Wildland-urban interface (WUI) fire modelling using PHOENIX Rapidfire: A case study in Cavaillon, France

In parts of the world with Mediterranean type climates, periodic wildfires are part of natural processes. Where such fires impinge on populated areas, there is the potential for significant losses of lives and property. The interface between vegetated areas and human settlements is described as the Wildland Urban Interface (WUI) and is typically where most wildfire losses occur. The threat of fire is becoming an important issue due to increasing populations and the potential for increased fire frequency with climate change. Understanding how wildfires behave in and adjacent to the WUI is important, as it can enable appropriate fire preparedness and response actions to be taken. Dynamic fire spread simulators are widely used to characterise large-scale fires to provide rapid predictions of potential fire spread and impacts, however these scales are generally much greater than those of interest in the WUI. There are currently few options for rapidly characterising fire in the WUI. PHOENIX Rapidfire (PHOENIX) is a wildfire simulator designed to characterise large, fast moving fires in Australia. It is unique in that it incorporates the contribution of firebrand transport and ignition to fire spread. We present a case study where PHOENIX is evaluated as for fine scale WUI fire spread prediction using a fire that occurred in 2012 in Cavaillon, France. This small fire burnt a total of 33 ha in heterogeneous fuels and hilly terrain, which resulted in a variety of impacts within the urban area. A significant feature of this fire was that spotting was observed to be an important part of fire spread; embers were launched from successive ridge tops, carried by winds to ignite fires in adjacent gullies that then ran up subsequent slopes. PHOENIX requires inputs of fuel, topography and weather. The fuel at Cavaillon was surveyed at a high resolution (1 m) and defined using the Fire Paradox fuel classification. These classes were converted to equivalent fuel loads (t/ha) for different fuel strata as used in PHOENIX. The spotting and ember module of PHOENIX was designed for long distance (>200 m) spotting. Subsequently the spotting model thresholds were manually re-calibrated to provide a comparable spotting pattern to those observed at Cavaillon. Weather inputs were sourced from a nearby automatic weather station. Simulations of fire spread were run at resolutions of 5, 10, 15, 20 m. Observations of the Cavaillon fire supplied by the local fire brigade were used to reconstruct the actual fire progression. A simulation of the fire using the best data available was compared to the observed perimeter at 18:00. PHOENIX was effective in predicting spread in the WUI, however testing against other fire events is required before its widespread use. The simulation of small scale spotting appears to be a necessary step in the simulation of such fires where it is a key spread mechanism. In addition modelled ember densities can be used to predict asset impacts, which can supplement standard intensity threshold approaches. The ability to evaluate fire spread characteristics at the WUI scale would provide significant opportunities to estimate impact risk, reconstruct fire behaviour, undertake vulnerability modelling, evaluate fire mitigation strategies and assist suppression planning. The results of our investigation indicate that further research in this area is warranted.