Storm Surge

All tropical cyclones crossing or passing near the coastline are capable of producing a damaging “storm surge”, which can increase the sea level for periods of several hours and affect 100’s of km of coastline either side of the storm track. The storm surge is caused by the extreme winds surrounding the eye of the cyclone causing strong currents, the low pressure in the centre and how it interacts with the profile of the continental shelf and the coastline. Wide shallow bays are likely to experience the highest surge, especially if they then narrow into river estuaries, while headlands or islands in deepwater are generally less affected.

The following schematic diagram shows that the full impact of the cyclone on the sea level will be made up from a number of different elements:

Schematic diagram - impact of cyclones on sea levels. Copyright 2011 Systems Engineering Australia Pty Ltd

Firstly, the surge generated by the cyclone will be added onto the normally expected tide level at any location and so the timing of the arrival of the peak surge and the high tide has a large effect on the final storm surge level.

Next, the extreme winds can also create abnormally high waves that ride on top of the combined tide and surge level, sometimes reaching into areas that do not normally experience ocean swell. When these waves finally break against the shoreline their energy can create a localised extra increase in the water level at a beach, which is called breaking “wave setup”.

The combination of the tide, plus the surge, plus the wave setup is termed the “total storm surge” level and this is capable of flooding inland over low lying land. The inundation extent based on the predicted water level is shown on the accompanying inundation maps.

Finally, some individual waves will always rush up the beach or a cliff face and can cause localised impacts and erosion at heights sometimes well above that of the total storm surge level. This is called “wave runup” and is not presently mapped because it can vary greatly depending on the shoreline conditions and it does not form a continuous flood-height, although wave over-wash will occur in some areas.