MCS 7 Chia Laguna, Cagliari, Sardinia, Italy, September 11-15, 2011 THE IMPORTANCE OF FIRE FRONT WIDTH IN THE ANTICIPATION OF ERUPTIVE FIRES F.J. Chatelon*, J.H. Balbi*, J.L. Rossi*, J.B. Filippi*, T. Marcelli*, C. Rossa** , ***, D.X. Viegas** chatelon@univ-corse.fr *Università di Corsica, Systèmes Physiques pour l’Environnement, UMR-CNRS 6134, Campus Grossetti, BP 52, 20250 Corte, France **Centre of Studies on Forest Fires/ADAI, University of Coimbra, Portugal ***School of Technology and Management, Polytechnic Institute of Leiria, Portugal Abstract Fire eruption is a significant threat to the safety of wildland fire fighters. Over the last few years, several accidents have caused important human losses. Generally, these fire fighters are surprised by the sudden acceleration of the head fire and they are not prepared for facing it. We characterize eruptive fires by an unexpected increasing of the head fire in a short lapse of time. Despite the danger of the phenomenon, fires rarely erupt and consequently studies of the mechanisms contributing to fire eruption are not so numerous. A recent paper [1] reviewed them in two ways: explanations are mainly based on a variation of the external conditions or on the spreading fire’s own properties. Our interpretation about the mechanism leading to a possible eruption consists in an induced air flow created by the fire. We use a semi-physical steady-state model with two equations, which are coupled by this induced wind. This coupling is solved thanks to a fixed-point method and it converges under certain conditions (or does not). In the divergence case, the rate of spread grows infinitely. So the model is not able to describe the fire behaviour during the eruption but it is possible to give the precise conditions for the occurring of the eruption. When meteorological conditions and fuel bed properties are known, we can find the slope angle at which a fire, with given dynamic parameters (fire front width, flame length), erupts. In particular, we will pay interest in the influence of the fire front width on the condition leading to eruption. We validate the model on two sets of experiments at the laboratory scale and a real accident, happened on the Kornaty Islands, Croatia, 2007. Additional Keywords: Eruptive fire, unsteady solutions, fire spread, physical model. Introduction The spread of a wildfire is more important up a slope than it will on a flat terrain. But sometimes, this usual behaviour can turn into an extreme situation with a sudden and important acceleration of the head fire. Some explanations are given in the literature to define this significant change. The pioneering interpretation, proposed by Viegas [2], consist in a positive feedback from the fire. The convective flow induced by the fire front in the presence of wind or on a positive slope transports oxygen to the reaction zone. This one intensifies the combustion process and consequently the flame length and the rate of spread. So the reaction needs more and more ambient air. This feedback process will increase continuously and the rate of spread could reach very important values if it is not inhibited by some external mechanism.