Storm Surge Preparedness and Safety

What is Storm Surge?

A storm surge is a rise above the normal water level along a shore resulting from strong onshore winds and / or reduced atmospheric pressure. Storm surges accompany a tropical cyclone as it comes ashore. They may also be formed by intense low-pressure systems in non-tropical areas.

Storm Surge + Normal Tide = Storm Tide

The combination of storm surge and normal (astronomical) tide is known as a 'storm tide'. The worst impacts occur when the storm surge arrives on top of a high tide. When this happens, the storm tide can reach areas that might otherwise have been safe. On top of this are pounding waves generated by the powerful winds.

The area of sea water flooding may extend along the coast for over 100 kilometres, with water pushing several kilometres inland if the land is low lying.

The combined effects of the storm tide and waves can knock down buildings, wash away roads and run ships aground. If you are caught in your home or in a car when a significant storm surge arrives, you may not survive.

The paths of cyclones are often erratic, which makes it hard to forecast exactly when and where a cyclone will cross the coast. This makes it difficult to predict how high the astronomical tide will be when the storm surge strikes, since the time difference between high and low tide is only a few hours.
As a result the Bureau of Meteorology, in its warnings to the public, makes the 'worst case' assumption that the cyclone will cross the coast at high tide.

Had Cyclone Tracy arrived in Darwin during a high tide, the devastation would have been even worse. Similarly, a low tide saved Townsville from a dangerous storm surge that accompanied Cyclone Althea in 1971.

Storm surges and tsunami

Storm surges and tsunamis are generated by quite different phenomena. While both can cause inundation and significant damage in coastal regions, they have quite different characteristics.

A storm surge is generated by weather systems forcing water onshore over a generally limited stretch of coastline. It will normally build up over a time frame of a few hours, as the cyclone or similar weather system approaches. Normally wind-waves on top of the surge will contribute to its effect.

A Tsunami is generated by earthquakes, undersea landslides, volcanic eruptions, explosions or meteorites. These travel great distances, sometimes across entire oceans affecting vast lengths of coastal land.

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How high will the storm surge be?

Every cyclone that affects the coast produces a storm surge. But not all storm surges rise to dangerous levels. The height of the surge depends on:

• The intensity of the cyclone - as the winds increase, the sea water is piled higher and the waves on top of the surge are taller.
• The forward speed of the cyclone - the faster the cyclone crosses the coast, the more quickly the surge builds up and the more powerfully it strikes.
• The angle at which the cyclone crosses the coast - in general, the more head?on the angle, the higher the surge. However other angles can lead to local zones of enhanced surge in areas such as narrow inlets and bays.
• The shape of the sea floor - the surge builds up more strongly
  if the slope of the sea bed at the coast is shallow. If the sea bed slopes steeply, or if fringing reefs are present, then the surge will be less.
• Local topography - bays, headlands and offshore islands can funnel and amplify the storm surge.

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