Effect of Urban Landscapes on the Transport and Spotting Risk of Firebrands

Firebrand spotting is a major mechanism associated with the propagation of wildfires, and it is caused by the transport and eventual settling of flying burning materials (known as firebrands or flying embers). Upon landing, firebrands can ignite local fuel, causing secondary fires, which can rapidly spread to unburned areas far from the main fire. This makes fire spreading behavior more erratic and difficult to predict, therefore reducing the effectiveness of fire mitigation methods. It is known that firebrand transport depends heavily on the characteristics of the ambient flow. Therefore, a better understanding of firebrand transport in turbulent flows over urban landscapes will allow to better guide fire mitigation methods and reduce fire losses associated with wildland-urban interface (WUI) fires.

In this study, the effect of topography-induced turbulence on the transport, smoldering lifetime and spotting risk of spherical firebrands is investigated. Large Eddy Simulation is used to simulate the turbulent flow over an idealized urban region. Smoldering particles of different sizes are released within the boundary layer and the evolution of their trajectories, dispersion, and combustion state are obtained using a Lagrangian transport model. When comparing these results with a case without obstacles, it is observed that firebrand dispersion and spotting potential are significantly enhanced under the presence of the complex turbulent flow induced by the non-flat topography.