Revisting Intensity of Combustion Waves to Address Outstanding Issues in Wildfire Modelling

The global wildland fire management community faces pressing climate change and operational challenges and requires improved capabilities in existing modelling tools or the development of novel decision support tools to limit the negative impact of wildfires and to increase use of prescribed burning where appropriate. This presentation will discuss limitations in the existing approaches to incorporating fuel structure effects in different model types (empirical, semi-empirical, detailed physics-based). In particular, novel experimental data will be presented addressing previously identified limitations [1] in the description of surface fuel beds in one of the most widely-used semi-empirical models; the Rothermel model, which underpins many current operational models.

The Rothermel model [2] involves a conservation of energy approach, incorporating separate terms to describe energy release rate in the combustion zone (reaction intensity) and energy transferred to the unburnt fuel (propagating flux), and incorporates a number of empirical closure terms.  The reaction intensity is empirically based, with the underpinning experimental measurements described in Frandsen and Rothermel [3]. By measuring the mass loss rate in a section of a fuel bed, Frandsen and Rothermel were able to characterize the intensity distribution within the combustion zone. However, the interacting effects of simultaneously varying fuel loading and packing ratio were not systematically considered, complicating efforts to understand the interacting effects of fuel loading and bulk density.

This study presents a series of laboratory-based flame spread experiments (no wind) involving excelsior fuel beds of varying structural conditions (Fuel Height: 0.02 to 0.12 m, Bulk Density: 3.3 to 20 kg/m³, Fuel Loading: 0.2 to 0.4 kg). The reaction intensity was calculated via a similar procedure to that described by Frandsen & Rothermel [2] as ‘Method 2’, in which the longitudinal length of the mass measurement region is greater than or equal to the combustion zone depth.

Clear trends in the peak mass loss rate and profile with bulk density were observed with a significant reduction at lower fuel loadings (0.2 kg/m²), and the reaction time was observed to increase at higher bulk densities along with a lengthening in the reaction intensity distribution region (further behind the combustion wave front). These results, along with existing observations of the trailing, in-depth combustion region in porous fuel beds, can be used to further investigate the observed tendency for underprediction of spread rates when the Rothermel model is applied to compressed fuel bed scenarios and has practical implications for other fire behaviour modelling applications. For example, improved characterisation of the overall combustion wave may enable improved modelling of smoke generation, surface-to-crown fire transition, and fuel consumption (e.g. to evaluate prescribed fire effectiveness).

[1] Z. Campbell-Lochrie, M. Gallagher, N. Skowronski, R.M. Hadden, The effect of fuel bed structure on Rothermel model performance, Int. J. of Wildland Fire. 33 (2023).

[2] R.C. Rothermel, A Mathematical Model for Predicting Fire Spread in Wildland Fuels, Research Paper INT-115, USDA Forest Service.,1972.

[3] W.H. Frandsen, R.C. Rothermel, Measuring the energy-release rate of a spreading fire, Combust Flame 19 (1972) 17–24.