Unravelling wildland fire morphology and structure using image velocimetry

Wildland fires have been increasing in size, frequency and intensity during recent decades, affecting entire ecosystems and societies even in regions historically not considered fire prone. Some of those fires display dynamics of extreme fire behaviour, which chaotic and large-scale nature make them challenging to study. Thus, there is a need of new methodologies for wildland fire analysis, capable of capturing spatiotemporal characteristics suitable for this application. This work presents two applications of the image velocimetry technique applied to wildland fires, offering new details on the morphology and structure of large-scale medium-intensity prescribed shrubland fires, as well as an outlook on new applications in more complex scenarios. Fire flow displacement vectors and streamlines were calculated and mapped from a high-resolution overhead visible-spectrum (RGB) video acquired during a four-hectare prescribed gorse fire. This method allowed for identification of spatially interleaved flow convergence and divergence regions, providing insight on the high-level structure of the fire front and flaming zone. The method was further expanded to identify what we refer to as “fire sweeps”, via the application of a 2D convolution operation on the displacement vector based upon a kernel carefully designed to highlight the characteristics highly divergent fire flows.