Investigating the dynamic behaviour of merging fire fronts

Alexander I. Filkov; Brendan Holyland; Brett Cirulis; Khalid Moinuddin; Duncan Sutherland; Jason Sharples; James Hilton; Craig B. Clements; Trent D. Penman

doi:10.1071/WF24126

RESEARCH ARTICLE (Open Access)

Next Contents Vol 34(7)

Investigating the dynamic behaviour of merging fire fronts

Alexander I. Filkov

^A ^* , Brendan Holyland ^A , Brett Cirulis ^A , Khalid Moinuddin ^B , Duncan Sutherland ^C , Jason Sharples

^C , James Hilton

^D , Craig B. Clements ^E and Trent D. Penman

^A

+ Author Affiliations

- Author Affiliations

^A School of Agriculture, Food and Ecosystem Sciences, Faculty of Science, University of Melbourne, 4 Water Street, Creswick, Vic 3363, Australia.

^B Institute of Sustainable Industries and Liveable Cities, Victoria University, Melbourne, Vic 8001, Australia.

^C School of Science, University of New South Wales Canberra, Canberra, ACT, Australia.

^D Covey Associates Pty Ltd, Maroochydore, Qld, Australia.

^E Wildfire Interdisciplinary Research Center, San José State University, San José, CA 95192-0104, USA.

^* Correspondence to: alexander.filkov@unimelb.edu.au

International Journal of Wildland Fire 34, WF24126 https://doi.org/10.1071/WF24126

Submitted: 31 July 2024 Accepted: 31 May 2025 Published: 28 June 2025

© 2025 The Author(s) (or their employer(s)). Published by CSIRO Publishing on behalf of IAWF. This is an open access article distributed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND)

Abstract

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.

Keywords: fire behaviour, junction fires, merging fire fronts, rate of spread, spot fire coalescence, temperature, thermal characteristics, wildfires.

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