New Laboratory Results on Ignition and Propagation in Live Vegetation Paving the Road to a Semi-Empirical Model

Wildfires arriving at a brush-cleared vegetation near vulnerable assets in the Wildland-Urban Interface (WUI), experience significantly reduced fire propagation, intensity, and associated risks. However, assessing the effectiveness of fuel reduction on fire behavior remains challenging due to limited understanding of ignition and propagation mechanisms in live vegetation. To fill this gap, a simplified approach was designed, focusing more on the combination of physical modeling with new empirical data rather than providing new insight in the physical process modeling. Burning experiments were conducted on cypress trees at two scales of live vegetation, the “laboratory scale” and the “real scale,” to gather data on fire behavior in cypress canopies with varying fuel moisture content (FMC) and bulk density (BD), using two ignition methods. A semi-empirical model, based on the physical model Fire Dynamic Simulator was developed, using the “laboratory scale” data as inputs, while the data recorded at “real scale” were used to validate the model. Laboratory-scale experiments showed consistent results when ignition was initiated by flame contact. In contrast, indirect radiant heat ignition was highly variable due to the influence of gaps between leaves. At the real scale, BD had a significant impact on fire behavior. The model evaluation showed it could simulate fire auto-propagation in live vegetation much more precisely compared to current physical models, leveraging the precise fire behavior data obtained at the laboratory scale.