Fuels breaks are essential components of forest fire prevention and suppression policy in the French Mediterranean area. Beyond this specific policy, fuel breaks are a widespread tool in the management of wild land - urban interface. In the applied context of fuel break management, the following questions frequently arise from forest managers and firefighters: how the fuel remaining at the ground level (shrubs, grasses) will burn? when trees are maintained on the fuel break, will fire transitions to tree crowns be possible? what will be the effect of a given fuel treatment on fire behavior? We tackled these questions using numerical simulations of a physics-based model. The mathematical formulation of the model is derived from a multiphase approach that consists in solving the conservation equations (mass, momemtum, energy) of the coupled system formed by the vegetation and the surrounding gas mixture. The vegetation is represented by a set of families of solid fuel particles defined by their main physical properties (shape, size, moisture content). The spatial distribution of each family is given through its volume fraction field. Both convection heat transfer between the hot gases and the vegetation and radiation heat transfer from the soot particles of the flame and from the embers, are taken into account in the energy balance that controls the fire propagation. Equations are solved in a two-dimensional physical domain (vertical plane). We performed numerical simulations for an heterogeneous vegetation layer of various heights (from 0.25 to 1 m height) composed of shrubs (Quercus coccifera) and grasses (Brachypodium ramosum) and for wind speeds ranging between 1 and 10 m/s at 2 m height above the ground level. The analysis of the results revealed two modes of fire propagation depending on a characteristic Froude number based on wind velocity and flame height. For low Froude numbers, the flame front was nearly vertical and the fire behavior was controlled by radiation (plume dominated fire). For high Froude numbers, hot gases were pushed towards the unburned fuel ahead of the fire front and convection became the dominant mode of heat transfer (wind-driven fire). Simple formulas describing the numerical rate of spread as a function of wind speed and vegetation height are researched. Additional simulations of shrub land fires were also performed to demonstrate the ability of the model to simulate fire transitions to tree crowns.
Short term perspectives are to solve the same equations at a larger spatial scale (around 200 m length and 50 m height). First computations have already been run at this scale to simulate a fire spreading in a Pine stand that reaches a fuel break and have already demonstrated that the model allows to test the effects of fuel reduction on fire behavior.

(*)Fuel breaks are intended to be areas where the wild land fuel has been reduced (shrubs and/or trees), but not completely removed.