Fire-spotting generated fires. Part II: The role of flame geometry and slope

Abstract

This is the second part of a series of two papers concerning fire-spotting generated fires. While, in the first part, we focus on the impact of macro-scale factors on the growth of the burning area by considering the atmospheric stability conditions, in the present study we focus on the impact of meso-scale factors by considering the effects of the flame geometry and terrain slope. First, we discuss the phenomenological power law that relates flame length and fireline intensity by reporting literature data, analysing a formula originally proposed by Albini, and deriving an alternative formula based on the energy conservation principle. Subsequently, we extend the physical fire-spotting parametrisation RandomFrontadopted in the first part by including flame geometry and slope. Numerical examples show that fire-spotting is affected by flame geometry and, therefore, cannot be neglected in simplified fire-spread models used in operational software codes for wildfire propagation. Meanwhile, we observe that terrain slope enhances the spread of a fire at a higher rate than the augmentation of fire-spotting generated fires, such that a rapid merging occurs among independent fires.