Part 3: Bushfires SENIOR SCIENCE DISASTERS

IDENTIFY SOME OF THE CONDITIONS THAT CAN COMBINE TO TRIGGER A BUSHFIRE INCLUDING DRY WEATHER, HIGH TEMPERATURES AND FLAMMABLE VEGETATION

CONDITIONS OF BUSHFIRES Three things that are necessary for a fire to burn: 1. Dry fuel 2. Heat 3. Oxygen Conditions that can combine to trigger a bushfire include: – dry flammable vegetation (dead leaves, bark and twigs) – high temperatures (and a fire that has already ignited) – dry weather (low humidity air).

Fire weather is most dangerous when hot and dry winds blow from the inland, particularly if coming from a westerly or a north-westerly direction. The worst conditions occur when deep low-pressure systems near Tasmania bring strong, dry, westerly winds to the coast. This occurred in both 1994 and Other factors affecting the danger and behavior of bushfires include: quantity, type, condition and distribution of fuel air temperature and sunlight relative humidity wind direction and velocity rainfall topography

DESCRIBE THE EFFECT OF THE SLOPE OF THE LAND AND INTENSITY OF THE WIND ON THE SPEED OF THE BUSHFIRE

SPREAD OF BUSHFIRES Spotlight – Pg 315 – summarise

IDENTIFY AND DESCRIBE SOME OF THE ENERGY TRANSFERS AND TRANSFORMATIONS ASSOCIATED WITH BUSHFIRES

ENERGY TRANSFERS The main energy released in bushfire is heat. Heat energy can be transferred in three main ways: 1. conduction where vibration of particles in solids and liquids is transferred slowly from particle to particle 2. convection in which heated liquid or gas moves in what are called convection currents 3. radiation in which electromagnetic radiation (mostly infra-red) travels at the speed of light through gas or a vacuum.

1. CONDUCTION Conduction causes major damage to the trunks of trees from ground level to the first branches and this may cause collapse of the trees after the bushfire has passed. Conduction of heat through the ground and as a danger to bushfire fighters is negligible.

2. CONVECTION Convection currents move upwards. They can be very dangerous to firefighters as the heat moves up a slope. Large amounts of heat can be transferred to the firefighters and the fire can move rapidly.

3. RADIATION About 25% of the energy release in a bushfire is released as radiation and this mostly moves horizontally. Radiation is very effective in drying out and preheating fuel ahead of the fire front. Firefighters must have protection against radiation.

ENERGY TRANSFORMATIONS In a bushfire chemical energy from reaction of the fuel with oxygen is changed to heat energy, light energy and kinetic (movement) energy of particles ejected by the fire. Australian bushfire fuels release about kJ per kg but a significant amount of this heat is absorbed into drying the vegetation so that the net release of heat energy is about kJ per kg. Grass and crop fuels release about half as much energy as trees. Fuel + oxygen  carbon dioxide + water + heat

DISCUSS THE REDUCTION OF FUEL BY CONTROLLED BURNS AND BACKBURNS IN REDUCING THE RISK OF BUSHFIRES

TEXTBOOK Pg Controlling Bushfires