Carlos G. Rossa

The effect of fuel moisture content on the spread rate of forest fires in the absence of wind or slope

Most studies on the effect of fuel moisture content (FMC) on forest fire behaviour focus on dead fuel moisture; mechanisms of fire spread in live vegetation are considered to remain unexplained by current theory and modelling. In this work, an empirical model for quantifying the effect of FMC on the ratio between spread rate and fuel bed height of fires in the absence of wind or slope was proposed. The model was fitted using data from laboratory experiments, carried out in fuel beds representative of natural litter and shrubland fuel complexes in a wide range of FMC (6–179%), and tested against data from field experiments and wildfires. The pattern of spread rate variation with FMC, namely its reduced rate for values above ~80%, was explained by the ratio between fuel low heat content and energy required for ignition.

A laboratory-based quantification of the effect of live fuel moisture content on fire spread rate

Observational evidence of an effect of live vegetation moisture content on fire spread rate remains extremely scarce despite the significance of fire activity in fuel complexes dominated by live components. This study assessed the moisture content effect of quasi-live fuels on fire spread rates measured in laboratory experiments. Fuel beds were built by vertically placing vegetation clippings to reproduce the natural upright fuel structure. The fuel drying process during storage resulted in a wide moisture content range (13–180%). An exponential damping function was fitted to rate of spread observations in four fuel types, indicating that rate of spread is halved by an increase in live moisture content from 50 to 180%. This effect, especially at higher moisture contents, was weaker than that predicted by theoretical formulations and from studies in mixtures of dead and live fuel.

Study of the jump fire produced by the interaction of two oblique fire fronts. Part 1. Analytical model and validation with no-slope laboratory experiments

When two fires approach each other, convective and radiative heat transfer processes are greatly enhanced. The interaction between two linear fire fronts making an angle θoi between them is of particular interest as it produces a very rapid advance of their intersection point with intense radiation and convection activity in the space between the fire lines. This fire is designated here as a ‘jump fire’ for when the value of θoi is small, the intersection point of the fire lines can reach unusually high rate of spread values that decrease afterwards in the course of time. A very simple analytical model based on the concept of energy concentration between the fire lines is proposed to explain this behaviour, which in large-scale fires can be of great concern to personnel and property safety. Experimental tests performed at laboratory scale on a horizontal fuel bed confirmed the basic assumptions of the model and provide a framework to extend the present analysis to more general conditions, namely to explain the behaviour of real fires. Given the rapid changes in fire behaviour, ‘jump fires’ can be considered as a form of extreme fire behaviour.

Behaviour of slope and wind backing fires

Laboratory experiments of backing fires with slope (–60 to 0°) and wind (–4.5 to 0 m s–1) were carried out in fuel beds of dead Pinus pinaster Ait. needles and straw at a 0.6-kg m–2 fuel load, evaluating rates of spread and flame geometry. Wind velocity measurements inside and above the fuel beds were also carried out. Increase in fuel moisture content decreased the ratio between downslope and level-ground rates of spread (ROS). The ROS decrease with slope angle followed by an increase agreed well with flame geometry data that provided an estimation of the amount of radiation reaching the fuel bed. Features of slope backing fire behaviour could be reasonably estimated based on no-slope fire spread rate. Evidence was found that fuel moisture influenced the ROS of backing fires with wind, despite with an effect opposite to that of slope. Reduced penetration of air into the fuel beds explains the small ROS variation and results suggest that for an increasingly deep fuel bed, the mean ROS tends asymptotically to the no-wind ROS.

ON THE FIRE-SPREAD RATE INFLUENCE OF SOME FUEL BED PARAMETERS DERIVED FROM ROTHERMEL’S MODEL THERMAL ENERGY BALANCE

We analysed the role of some fuel bed properties on forest fire-spread rate based on the thermal energy balance upon which the well-known fire-spread rate model of Rothermel (1972) was developed, showing that neither fuel bed height, load or density directly influence the thermal energy balance. The influence of such parameters, often inferred from empirical descriptions of spread rate, must result from indirect effects on heat transfer mechanisms. The fraction of heat transferred from the flame to the unburned fuel depends mostly on fuel moisture content and is independent of spread rate and flame geometry. Because empirical models usually implicitly assume the underlying mechanisms of fire spread for describing fire behaviour, this study results can assist at idealizing and delineating future experiments and approaches.