Investigation of slope thresholds for flame attachment

Abstract

It is well known that wildfires, burning in the landscape, spread more rapidly as the slope of the terrain increases. Typically rules of thumb are employed to provide guidance on expected rates of spread on slopes up to about twenty degrees. For slopes inclined above twenty degrees, however, fire behaviour is more difficult to predict. Currently, the reasons for this are not well understood and there is no definitive model (operational or otherwise) that can account for the types of fire behaviour observed on very steep or confined topography. This behaviour includes so called eruptive fire behaviour, which is characterized by dynamic fire propagation - the fire can accelerate up the slope to very high rates of spread, sometimes with tragic consequences. In this paper we present the results of computational fluid dynamics analyses of convective plume dynamics in a rectangular trench similar to those involved in the Kings Cross Underground disaster. We demonstrate the existence of a threshold angle of inclination, above which the convective plume attaches to the floor of the trench. We discuss the implications of our results for wildland fire safety.