Physics-Based Modelling for Mapping Firebrand Flux and Heat Load on Structures in the Wildland-Urban Interface’

Abstract

The mechanisms of structure ignition by wildfires are classified into direct flame contact, radiant heat, firebrand attack and a combination of two or all of them. Arguably, airborne firebrands play a vital role as the main cause for structure ignition and fire propagation by forming spot fires far from the fire front. Firebrand flux and the heat load are important parameters to calculate the wildfire risk on structures. Australian Building Standard AS3959 is developed based on radiation heat flux and it does not quantify the effects of firebrand landing flux on structures to assess the wildfire risk completely. To improve the assessment of the Bushfire Attack Level (BAL) in AS3959, there is a need for firebrand flux quantification at different scales of wildfires. Lacking information about firebrand generation from various vegetation species at different environmental conditions creates a gap to estimate the firebrand flux accurately. In this study, we aim to use a physics-based model to quantify the firebrand generation rate of Eucalyptus dominant forest vegetation at different severities of wildfires expressed by the Fire danger indices (FDI) of 100, 80, 50. The wind speed is adjusted while keeping the temperature, relative humidity, and drought factor as constants to obtain the focused FDIs. A 40 m height Eucalyptus forest is modelled with 25 t/ha understorey and 10 t/ha canopy fuel loads as per AS3959 forest vegetation classification. The forest fires are prescribed with the intensities of 53.4, 43.1, and 27 MW/m with 100 m length to replicate the fire events explained by FDIs. The depth of the fireline is approximated according to the fire residence time and the spread rate. The firebrand size, shape, and quantity are taken from our previous firebrand generation study (Wickramasinghe et al. 2022) and the particles are injected randomly through the forest volume which is engulfed by the fire. The distances between the modelled structure that follows an Australian standard house design and the vegetation are maintained according to the BALs. We obtained the radiative heat flux on the houses close to the algorithm provided in AS3959 for each BAL. In this study, both firebrand and heat flux are quantified at strategic locations of the house. We find a logarithmic relationship exists between firebrand flux and radiative heat flux in the range of R2 0.96 to 0.99. Hence, for a certain BAL, the firebrand flux increases with the FDI similar to radiative heat flux. Results from this study can be used to quantify the firebrand flux on houses from different vegetation fires, which may improve the design standards and construction requirements of buildings to mitigate the vulnerability of wildfires at the wildland-urban interface (WUI).