Investigating the dynamic behaviour of merging fire fronts

Background Merging fire fronts have been associated with rapid fire spread and extremely destructive wildfires, yet few studies have characterised these behaviours outside the laboratory. Aims This study aimed to improve our understanding of merging fire fronts using two experimental harvested crop burns in Victoria, Australia, in 2021. Methods Unmanned aerial vehicles (UAVs) were used to capture the propagation of fire fronts exhibiting different merging behaviours, including forward and backward (V-shaped) junction fires, linear and coalescing fronts. Key results A fourfold increase in fire length of forward junction fires led to a doubling of the mean rate of spread (ROS). Forward junction fires spread, on average, three times faster than linear fire fronts, and they also exhibited higher temperature peaks during merging. An increase in the junction angle during merging was observed for all initial junction angles except 60°–90°, but in contrast to laboratory studies, no significant decrease in the ROS was found as the junction angle increased. Conclusions Our findings suggest that junction fires may lead to other dynamic fire behaviours (DFBs) involving pyroconvective interactions, firebrand showers, fire whirls and other effects. Implications These results demonstrate the need for further research into merging fire fronts dynamics, to improve operational fire behaviour models.