provide data for next-generation fire models, it is therefore necessary to collect detailed time-resolved observations of these processes. To accomplish this, we conducted a series of small-scale field experiments in pine needle litter. 10 m x 10 m plots were constructed with fuel loads of either 0.5 kg/m2, 1.0 kg/m2, or 1.5 kg/m2. Wind was measured with an array of 17 anemometers and flame spread and combustion mode were monitored with an overhead infrared camera and thermocouple arrays. The light and gusty wind conditions resulted in unsteady fire behavior and a single relationship between wind speed and spread rate is unable to capture the range of behavior observed. A novel technique is introduced for identifying regions of flaming versus smoldering combustion in the infrared imagery. This approach indicates that flaming and smoldering times can be related directly to fuel loads, rather than wind conditions, though more work is required to link these times to burning rates. The implication is that dynamic flame spread in these conditions is difficult to predict without models that account for fire-atmosphere interactions. However, fuel consumption, and therefore prescribed fire effects, may be more straightforward to predict. This work has also resulted in a uniquely detailed dataset for testing fire models.