What is a Sea (Storm) Surge?

A Sea (Storm) Surge is water that is pushed toward the sea shore by the force of the winds swirling around the storm. This advancing surge combines with the normal tides to create the hurricane storm tide, which can increase the mean water level by up to 4 metres or more in height.

In addition, wind-driven waves are superimposed on the storm tide. This rise in water level may cause severe flooding in coastal areas, particularly when the storm tide coincides with the normal high tides. Storm surge also affects rivers and inland lakes, potentially increasing the area that may have to be evacuated.

The level of surge in a particular area is also determined by the slope of the continental shelf. A shallow slope off the coast will allow a greater surge to inundate coastal communities. Communities with a steeper continental shelf will not see as much surge inundation, although large breaking waves can still present major problems. Where sea walls and barriers are unable to protect harbours, storm tides, waves, and currents inside these confined harbours may severely damage ships, marinas and pleasure boats.

In general, the more intense the storm, and the closer a community is to the right-front quadrant, the larger the area that must be evacuated. The problem is always the uncertainty about how intense the storm will be when it finally makes landfall. Emergency managers and local officials balance that uncertainty with the human and economic risks to their community. This is why a rule of thumb for emergency managers is to plan for a storm one category higher than what is forecast which is a reasonable precaution to help minimize the loss of life from major storms.

Wave and current action associated with the tide also causes extensive damage. Water is heavy therefore extended pounding by frequent waves can demolish any structure not specifically designed to withstand such forces.

The currents created by the tide combine with the action of the waves to severely erode beaches and coastal highways. Many buildings withstand hurricane force winds until their foundations, undermined by erosion, are weakened and fail. In estuaries, intrusions of salt water may also endanger the public health and disrupt the eco-system.

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What is a tsunami?

A tsunami (which is Japanese for “harbour wave”) are a set of very large waves, which are created by any rapid, large-scale disturbance of the sea, including earthquakes, landslides, volcanoes, asteroid impacts or extreme weather events (meteo-tsunamis). They generally have very long wavelengths (up to 100 km or more) and high velocities (sometimes exceeding 800 km/hr), but may have small wave heights (few decimetres) in deep ocean.    However, once they reach the coast and shallower water, their velocity decreases therefore increasing their wave height.   

The most destructive tsunamis are often triggered by sudden vertical displacements over large areas due to large shallow earthquakes along geological plate boundaries or large fault zones, which may cause a vertical displacement in the water column.

The tsunami probability risk is low for the coastal areas of Cape Town but the hazard is not totally excluded due to the mid-Atlantic and Indian Ocean faults and their related earthquake potential. Listen to the radio or TV for any tsunami warnings, and if you observe any unusual sea conditions move inland to a higher ground elevation.

By drafting your own Family Emergency Plan and identifying all your risks, including those hazards affecting your own environment, you can make a positive contribution to preparedness.