Behaviour of slope and wind backing fires

Abstract

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.