Wildland Urban Interface
Direct flame contact, radiant heat, and burning firebrands (or embers) have been identified as three principal ways that cause fire spread in the wildland and Wildland-Urban Interface (WUI). However, only burning firebrands can initiate a new spot fire at distances further than 60-m away from the main fire front. During extreme weather events, spotting due to firebrands (referred as the firebrand phenomenon) can overpower fire suppression efforts and become the dominant fire spread mechanism. The spotting process includes three phases: firebrand generation, transportation, and ignition of the recipient fuel. Considerable work on ember transport has been conducted, but much less work has been done to understand fire ember production. Fire ember production from various fuel types under different conditions is the basis for understanding firebrand transport and ignition, and for validating fire behavior models and developing mitigating strategies in the wildland and WUI.
The purpose of this project was to investigate ember production from selected burning wildland and structural (construction materials) fuels under a range of environmental conditions through full-scale and small-scale laboratory experiments. Specific objectives included the determination of thermal decomposition and combustion properties at small scale of selected fuels under a range of heating rate, radiant heat flux, and moisture content (MC) levels, the investigation of firebrand production (including mass, size and flying distance) at full-scale from burning wildland and structural fuels under a range of conditions, the study of burning duration and intensity of embers under a range of conditions, and the evaluation of the impact of key firebrand properties on ignition potential and fire spread in the WUI. Firebrand production is a stochastic phenomenon, thus require a statistical approach to the problem. The outcome variables were ember production properties (such as size, mass and shape, travel distance, burning duration and intensity). The controlling factors were fuel type, fuel MC, fuel geometry and dimension and environmental conditions (e.g., wind speed). Our hypothesis was that the ember production characteristics could be described using thermal and combustion properties and geometry factors of the fuel and would be functions of these controlling factors. We further hypothesized that interaction existed between certain variables involved in the ember production process. Hence correlation among the outcome variables was evaluated. The problem involved observation and analysis of more than one variable at a time, thus linear or non-linear statistical modeling with multiple factors was used.
Results from this project can help us answer the following questions: (1) “What is the rate of ember production from burning wildland and structural fuels in the WUI under a range of wind speed and moisture conditions?”, (2) “What is the characteristic size and shape of embers produced from burning wildland and structural fuels in the WUI under a range of wind and moisture conditions?”, (3) “How far can embers of characteristic size and shape travel under a range of wind speeds?”, (4) “How long can embers of characteristic size and shape burn and at what intensities?”, and (5) “What is the role of ember production from wildland and structural fuels in fire spread in the WUI?”