Risks Associated with Tropical Cyclones
Many people know that tropical cyclones contain damaging winds but most do not realise the other hazards that come with them. These hazards are listed below:
Storm Surge:
Storm Surge is water that is pushed towards the shore by the force of the winds that are swirling around the storm. This surge combines with normal tides to create a cyclone storm tide, this can then increase the water level by 4.5m or more. The wind driven waves are then rolled over one another onto the storm tide. This rise in the water level can cause severe flooding in coastal areas and particularly when the storm tide occurs at the same time as the normal tide.
The height of the surge in particular areas can be determined by the continental shelf's that are surrounding it. A shallow slope off the coast will allow a bigger surge to cover the coastal communities, where as a steeper slope off the coast will not see as much surge cover their community but large breaking waves can present major issues. The storm tides, waves and currents can cause severe damage to ships, marinas and pleasure boats if they are caught in any confined areas.
Storm Surge:
Storm Surge is water that is pushed towards the shore by the force of the winds that are swirling around the storm. This surge combines with normal tides to create a cyclone storm tide, this can then increase the water level by 4.5m or more. The wind driven waves are then rolled over one another onto the storm tide. This rise in the water level can cause severe flooding in coastal areas and particularly when the storm tide occurs at the same time as the normal tide.
The height of the surge in particular areas can be determined by the continental shelf's that are surrounding it. A shallow slope off the coast will allow a bigger surge to cover the coastal communities, where as a steeper slope off the coast will not see as much surge cover their community but large breaking waves can present major issues. The storm tides, waves and currents can cause severe damage to ships, marinas and pleasure boats if they are caught in any confined areas.
Wind and Squalls:
Tropical Cyclones are known for their damaging and powerful winds. When the cyclones are categorised into one of the 5 categories they are actually based on the strength of the wind in the storm.
Gust are short but rapid bursts in wind speed and they are mainly caused by turbulence over the land that mixes with the air that is rising to the surface. Where as Squalls are long periods of increased wind speeds and they are usually associated with the bands of the thunderstorms (these make up the spiral bands around tropical cyclones).
A tropical cyclone's winds damage and destroy structures in two different ways:
- First, many homes are damaged or destroyed when the high winds lift the roof off of its dwellings. This process is called Bernoulli's Principle which implies the faster the air moves the lower the pressure within the air becomes. The high wind moves over the top of the roof and creates lower pressure on the exposed side of the roof structure. This high pressure in the attic helps to lift the roof, then once it is lifted and it acts as a sail as it is lifted clear of its dwellings. After the roof is gone the walls are much easier to be blown down by the cyclone's strong winds.
- The second way that wind destroys buildings can also be a result of the roof becoming airborne. The wind picks up the debris (wood, metal, toys, tree branches, etc...) and it sends them flying off at high speeds into other structures.
Through many investigations people have realised that majority of the damage from tropical cyclones is due to all of the flying debris that is tossed around.
Inland Flooding:
Tropical Cyclones not only threaten Northern Australia with storm surges and strong winds but also with torrential rain and flooding. Flooding poses a huge issue on the communities affected as it lasts for many days before it finally recedes. But even though it might recede quickly it will not replace all of the damaged items / homes that were inundated with water.
Many people believe that the stronger a Tropical Cyclone is the greater chance there is for flooding. However this does not always happen. A weak Tropical Cyclone that is moving at a slow pace can cause more damage due to flooding than a powerful Tropical Cyclone that is moving quite quickly.
There are also many different types of flooding. They are listed below:
- Flash Flooding:
Flash Floods are rapid flood waters that can inundate a community very quickly. They can begin after a few minutes or a few hours of very heavy rainfall.
This fast moving water can reach heights of 10m or more and can roll boulders, rips trees from the ground and destroy any buildings or bridges in its path. - Urban/Area Floods:
Urban/Area floods also have quite rapid flood waters but they are not quite as severe as flash floods. Streets will become fast moving rivers and basements can become death traps once they fill with water.
The main cause of this is due to the conversion of fields or woodlands to the roads and parking lots. - River Flooding:
River Floods are longer term events that can occur in just a few hours after a lot of runoff torrential rain from Tropical Cyclones reaches the rivers. A lot of the water in river floods may also begin a flash flood.