By Michael Kuhne, AccuWeather staff writer
While the powerful, destructive wind speeds of an approaching cyclone are used to estimate the storm's category, storm surge is often the greatest threat to both lives and property.Storm surge is an above-normal rise in sea water along the coast, which is generated by a tropical storm or hurricane and exceeds normal astronomical tides.
"These tropical cyclones generate enough wind and wave action to pile up water at the leading edge of their forward motion," AccuWeather Hurricane Expert Dan Kottlowski said. "A moving hurricane causes a large pile up, or a building area of water ahead of its path."
If large enough, a storm surge could wash away entire structures and permanently alter the shape and size of coastal areas.
"Any structure, including buildings and houses, in the path of a hurricane and a rising surge of water can be seriously damaged or completely destroyed by storm surge," Kottlowski said.
The power that a large volume of moving, rising water exerts on all structures right at or near the coast is immense, he added.
In addition, high winds in front of the storm can cause damage to structures preceding a storm surge, creating conditions that make the structures even more vulnerable to its impact.
"In some storms, the storm surge is so great that no object or structure can survive this force of water," Kottlowski said.
As this building level of water starts to move into the coast, the water will rise to a certain height depending upon the slope of the coast or continental shelf, he added. The more gradual the slope, the higher the storm surge.
"The height of the storm surge is also dictated by the shape of the coast," Kottlowski said, citing differences between a concave coast versus a convex, or almost straight, coast.
Coastal areas that are more concave, such as those with coves or inlets, will experience higher storm surge events because of a funneling effect associated with the large volume of sea water.
"If the coast has more of an abrupt slope, and is oriented more in a straight line, it will disperse the water, leading to a lower storm surge," he said.
The size of a storm surge is also impacted by the size of the storm itself, Kottlowski said, stating large storms lead to more water being churned up, intensity and forward speed.
Winds speed and the speed of the forward motion of a storm or hurricane determines just how much water piles up ahead of a storm.
Often the strongest winds are in the right front quadrant of a moving storm. This is the area to the portion of a cyclone's eyewall to the right of the eye, or center of the storm.
"The greatest storm surge will often be found at where the leading edge of the storm, and right front quadrant of the storm, move onshore," Kottlowski said.
There are many variables that determine storm surge impact including the depth of the water, tidal interactions and wave action.
"Hurricanes making landfall at high tide and with very rough seas can bring total inundation values well over what the storm surge is," he said.
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One of the most recent examples of the devastating, costly impact of storm surge occurred in 2005 when Hurricane Katrina flooded the New Orleans area and Mississippi coastline, according to NOAA. Katrina's storm surge flooding was between 25 and 28 feet above normal tide levels. Katrina caused an estimated $75 billion in damages.
"A large storm surge will change the shape and size of coastal areas, especially beaches," Kottlowski said. It is extremely expensive to restore beaches and coastal areas to their original shape after a hurricane, he said.
"In most cases, it’s probably best just to let Mother Nature take its course," he said. "After all, most places that have experienced massive destruction from storm surge were probably shaped by previous hurricane hits."
Kottlowski said this is one reason why local and federal officials often encourage residents with surge-prone properties to not rebuild structures close to the coast, or if at all, if they were impacted by hurricanes.
For more safety and preparedness tips, visit AccuWeather.com/Ready.