Fire behaviour in dry eucalypt forests in Australia is characterised by the occurrence of spotfires—new fires ignited by the transport of embers ahead of an existing fire. Under most burning conditions, spotfires play little role in the overall propagation of a fire, except where spread is impeded by breaks in fuel or topography. Spotfires allow these impediments to be overcome.

However, under conditions of severe bushfire behaviour, spotfire occurrence can be so prevalent that spotting becomes the dominant propagation mechanism and the fire spreads as a cascade of spotfires forming a ‘pseudo’ front.

It has long been recognised that the presence of multiple individual fires affects the behaviour and spread of all fires present. The converging of separate individual fires into larger fires is called coalescence and can lead to rapid increases in fire intensity and spread rate, leading to the phenomenon of a ‘fire storm’. This coalescence effect is frequently used in prescribed burning, with multiple point ignitions used to rapidly burn out large areas.

This project is focusing on:

• Fire coalescence to provide better predictions of fire propagation
• The intrinsic dynamics of flame front propagation as a contributor to fire spread across different spatial and temporal scales
• Within a simulation framework an end-to-end model of the behaviour of mass spotfires, from firebrand/ember launch to fire coalescence.

The modelling and simulation aspects of the project have contributed to understanding the processes that drive fire coalescence and dynamic fire spread. In particular, the research has addressed the role that fire-line geometry (especially curvature) plays in the dynamic propagation of bushfires.

The team has demonstrated the performance advantages of fire propagation models incorporating curvature dependence when applied to simple wind-driven fires at both laboratory and field scales. The research has also produced fundamental insights into how the shape of the fire line affects the dynamic behaviour of the fire as a whole. Coupled fire-atmosphere modelling was used to investigate how fire-induced air movements (pyroconvection) can produce significantly enhanced rates of spread for certain fire shapes.

Utilising the research outcomes will include development of education and training materials relating to dynamic fire behaviour and extreme fire development, which will incorporate the research findings on fire coalescence and mass spotfires.

Research findings will also be used to develop metrics of relevance to the National Fire Danger Rating Project. In particular, existing measures of ‘convective fire power’ based solely on information relating to the fire perimeter will be extended to include contributions from within flaming zones where spot fire coalescence can contribute significantly to pyroconvective release.