An Empirical Model for the Effect of Wind on Fire Spread Rate

Abstract

Predicting wind-driven rate of fire spread (RoS) has been the aim of many studies. Still, a field-tested model for general use, regardless of vegetation type, is currently lacking. We develop an empirical model for wind-aided RoS from laboratory fires (n = 216), assuming that it depends mainly on fire-released energy and on the extension of flame over the fuel bed in still air, and that it can be obtained by multiplying RoS in no-wind and no-slope conditions by a factor quantifying the wind effect. Testing against independent laboratory and field data (n = 461) shows good agreement between observations and predictions. Our results suggest that the fuel bed density effect detected by other work may be a surrogate for the amount of fuel involved in combustion, which depends on fuel load. Because RoS under windless conditions is unaffected by fuel load, the involved mechanisms differ from wind-aided propagation. Compared to shallow fuel beds, the wind effect is usually modest in deep vegetation, because tall fuel complexes are dominated by live fuels (high moisture content) and flames extend less above the vegetation when fuel moisture is high. The present work warrants further inspection in a broader range of field conditions.

The Effect of Ecophysiological Traits on Live Fuel Moisture Content

“…Live fuel moisture content (LFMC) is a landscape-level management metric that, along with weather and topography, is incorporated into rate-of-spread models and fire danger ratings [1][2][3][4]. LFMC is expressed as the ratio of water content in fresh plant tissue to the dry weight and represents the amount of moisture that needs to evaporate from a fuel source before ignition can occur.…”

Live Fuel Moisture Content: The ‘Pea Under the Mattress’ of Fire Spread Rate Modeling?

“…This range overlaps roughly with that used in the experimental apparatus of Fletcher et al (2007) [11] (80-140 kW m −2 ). Still, in the laboratory fire spread tests of [13], overall FMC was shown to be a good predictor for RoS. Thus, different heat flux magnitudes leading to distinct ignition mechanisms do not seem to explain the apparent conflict in the influence of live FMC on RoS between laboratory and field fires, and another explanation must be provided.…”

“…There are studies confirming that ignition mechanisms differ between live and dead fuels [8][9][10][11]. We discuss the hypothesis that current research, including recent work [12,13], is sufficient to establish that those differences in the ignition process do not preclude RoS modeling from overall fine FMC (live and dead fuels) such that the resulting accuracy is acceptable for operational purposes.…”

“…We will try to solve this "mystery" by following the quote of Sir Arthur Conan Doyle (the creator of Sherlock Holmes): "Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth." Rossa and Fernandes (2018) [13] reported RoS data for wind-assisted laboratory trials in shrub-like fuel beds (litter overlayered by live tree twigs, n = 45). Using the equations presented in [25] we can estimate the net horizontal heat flux through the fuel bed [3] in those trials, which yields an average of 76 kW m -2 , reaching a maximum of 121 kW m −2 .…”

“…Thus, different heat flux magnitudes leading to distinct ignition mechanisms do not seem to explain the apparent conflict in the influence of live FMC on RoS between laboratory and field fires, and another explanation must be provided. Adding to the uncertainty, RoS was predicted from the overall FMC in recent field-tested RoS models for fire spread in no-wind and no slope-conditions [12] and wind-assisted propagation [13].…”