Chapter 6 - Wind-Aided Fire Spread

Abstract

On the chaparral-covered hillsides of southern California, autumnal episodes of persistent strong low-humidity warm winds, after the vegetation is desiccated during the long dry summer, can result in wind-aided fire spread over large areas. An annual critical fire season is encountered in other Mediterranean climate locales in several continents. To predict the evolution of the firefront that typically separates burned and unburned areas of vegetation, one could advance arc segments of the firefront perimeter. Each segment is to be advanced along its normal, according to the equilibrium rate of spread holding for the local, instantaneous conditions. This is carried out at the back and flanks of the perimeter, but it seems especially important to quantify accurately the advance of the head of the wind-aided fire. Although the equilibrium rate of spread may not be fully achieved in the buildup phase of a firefront evolving from an ignition, for the high-spread-rate conditions of primary interest here, the fire, in minutes, typically achieves a size in which the equilibrium rate is an excellent approximation. Results from well-defined smaller scale experiments can be used to develop a capability to anticipate the evolution of the firefront configuration of a larger scale fire. It is this larger scale fire not suppressed in its early stage that challenges fire-management capacity at the urban-wildland interface. However, by themselves, a limited number of minimally instrumented field events conducted in heterogeneous, possibly discontinuous, incompletely characterized fuel beds, which are burned under uncontrollable conditions, constitute isolated, perhaps unrepeatable anecdotes, not a database to assist quantification.