Role of the ornamental vegetation in the propagation of the Rognac fire 2016

[Departement_IRSTEA]Territoires [TR1_IRSTEA]SEDYVIN [ADD1_IRSTEA]Adaptation des territoires au changement global<br/>Fire continuum conference: Preparing for the future of wildland fire, Missoula, USA, 21-/05/2018 - 24/05/2018

International audience

English. In August 2016, a fire due to negligence during leisure activities started in the wildland-urban interface (WUI) of Rognac (a community located to the NE of Marseilles, SE France) and, driven by a strong wind and by exceptionally dry and warm climatic conditions, burned almost 3000 ha, mostly in urbanized area, in a few hours (rate of spread up to 5.3 kmh-1). These severe meteorological conditions entailed difficult firefighting operations, already hampered by a lack of resources due to several aircrafts grounded for maintenance and by six other fires burning at the same time in the area. The fire impacted six communities, ending up in the outskirts of Marseilles, some 15 km away from its ignition point.Besides the exceptional size for a WUI fire, what mostly characterized the Rognac fire was its propagation through the WUI to the core of the urban areas, becoming more an "urban fire" than a "WUI fire". The fire propagation mainly occurred through ornamental vegetation such as the ornamental hedges located along the roads that led to buildings causing much damage. In total, 26 homes were destroyed and 155 partially damaged, often because of the poor positioning of ornamental trees or hedges (mostly big pine trees and cypress hedges) planted too close to the housing and sometimes combined with a lack of clear-cutting (however mandatory around housing located at least at 200m from forest/shrubland areas). The damage was due to strong radiant heat emitted by the burning vegetation surrounding homes as well as to the massive amount of embers generated by this vegetation.This exceptional fire event (since 2003) underlines the need to better understand the fire propagation through WUI vegetation that differs from wildland vegetation especially by its strong heterogeneous structure and by the presence of exotic species whose fire behaviour is unknown for the most part. In order to improve fire prevention in WUI, under climatic conditions more and more conducive to fire with the on-going global change, it is thus necessary to adapt the fire behaviour modelling to this type of vegetation.