Abstract
Spotting is thought to increase wildfire rate of spread (ROS) and in some cases become the main mechanism for spread. The role of spotting in wildfire spread is controlled by many factors including fire intensity, number of and distance between spot fires, weather, fuel characteristics and topography. Through a set of 30 laboratory fire experiments on a 3 m x 4 m fuel bed, subject to air flow, we explored the influence of manually ignited spot fires (0, 1 or 2), the presence or absence of a model hill and their interaction on combined fire ROS (i.e. ROS incorporating main fire and merged spot fires). During experiments conducted on a flat fuel bed, spot fires (whether 1 or 2) had only a small influence on combined ROS. Slowest combined ROS was recorded when a hill was present and no spot fires were ignited, because the fires crept very slowly downslope and downwind of the hill. This was up to, depending on measurement interval, 5 times slower than ROS in the flat fuel bed experiments. However, ignition of 1 or 2 spot fires (with hill present) greatly increased combined ROS to similar levels as those recorded in the flat fuel bed experiments (depending on spread interval). The effect was strongest on the head fire, where spot fires merged directly with the main fire, but significant increases in off-centre ROS were also detected. Our findings suggest that under certain topographic conditions, spot fires can allow a fire to overcome the low spread potential of downslopes. Current models may underestimate wildfire ROS and fire arrival time in hilly terrain if the influence of spot fires on ROS is not incorporated into predictions.
Highlights
Spotting has played a major role in some of the largest and most destructive wildfires on record [1,2,3]
For each experiment we used 0.8 kg per m2, or 9.6 kg total, dry weight of fuel, meaning weight of fuel if all moisture was removed. This dry weight of fuel is above the approximate threshold that allows for continuous fire spread, rather than patchy fire spread, that has been observed in previous experiments in the same laboratory at lower fuel weights [30]
The spot fires had only a small amount of lateral spread prior to merging with the main fire
Summary
Spotting has played a major role in some of the largest and most destructive wildfires on record [1,2,3]. In comparison to a continuous line of fire, spotting during wildfires creates complex discontinuous patterns of fire spread through the ignition of “spot fires”.
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