Hazards, Disasters and Survival

A booklet for Students and the Community

Chapter 7

Cyclone Hazards and Disasters

AUSTRALIAN TROPICAL CYCLONES

Tropical cyclones (or hurricanes in North America, typhoons in Asia) are like giant whirlwinds of air and dense cloud spiraling at over 120 km/h around a central ‘eye’ of extreme low pressure. 

Cyclone Season and Effects

Australia’s cyclone season is usually November to April.  Cyclones mainly affect coastal areas north of Perth along the WA and NT coasts, most of the Qld coast and occasionally the far northern NSW coast.  The greatest threat lies north of the Tropic of Capricorn. 

Frequency - Cyclones occur frequently in the southern hemisphere, with an average of ten per year being tracked by the Bureau of Meteorology in the Australian region alone.  Of these, six may be expected to cross the Australian coast each year.  Cyclones further out in the Indian and Pacific Oceans are monitored in case they threaten Australian islands or nearby countries.

Severity Categories - These range from ‘1’ for weak cyclones (strongest wind gusts less than 125 km/h), through ‘2’ (125-169 km/h), ‘3’ (170-224 km/h), ‘4’ (225-279 km/h), to ‘5’ for the most severe cyclones (wind gusts more than 280 km/h).

Effects - Cyclones approach from the sea bringing with them torrential rains, extreme winds and sometimes storm surges (explained later).  Damage caused by each cyclone varies widely depending on its path, but can include buildings, crops and boats at sea.  Most deaths from cyclones occur as a result of drownings (both at sea and during floods), collapsed buildings, or debris which become lethal projectiles carried along by the extreme winds.

Our Worst Cyclone Disasters

Cyclone Mahina - This terrible storm in March 1899, which was accompanied by a storm surge of over 10 metres, caused by far the highest death toll of any cyclone before or since, claiming over 400 lives as it completely destroyed a pearling fleet at Bathurst Bay near Cape Melville, Queensland.  Deaths included the crews of 100 vessels and an estimated 100 local aborigines.

Cyclone Tracy - To date, TC Tracy has been Australia’s most destructive in terms of damage to a community.  It struck Darwin on Christmas Eve, 1974.  Rainfall totalled 195 mm in 8½ hours with estimated winds of up to 250 km/h literally tearing the whole city apart and killing 65 people.  (Full details follow in Case Study 1).

THE BIRTH AND ANATOMY OF A CYCLONE

The ‘life-cycle’ of the average tropical cyclone (or hurricane/typhoon) is about seven days but can extend to over three weeks.  They form in the atmosphere over warm ocean areas with at least 26°C water temperature (mainly in latitudes 5° to 20° north or south) although their exact trigger-mechanism is not fully understood.  If conditions are right, an ordinary tropical depression, or ‘low’ can develop into a tropical cyclone.  In the southern hemisphere, in a ‘low’, the winds spiral in a clockwise direction towards its centre, where they rise and spill over in an outward flow at high altitude. Summer heat beating on the warm ocean evaporates water, creating a deep layer of moist air. The uplift of this moist air in the centre of a ‘low’ cools it, causing the intense rain characteristic of tropical cyclones.  Higher in the upper levels the rising air spirals outward, removing air faster than it flows in, resulting in a fall in barometric pressure.

The Central ‘Eye’

Cyclones vary greatly in character but the one feature they all have in common is a virtually calm centre with clear sky.  This centre, or ‘eye’ is generally about 16 - 30 km across, but even this can range from 10 - 100 km.  Around this eye are bands of heavy cloud, associated with the moist air which spirals in towards the centre of the cyclone.  These clouds, known as the ‘eye wall’, may be 15 km high and mark the ring of strongest winds and heaviest rainfall.

CYCLONE BEHAVIOUR AND WARNING TIME

Research shows that tropical cyclones in the Australian region have more erratic tracks than those in other parts of the world.  Cyclones may exist for a few days, to over three weeks!  They may move forward, double-back, stay motionless for periods, or move in circles, and therefore need to be tracked carefully by weather observers (see map below).  If they reach land, the friction of the earth and the loss of sustaining heat energy from the ocean, cause cyclones to ‘fill’ and drop most of their rain.  As cyclones move forward at only 15 - 25 km/h, there is usually sufficient warning time for people to prepare for their onset.

