Atmosphere And Weather

Anatomy of a Storm Surge



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"Anatomy of a Storm Surge"
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As fearsome as the high winds of a hurricane can be, the storm surge (or storm tide) often causes the most death and destruction. A storm surge is the above-normal rise in sea level produced by the storm and in severe cases can become a wall of water 20 feet high sweeping inland, sometimes for miles. There are several factors that contribute to a storm surge: wind, barometric pressure, the shoreline topography, and tides. The two that are most important are wind and topography.

The main factor responsible for a storm surge is the cyclonic wind of a hurricane. The winds create great waves stretching out from the storm. One result of this is that "forerunner waves," large swells, will strike land well in advance of the storm. In the days before modern meteorology and satellites, this was an important warning sign of an approaching hurricane. The height of the surge is raised even further by the low barometric pressure within a hurricane. With reduced air pressure, the sea level tends to rise.

The other major factor is the shape, or topography, of the seabed and oceanfront land. Shallow water, or a restricted area like the mouth of a river mouth, will confine the water, causing it to pile up into a huge wall of water rushing inland. If the worst of a storm hits at high tide, the height of the surge will be higher still.

A storm doesn't have to have hurricane strength to produce a storm surgein fact, any storm, even a very moderate one, will produce wave action that can drive some water ashore. It is the terrific power of the hurricane force winds that makes their storm surges so destructive. A tropical storm (sustained winds between 39 and 74 miles per hour) will produce storm surges of 3 to 4 feet. Hurricanes are another matter. Even a category 1 hurricane may have a storm surge of up to 5 feet, while the monster category 5's (sustained winds of over 155 miles per hour) have storm surges exceeding 18 feet. When the topography acts to intensify the surge, storm surges reach 20 feet, and in rare cases as much as 30 feet.

A hurricane storm surge was responsible for the worst natural disaster in United States history. In 1900 a category 3 storm struck Galveston, Texas, which was not protected by the high levee that safeguards the city today. A storm surge estimated at 8 to 15 feet inundated the city and killed over 8000 people. In 1938 a category 3 storm known as the "Long Island Express" unexpectedly hit Connecticut and Long Island with a 10-12 foot storm surge that caused over 600 deaths. In 1969 the category 5 storm, Camille, devastated the Gulf coast with winds of 200 miles per hour and a storm surge measured at an incredible 24.6 feet! Most recently, the 10-20 foot storm surge from hurricane Katrina flooded New Orleans and caused over 1200 deaths and widespread destruction along the Gulf coast.

The best way to survive a storm surge is to not be thee when it hits. The National Weather Service points out that most of the fatalities in hurricanes are due to being caught in the storm surge. It is now standard procedure for local, state, and federal authorities to declare an evacuation when a major hurricane is approaching a populated shoreline. In the event you ever find yourself having to evacuate ahead of an approaching hurricane, here are the steps to follow:
1) Identify the nearest evacuation facility, a friend's house, or a hotel you can stay at outside the danger zone. Map out the route you need to take.
2) Fill up your car with gas and make preparations like boarding up windows ahead of time.
3) Expect traffic congestion and leave early.
4) Make arrangements for pets. Emergency shelters usually will not accept pets.

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