Fire Behavior of Some Southern California Live Chaparral Fuels

Wildfire spread in living vegetation, such as chaparral in southern California, often causes significant damage to infrastructure and ecosystems. In order to study wildfire spread in living vegetation, four of the most common chaparral in southern California, chamise, manzanita, scrub oak and ceanothus, were burned and compared. The observed fire behavior included mass loss rate, flame height, temperature structure and velocity field above the burning fuel bed. It was observed that flame height increases mainly with heat release rate. By using successive images of the temperature field, a recently developed thermal particle image velocity (TPIV) algorithm was applied to estimate flow velocities in the vicinity of the flame. The results are generally in agreement with other experimental results obtained on gas and liquid fuels. * Corresponding author: [email protected] Proceedings of the Third Joint Meeting of the U.S. Sections of The Combustion Institute INTRODUCTION Periodic fire has been a part of the southern California landscape for centuries. The mountain slopes surrounding the Los Angeles basin are covered at lower elevations (below 1600 m) with chaparral vegetation characteristic of Mediterranean climates [1]. Studies of fire spread mainly under laboratory conditions with simplified wooden fuels over many years have produced good quantitative descriptions [2-7]. Rothermel and Philpot [8] applied Rothermel’s model to chaparral and Albini and Stocks [9] tested Albini’s 1985 model with jack pine (Pinus banksiana) crown fire data. However under natural conditions, the free-burning wildfires are not only unplanned and unexpected but occur in many different and complicated situations. For predicting fire spread rate, existing operational models utilized by the Forest Service are based on the semi-empirical model developed originally by Rothermel [3]. It is applicable to quasisteady burning of surface fires with uniform fuel loading and assumes that the fuel is dominated by dead material and in close proximity to the ground. Crown fire, transition from ground-to-crown-fire, fire spread through live fuels, fire spread by spotting, and modification of meteorological conditions by the fire are some of the phenomena not accounted for in the Rothermel model. Wilson [4-5, 10] suggested addition of a term in the spread model involving wind and fuel moisture. However, the application of this to shrub fuel beds such as chaparral is currently unknown. Chaparral is a fireprone, shrub dominated plant community characterized by evergreen sclerophyll shrubs such as chamise (Adenostoma fasciculatum), manzanita (Arctostaphylos densiflora), scrub oak (Quercus berberidifolia) and hoaryleaf ceanothus (Ceanothus crassifolius), which are the most hazardous and popular brush fuels (Fig. 1 and Fig. 2). Usually two or more species are mixed and can be found in the foothills of southern California’s mountains. Fire spread in chaparral fuels has been described as a crown fire as well as a surface fire with a deep fuel bed. In varying degree, physical characteristics of fuels and fuel beds, such as fuel moisture content, fuel loading, fuel surface area-to-volume ratio and fuel bed porosity, influence ease of ignition, rate of spread, burning time and fire intensity. How these variations affect fire behavior is only known in a general way. However few explicit relationships have been established, particularly for the combinations of these factors as they appear naturally in chaparral fuel beds. For the purpose of studying wildfire spread in natural chaparral fuel beds, this paper focuses on the experimental investigation of flame plume proprieties of live shrub fuels. Here a fire starts in a circular shrub fuel bed in still air (Fig. 3). The solid surface of shrub fuel is heated, by radiation and convection from the burning products, and pyrolyzes to form vapour-phase fuel. Combustion of the pyrolyzate forms a flame plume above the fuel bed that is made visible by radiation from soot particles generated by the diffusion flame. Ambient air is entrained from the environment into the fire plume to oxidize the pyrolyzate fuel. Because the momentum flux of the pyrolyzate is very small, buoyancy forces control the fire plume. As heat transfers from the fire plume to the fuel bed, the fire spreads across the fuel bed. The