Lethal Energy

Tropical cyclones are the greatest storms on earth, releasing in one day as much condensation heat energy as up to four hundred, 20 megaton atomic bombs.  Every cyclone is dangerous and must be regarded as a real threat until the danger has clearly passed.  Where tropical cyclones are concerned, because of their erratic paths, there is no such thing as a ‘false-alarm’ - the community which has been by-passed by a cyclone knows it has been lucky this time.

Beware the ‘Eye’

It is important for people in cyclone areas to be aware that if the ‘eye’ of the cyclone passes over them, there will be a sudden lull in wind and clearing of skies, which may last anywhere from a few minutes to an hour or two.  Then the other side of the cyclone will strike and the winds will resume with equal strength, but blowing from the opposite direction.  It is, therefore, most important that people should remain in shelter during and after the passing of the ‘eye’.

STORM SURGE

One other effect of a cyclone is that it can produce a storm surge (or tide).  A storm surge may be caused by cyclonic winds blowing across the water and a fall in atmospheric pressure.  The lower pressure at the eye of the cyclone actually allows a sea-level rise in, and close to, that centre.  This raised dome of water is usually 60-80 km across and 2 to 5 metres higher than normal sea-level.  As a cyclone nears the coast, low-lying areas may suffer flooding because strong on-shore winds have displaced the storm surge ahead of the cyclone’s centre.  The amount of flooding that occurs depends very much on the height of the tide when the cyclone crosses the coast.  If the tide is fairly low, flooding may not occur.  The diagram shows how sea-level can be affected by surrounding wind and air pressure as the cyclone’s centre crosses the coast.

Actual Occurrences

Cyclone Althea - When it struck Townsville in 1971 there was a surge of up to 3.6 metres in the area but little flooding occurred as the tide was low.  (Extensive wind damage did occur).

Cyclone Tracy - This had a surge of about 4 metres but caused no flood damage as the combination of surge and tide did not reach flood-level in Darwin.

Cyclone Aivu - A moderate storm surge accompanied TC Aivu in 1989 affecting a mainly unpopulated section of the Qld coast.  The village of Wunjunga near Homehill, however, suffered severe damage (see photo), mainly to unoccupied holiday cottages, although one man died in a caravan.

Warnings  - People shelter-ing in low-lying coastal areas are potentially more at risk from a storm surge than from cyclonic winds, and should listen for storm surge (or tide) warnings.  If a storm surge coincides with a high tide, massive flooding and additional destruction is likely to occur.

Overseas - Storm surges are fairly rare in Australia, but one alone in East Pakistan (now Bangladesh) in 1970 caused the deaths of 500,000 people when a cyclone struck the area. 

AUSTRALIAN CYCLONES - CASE STUDIES

1.       Tropical Cyclone Tracy, NT, December 1974 (Chronology of a Disaster)

Deaths and Damage - ‘Tracy’ claimed 49 lives in Darwin itself, while a further 16 were lost at sea on several small vessels which were in the path of the (Category 4) cyclone.  Approximately 650 people were treated for injuries on Christmas Day, while more than 35,000 people were evacuated in the days that followed.  This was necessary as power, water, sanitation and communications were lost; over 80% of all buildings were destroyed; potential for disease was great; and there were only 400 of Darwin’s 11,200 homes remaining even reasonably intact.  Evacuation and relief efforts were coordinated by the Natural Disasters Organisation (now Emergency Manage-ment Australia).  Insured losses were $837 million and total estimated costs were $4,180 million (1997 values).

2.     Tropical Cyclone Ada, Qld, January 1970

This ‘Category 4’ cyclone severely damaged resorts on the Whitsunday Islands of ‘Daydream’, ‘South Molle’ and ‘Hayman’ as well as boats and homes near Proserpine where floods also occurred.  14 people died and total estimated costs were $390 million (1997 values).

3.     Tropical Cyclone Winifred, Qld, January 1986

At ‘Category 3’, it struck Cairns to Ingham, destroying 50 homes and damaging hundreds of others and larger buildings.  There were three deaths, 20 injuries and heavy sugar cane, fruit and vegetable crop losses.  Total costs were $325 million (1997 values).

