Along the coast, storm surge is often the greatest threat to life and property from a hurricane. In the past, large death tolls have resulted from the rise of the ocean associated with many of the major hurricanes that have made landfall. Hurricane Katrina (2005) is a prime example of the damage and devastation that can be caused by surge. At least 1,500 persons lost their lives during Katrina and many of those deaths occurred directly, or indirectly, as a result of storm surge.
Storm Surge vs. Storm Tide
Storm surge is an abnormal rise of water generated by a storm, over and above the predicted astronomical tides. Storm surge should not be confused with storm tide, which is defined as the water level rise due to the combination of storm surge and the astronomical tide. This rise in water level can cause extreme flooding in coastal areas particularly when storm surge coincides with normal high tide, resulting in storm tides reaching up to 20 feet or more in some cases.
Factors Impacting Surge
Storm surge is produced by water being pushed toward the shore by the force of the winds moving cyclonically around the storm. The impact on surge of the low pressure associated with intense storms is minimal in comparison to the water being forced toward the shore by the wind.
The maximum potential storm surge for a particular location depends on a number of different factors. Storm surge is a very complex phenomenon because it is sensitive to the slightest changes in storm intensity, forward speed, size (radius of maximum winds-RMW), angle of approach to the coast, central pressure (minimal contribution in comparison to the wind), and the shape and characteristics of coastal features such as bays and estuaries.
Adding to the destructive power of surge, battering waves may increase damage to buildings directly along the coast. Water weighs approximately 1,700 pounds per cubic yard; extended pounding by frequent waves can demolish any structure not specifically designed to withstand such forces. The two elements work together to increase the impact on land because the surge makes it possible for waves to extend inland.
In confined harbors, the combination of storm tides, waves, and currents can also severely damage marinas and boats. In estuaries and bayous, salt water intrusion endangers the public health, kills vegetation, and can send animals, such as snakes and alligators, fleeing from flooded areas.
Surge Vulnerability Facts
* From 1990-2008, population density increased by 32 percent in Gulf coastal counties, 17 percent in Atlantic coastal counties, and 16percent in Hawaii (U.S. Census Bureau 2010).
* Much of the United States' densely populated Atlantic and Gulf Coast coastlines lie less than 10 feet above mean sea level
* More than half of the Nation's economic productivity is located within coastal zones.
* Seventy-two percent of ports, 27 percent of major roads, and 9 percent of rail lines within the Gulf Coast region are at or below 4-foot elevation.
* A storm surge of 23 feet has the ability to inundate 67 percent of interstates, 57 percent of arterials, almost half of rail miles, 29 airports, and virtually all ports in the Gulf Coast area.