Size and energy distribution of firebrands produced from burning trees

Abstract

Firebrands generated from wildfires can contribute to wildfire spread and are a threat to structures in the wildland-urban interface (WUI). Understanding the characteristics such as the firebrand size, mass and heat flux to the recipient fuel are key for evaluating wildfire spread by firebrands. The goal of the current study was to evaluate the characteristics of firebrands produced from different species of trees and to assess the threat due to firebrands to several WUI related fuel sources. To this end, a total of 71 trees/shrubs of Douglas-fir, ponderosa pine and sagebrush tees were burned, and firebrands collected. “Hot” firebrands with temperatures greater than 300 °C were captured using fire-resistant fabrics. These “hot’ firebrands left char marks on the fabric and the char marks are imaged. A novel methodology to estimate the heat flux deposited by firebrands based on the sizes of the char marks is presented. The average heat flux of firebrands from Douglas-fir and sagebrush species (short needles) were an order of magnitude higher when compared to the heat flux of firebrands from ponderosa pine species (long needles). A further analysis on the ignition potential of firebrands indicated that the Douglas-fir firebrands are more likely to cause longer range spot fires whereas the firebrands from sagebrush species are more likely to contribute to wildfire spread in the immediate vicinity. Further, it was established that increasing the Critical Heat Flux of the potential fuels in the WUI can lead to exponential reduction in the risk of ignition due to firebrands.