Monday, December 10, 2012

Geography in the News: Storm Surge Threats

From National Geographic:  Geography in the News: Storm Surge Threats

Superstorm Sandy sent a storm surge of 13 feet (4 m) onto New Jersey’s and New York’s fragile shorelines, creating chaos and widespread misery for coastal inhabitants. Examining experiences with hurricanes Katrina and more recently Irene, storm surges clearly create more damage than wind and rain during these tropical and middle latitude cyclones.
Storm surges are associated with high winds over water. Wind over open water can bring not only high water surging onto a shoreline, but the surge, combined with intense wave action, can pummel shoreline structures. Parts of demolished structures then become battering rams with the surging water and waves destroying even more structures.
Virtually all ocean swells and waves are created by wind over water. As wind speed increases, the height between the tops of swells and the troughs that separate them increases. As the swells reach the shallow waters of coastal areas, drag created at the bottoms of swells slows their movement, creating breaking waves along the shore.
The faster the wind, the larger the body of water and the longer the wind blows from a single direction, the larger will be the swells. As these large swells approach the shallower water along the shores, drag increases along their bottoms causing the tops of the swells to become closer together. Thus, the net effect is that the water literally piles up against the shore, creating a storm surge.
Therefore, hurricane-force winds blowing from the Atlantic Ocean toward the Eastern Shore for several days can push a devastating storm surge into low coastal areas, overwhelming natural protective dunes. This process is precisely what happened with Superstorm Sandy between October 27-31.
Not since Hurricane Katrina struck the Louisiana and Mississippi coasts in 2005 has there been greater coastal damage from a storm surge than Sandy’s damage. Katrina was one of the top five deadliest Atlantic hurricanes with a documented 1,833 dead. By far, the majority of Katrina’s fatalities were caused by the storm surge that overtopped the levies and flooded New Orleans’ low Ninth Ward. The protective dikes were overwhelmed by the surge that reached 25 to 28 feet (7.6 to 8.5 m) above normal sea level.
Additional factors played supporting roles in Superstorm Sandy’s increased storm surge. As the counterclockwise rotating storm made landfall on the New Jersey and New York coasts, the winds on the north side of the storm came directly onshore driving the full force of the surge onto the shoreline. In addition, the landfall coincided precisely with the high tide associated with a full moon (spring tide). As the moon lines up on the opposite side of the Earth from the sun during a full moon (syzygy), the gravitational pull between the two intensifies, resulting in higher tides.
Few who endured the direct affects of coastal flooding associated with Sandy’s storm surge will soon recover from the personal and emotional damage. Storm surges, however, are the real destructive agents and represent the most dangerous phenomena associated with tropical storms.
Superstorm Sandy was a perfect example of the perfect storm, battering the densely populated East Coast with impunity.


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