Impact of the WUI vegetation management on damage to building: comparing post-fire damage assessment and CFD modelling results.

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In most Wildland-Urban Interface (WUI) fires, damage to building can result from poor vegetation management, comprising a lack of mandatory brush-clearing around building or an unwise location of trees (e.g. too close to building or overhanging the roof). It was interesting to check if post fire damage can be predicted by CFD fire modelling in different scenarios of vegetation management (treated vs untreated) according to different past-fire events. Ultimately, this work will help to assess if the fuel treatment regulation in the framework of fire prevention is efficient and to pinpoint possible limitations. Different scenarios of vegetation treatments were tested on four study cases of dwellings surrounded by gardens juxtaposed to wildland vegetation (therefore submitted to the regulation on fuel management in WUI) that were affected by the Vidauban fire in 2003 or the Rognac fire in 2016 (SE France). In each case, comparisons of modeled and observed fire behaviour and impacts on buildings were performed using the Fire Dynamic Simulator model (FDS) and very accurate georeferenced ornamental vegetation mapping, especially around buildings. Despite problems to adapt FDS modelling to the high fuel moisture content (FMC) of some species, results showed that overall brush-clearing mitigated fire intensity and propagation and therefore damage to building and ornamental vegetation, sometimes highlighting that this fuel management measure could be softened (decreasing the radius treated) or, on the contrary, strengthened according to the topography and wind. The fuel biomass treatment involving vegetation residues left on site was also found as deleterious as the lack of treatment. Overall, the modelling using FDS, at the WUI scale and taking into account a very refined vegetation distribution, was mostly successfully validated by post-fire damage recorded during the past-fire events, which was rarely attempted in the complex environment of forest fires in the French Mediterranean area. The current scientific limitations of the model did not allowed obtaining realistic results in terms of heat flux received by the building, even if they were coherent in global intensity comparing treated and untreated vegetation, and have to be addressed in future improvements of the model.