Numerical characterization of structures heat exposure at WUI

In the forthcoming years, self-protection of communities will be a first priority over fire suppression in order to tend to more fire-resistant and resilient WUI communities. All around the world, countries facing WUI fires apply different recommendations or regulations often issued from post-fire surveys. Still, more efforts are necessary to understand how and why dwellings are damaged or completely destroyed under WUI fires attack. In particular, there is a need to quantitatively assess the effectiveness of the current legal prescriptions for homeowners concerning the defensible space around dwellings. Three-dimensional, time dependent, computational fluid dynamics fire behavior models can take into account the factors interacting and contributing to WUI fires (i.e., weather conditions, terrain configuration, fire, vegetation and structures). Moreover, they allow the spatially-explicit modelling of vegetation elements (i.e., trees, shrubs, etc.). Thus, they can be supporting tools to quantitatively assess the heat (radiative and convective) exposure of structures during the approach of a WUI fire, in order to investigate how the characteristics of the defensible space can protect a dwelling or not against such a fire. This study addresses the characterization of heat exposure conditions of a dwelling in common Mediterranean WUI scenarios by using the three-dimensional, time dependent, computational fluid dynamics forest fire behavior model WFDS. To this purpose, WUI fire simulations have been carried out at the landscape scale, taking into account the different zones that a fire burns before it might approach and reach a home structure. This is, a forested area and the defensible space or cleared area around a dwelling. Two different scenarios have been studied, where different spatial patterns for the raised vegetation at the defensible space have been considered. One vegetation pattern has a low level of aggregation corresponding to a sparse spatial distribution of plants, whereas the other vegetation pattern has a higher level of aggregation representative of a clumpy distribution of plants. Both scenarios, in agreement with the current regulations in Corsica, have the same amount of available fuel load, as well as, the same number and characteristics of raised vegetation elements. Fire simulations for these two scenarios have been carried out at different wind and ambient conditions representatives on one side of normal dry summer conditions and on the other side of particularly dry summer conditions. Heat exposure conditions have been characterized in terms of radiative and convective heat fluxes received by the structure. Special attention has been given on the role of fire – vegetation – wind interactions for the results and discussion.