The following information was taken from the BOM Storm Surge
Preparedness and Safety Website page.
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
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
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
- 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.