The risks associated with a bushfire is determined by a combination of three elements, the hazards that the fire generates, your level of exposure to these hazards, and your vulnerability to these hazards. Understanding and recognising each element will help you to prepare for bushfire.
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Hazards are defined as any event or natural phenomenon that can cause or contribute to the loss of life, injury, property loss, socio-economic disruption or environmental damage. The main bushfire hazards include, direct flame contact from the fire front, radiation exposure (heat), ember attack, tree strike, wind attack, smoke exposure, surface fire attack, and attack from consequential fire.
Exposure refers to the elements exposed to bushfire. This includes people, infrastructure, housing, production capacities and other tangible human assets located in hazard-prone areas. When building for bushfire protection, aim to limit your homes (and its access routes) exposure to potential bushfire hazards, such as classified vegetation and potential sources of consequential fire.
Vulnerability refers to the level of protection (or lack of protection) that people, infrastructure, housing, production capacities and other human assets might have to a bushfire hazard. The greater your level of exposure to a bushfire hazard, the greater the risk of injury, damage, or loss of life. More generally, vulnerability refers to the extent to which a community, building, services or location is likely to be damaged or disrupted by the impacts of a bushfire.
Vulnerability is determined by the type and number of weak points that, all other things being equal, make yourself, building, or community more likely to be damaged or disrupted. The greater your level of vulnerability to bushfire, the greater the risk. A buildings vulnerability, for example, refers to the weak points in the buildings design, construction, use of materials, or maintenance.
Never store combustible objects against the house
When flames from a bushfire physically contact a building they can flow and wrap around the structure, impacting the sides of the house not directly facing the bushfire. Flames can also flow into underfloor spaces and over roofs and are able to flow into small gaps and crevices. When this happens, there is a significant risk that combustible parts of the house will ignite and spread fire and toxic smoke to other parts of the property.
Radiant heat is the heat produced from combustion, or the burning of a fuel source. It is a significant threat to people and can threated the safety of occupants and firefighters during a bushfire. The heat from a bushfire is especially fierce and can be felt from over one hundred metres away.
This radiation can ignite combustible material such as doors and cladding, and can crack or break windows, creating a breach in the buildings envelope allowing flames and embers to enter the house, increasing the likelihood that the house will be damaged or destroyed. Heat can also melt and distort plastic elements around the house, including cladding and gutters.
Embers are burning twigs, leaves, bark and other debris that are carried by the wind. These debris can ignite surface materials such as walls and timber verandas, or enter and ignite vulnerable parts of the house, such as vents and roof cavities. Embers can land well ahead of the main bushfire front, and as such they represent an especially unpredictable hazard. Ember attack is the most common way that buildings ignite during bushfires.
Tree strike is the process by which a damaged tree or branch falls and causes damage to property or causes injuries or loss of life. Tree strike can occur at any time, as a result of prolonged or sudden damage from winds, disease, insect infestation or dry rot from drought. It is also a common occurrence during and proceeding a bushfire as significant fire and wind damage can weaken trunks and branches which may lead to them falling.
Residential homes are not typically built in a way that can resist a direct tree strike without sustaining major structural damage. This is likely to result in the exposure of internal cavities to the outside environment which increases the buildings vulnerability to other bushfire hazards.
Wind can damage parts of the house by applying direct air pressure or carrying debris (e.g. branches, roofing materials, tiles) which strike the building. Strong winds (gusting windspeeds above 75 kilometres per hour) are common during bushfires, and they can compromise the integrity of the buildings envelope by lifting or dislodging part of the roof or cladding. This damage can provide openings to the outside environment which increases the buildings vulnerability to other bushfire hazards. The strong winds that accompany a bushfire are also a significant hazard to people, as it can push debris or cause people to fall.
Smoke (including toxic gases) can exacerbate asthma and other respiratory conditions, as well as irritate the eyes, nose and throat and cause coughing, shortness of breath or suffocation. Fine particles can also penetrate deep into the lungs and cause long-term health problems. Smoke can also reduce visibility during a bushfire, which can increase the chances of trips, falls, and other accidents.
Surface fires are low intensity fires (with short flames) that burn along the ground. Surface fires consume low-lying fuels such as low-lying vegetation, ground litter, mulch and other debris. These fires are typically patchy and erratic, but they can be a serious threat to people and buildings when they occur immediately adjacent to the house, where they can give rise to direct flame contact, radiant heat exposure and ember attack.
Consequential fire. When bushfires spread into built-up areas, fire will impact homes and their surrounding elements (including fences, gardens, cars, and stored materials). This burning of surrounding elements is known as a secondary, or consequential fire. These consequential fires present additional impacts to the house, generating radiant heat, and spreading their own embers, flames, and toxic smoke.