Ember Attack: Its Role in the Destruction of Houses during ACT Bushfire in 2003

The ACT bushfire in 2003 led to enormous loss of natural resources and property, including more than 500 houses at the urban interface. In order to understand how the bushfire affected the adjacent urban area and the reason for the massive destruction of houses in the Duffy area, the fire behaviour in the bushland, especially the formation of embers and their transport, are studied in the current paper. Parameters such as fire intensity, maximum “loftable” size of firebrands, and minimum as well as maximum travel distances downwind, have been evaluated using existing knowledge and established mathematical models. It is found that under the circumstances considered, the area to receive the most effective ember attack should be between 50 and 400 m downwind of the fire source, which is in excellent agreement with the Γ-type distribution of the affected houses at the urban interface. Scatter in the locations of these houses is also a reflection of the random feature of the transport of firebrands and the subsequent spot fires. There is no doubt that ember attack, resulting from the formation of a high-intensity crown fire in the adjacent bushland and the presence of high wind speeds, was a major, though not necessarily prime, contributor to the massive loss of houses in Duffy urban area. Introduction The ACT bushfire in 2003 was severe in Australian bushfire experience, partially due to its impact on the urban areas. Most remarkably, late in the afternoon of January 18, 2003, the bushfire swept several through urban areas including Duffy, resulting in the loss of more than 500 houses. As revealed by video taped during the fire and direct observations of witnesses during the fire development in the Duffy area (Moran 2003), the fire was notable for the generation and transport of large amount of embers. Once the bushfire entered the bushland adjacent to the Duffy residential area, embers sourced from the fire field and carried by strong local wind crossed a low fuel zone and a main road, and landed on the houses and surrounding vegetations. This process led to the ignition of houses and their surroundings in many separate locations. The Duffy residential fire is a typical example of how the bushfire may affect an adjacent built environment. For the purpose of furthering the understanding on the role of embers in affecting the residential area at the interface, the formation of embers and their flow-dynamic features observed in the Duffy area are analysed using the information collected during the fire and previously established knowledge. The relationship between the features of generated firebrands and the fate of the affected residential area is then discussed in these terms. Fire Environment and Behaviours Vegetation located in the northern and western outskirts of the flat Duffy residential area are mainly composed of the Pinus radiata pine trees planted about 20 years ago, with the heights varying between 12 and 15 m in principle (e.g. Moran 2003). The wind speed recorded by the nearby weather station, was about 40 km h -1 between 15:00 and 18:00 on January 18, 2003, from northwest, at an angle of ~50° west from north (Blanchi and Leonard 2005). Before the bushfire approached the Duffy urban area, air temperature was above 35°C and the relative humidity was below 10% (Blanchi and Leonard 2005). These factors constituted an extremely high score on the forest fire danger index (FFDI) (McRae 2003, Chen and McAneney 2004). Driven by local wind, the fire moved towards the urban interface by igniting a strip of pine trees situated in the north and west boundaries of the Duffy residential area after 15:00. This fire was characterised by burning trunks and canopies of individual pine trees or groups of them (Moran 2003, Blanchi and Leonard 2005), as is typical of crown fires. It burnt fiercely, although the fire frontline normal to the wind direction was not extensive, as shown by the video taped during the bushfire attack on the urban interface (Moran 2003). Flame plumes engulfed the individual trees with the flame tip often higher than the top of the trees. The height of a flame tip varied between 15 and 30 m. It was also observed that, due to the presence of strong local wind from time to time, the flame plumes developed around the tree crowns inclined significantly, with the maximum tilt angle of ~40° from the vertical plane (Moran 2003). Similar results can be obtained for flame tilt angles under the observed conditions of wind speeds and flame lengths using the model of Thomas (Muraszew 1974). Given the impact of such highintensity crown fire on local weather conditions, it is reasonable to expect that the local wind speed may be sometimes higher than the wind speed elsewhere, with the maximum reaching 55 km h -1 or so (Pyne et al 1996, Chen and McAneney 2004). Bushfire Conference 2006 Brisbane, 6–9 June 2006 Life in A Fire-Prone Environment: Translating Science into Practice Paper No. 0073 2 The flame height data can be applied to estimate the fuel consumption rate using the well-known equation 5 2 4 . 10 f f m h & = , introduced by Albini (1979). The heat release rate of the crown fire is then determined by a multiplication of the fuel consumption rate with the heat of combustion adopted for usual wildland fuels, that is Hc=18,620 kJ kg -1 (Pyne et al 1996). The fireline intensity Ib is also evaluated using a correlation, i.e. ( ) 3 2 6 . 18 c b f H I l = (Thomas 1971). The estimated data of fire behaviour are summarised in Table 1. Table 1 Results of fire behaviour evaluation. Item Evaluated result Comments Fuel type 20 year-old pine trees Searched result over the internet Flame height (m) 15–30 Retrieved from the video information (Moran 2003) Flame length (m) 20–35 Results adjusted by considering the inclination effect of flame plumes Maximum flame tilt angle (°) ~40 Direct measurement from the recorded flame images; confirmed using the Thomas’s model (Muraszew 1974) Fuel consumption rate (kg s -1 ) 2.5–14 Determined based on a model of Albini (1979) Heat release rate (kW) 47,000–263,000 Calculated from the fuel consumption rate Fireline intensity (kW m -1 ) 21,000–48,000 Determined by the model of Thomas (1971) and the flame length varying from 20 to 35 m The maximum heat release rate determined by the current method is roughly a quarter of that reported by McRae (2003), for the burning of forest fuels in three hours on that day. However, the maximum fireline intensity is essentially in agreement with that reported by Gould, who suggested a number of 50,000 kW m -1 for the burning of pine trees in the Duffy area (Chen and McAneney 2004). Due to the high-intensity of combustion over the tree trunks and canopies, strong buoyancy flow may be formed above the burning fuels. As the result of the gaseous buoyancy flow and the presence of local wind, burning fuel debris may be lofted into the airstream, transported by local wind, and finally land on the surface of unburnt objects downwind, which may lead to the ignition of the objects contacted (Fig. 1). This phenomenon is called spotting. As observed by the on-site fire fighters, the embers formed in the fire sources, crossed the low fuel zone and a main road, flowed into the urban area and eventually ignited houses at various locations (Moran 2003). It has been found that pine needles, pieces of twigs or barks, often acted as embers during the fire (Livbom 2005). Features of Transportation of Firebrands and the Associated Spot Fires Using the model developed by Woycheese and co-workers (1999), the maximum loftable characteristic scale of the firebrands was evaluated and found to be between 20 and 40 mm under the determined range of the fuel consumption rate. Due to the presence of strong local wind, the embers would usually present as glowing rather than flaming (Tarifa et al 1965), as confirmed from the video taped by Moran (2003). Burning of an ember with such sizes may last from 3.5 to 14.6 min, in light of an empirical correlation, ( ) η 4 / 2 0 d tb ≈ , where η is evaluated as 0.435 mm 2 s -1 from experimental data (Tarifa et al 1965). The existing theory for the diffusion-controlled surface combustion of spherical firebrands also yields a relationship of proportionality of burnout time to the square of the diameter of a firebrand (Fu et al 1989, Woycheese et al Attacked house Trajectory of an ember ug Convection column