4.    Tropical Cyclone Bobby, WA, February 1995

A ‘Category 4’ cyclone, ‘Bobby’ passed near Onslow which suffered considerable damage to homes.  The worst effects however, were felt at sea where seven lives were lost as two fishing trawlers sank, and a bulk ore carrier ran aground.  Extensive floods then followed in the south, damaging roads and bridges and causing heavy agricultural and mining production losses.

5.     Tropical Cyclone Olivia, WA,  April 1996

Another ‘Category 4’ which destroyed power installations and 55 houses (plus 27 damaged) at the mining town of Pannawonica.  Several buildings and another 20 houses suffered roof damage at neighbouring Mt Tom Price.  Fortunately, only 10 minor injuries occurred.

6.     Tropical Cyclone Justin, Qld,  March 1997

Although only a ‘Category 2’ cyclone, it caused significant damage in the Cairns region which it approached on two occasions during its long (3½ week) life.  Houses were undermined by huge waves, a marina and boats were severely damaged, roads and bridges suffered from flood and landslide damage and huge losses were inflicted on sugar cane, fruit and vegetable crops. The death toll of 7 in Queensland included 5 on a yacht which sank. 26 died in Papua New Guinea which was also severely affected.  Total estimated costs in Australia were $190 million (1997 values).

CYCLONE SURVIVAL AND PROPERTY PROTECTION

In cyclone-prone areas of Australia, strict building codes exist for all new constructions.  In some areas public cyclone shelters are provided in strong buildings for people who live or work in sub-standard buildings.  A cyclone warning system is provided by the Bureau of Meteorology, and State/Territory Emergency Services run preparedness campaigns to support community emergency plans.  If you live in a cyclone-prone area you should heed this advice:

Before the Cyclone Season

• Know your community cyclone plan, and how the cyclone warning system works.

• Have a portable radio and torch with spare batteries.

• Check your house is in good condition, particularly the roof, and trim tree branches clear of your house.  Clear property of loose items likely to cause damage in high winds.

• In case of a storm surge warning, identify your nearest safe, high area in advance.

• Create an emergency kit of tinned food, water containers, emergency lighting, first aid kit, medicines, tape and plastic bags.

Upon a Cyclone Warning

• Listen to your local radio or TV for further warnings.

• Board or tape windows, store loose articles inside.

• Lock up pets, fill water containers, fuel car and place under cover.

• Check your emergency kit and put spare clothing and shoes in plastic bags.

On Warning of a Local Evacuation

• Switch off electricity, gas, etc and lock your house upon leaving.

• Don’t forget your emergency kit.  Follow instructions from emergency personnel.

When the Cyclone Strikes

• Stay inside and shelter in strongest part of the house (eg bathroom or cellar).

• Protect yourself with mattress, blankets and anchor yourself to a strong fixture (such as water pipes) or get under a strong table.

• Beware the calm ‘eye’.  Remain indoors until advised that the cyclone has passed.

After the Cyclone

• Don’t go outside until advised officially that the cyclone has passed.

• Listen to your radio for further information and advice.

• If you had to evacuate, don’t go home until advised.  Use recommended routes.

• Beware of fallen powerlines, damaged buildings, trees and flooded watercourses.

STUDENTS - FIND FURTHER INFORMATION

As a project on cyclones, find out more about:

• the Cyclone Warning System;

• Cyclone Orson, WA and Cyclone Aivu, QLD; and

• cyclone-proof buildings, or the Coriolis Force (or Effect).

	EMERGENCY MANAGEMENT AUSTRALIA, PO Box 1020 Dickson, Australian Capital Territory  2602, AUSTRALIA Telephone: + 61 (0) 2 6266 5402    Facsimile: +61 (0) 2 6266 5029    Email: ema@ema.gov.au AUSTRALIAN EMERGENCY MANAGEMENT INSTITUTE, Mount Macedon Road, Mount Macedon, Victoria  3441, AUSTRALIA Telephone: + 61 (0) 3 5421 5100    Facsimile: +61 (0) 3 5421 5272    Email: aemi@ema.gov.au
	This page was last updated May 31, 2001