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Understanding Storm Surge Damage PhotosStorm surge is the most dangerous part of a hurricane. When a hurricane makes landfall, the storm surge causes the most destruction. The high water level causes flooding and also effectively brings the coast inland. Then, the pounding of the waves causes more destruction beyond flooding. Storm surge is primarily caused by the wind blowing inland, leading to water piling up. In the open ocean, the water can flow away underneath the storm. (This also creates a strong undersea current.) When the hurricane approaches shallow water, there is no room for the water to flow under the storm so it builds into the major part of the storm surge. The smaller part of the storm surge is caused by the low pressure in the middle of the storm. The pressure in the eye is significantly lower than the surrounding atmosphere, so the atmospheric pressure causes the water in the eye to rise. In general, for a 1 mb drop in pressure there is a 1 cm rise in water level. You can think of the eye as a drinking straw. When you drink through a straw, you create a lower pressure so the atmosphere can push the liquid up the straw to your mouth.
Figure 5. Schematized depiction of storm surge impact
Tide levels add to the storm surge, as seen in figure 5, and tides seem to increase as you go north; around Key West they are about 1 ft, the Gulf Coast 1-3 ft, the Carolinas 4-5 ft, New Jersey 6 ft, Boston 10 ft, Maine 20 ft, Alaska 30 ft, and Nova Scotia 50 ft.
Figure 6. A huge casino barge tossed about 100 yards over Highway 90 in Biloxi by Katrina’s storm surge The storm surge force can be quite substantial, as can be seen by the above image in figure 6. Effects of storm surge on homes with anchoring foundations reveal a pattern different from that seen with the barge example. Damage patterns for anchored homes show that the first floor is especially vulnerable.
Wave and current action associated with the storm surge causes extensive damage. 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.
Before Katrina After Katrina storm surge The famous concrete Muslim Mosque in Indonesia that survived the 2004 tsunami amidst total destruction had an open 1st floor. Below right is a home where the 1st floor was almost removed by storm surge damage. The following figures show similar effects.
The lower levels of The First Baptist Church in Gulfport, Miss. experienced significant damage from Hurricane Katrina's storm surge. The 1st floor of the Beachfront Holiday Inn is completely gutted. The peak surge reached 12 feet into this hotel with 1-2 foot waves on top of that. The "Official" elevation of this hotel is 16 feet above sea level, 16 feet + 12 feet = 28 feet. This means the storm surge reached 28 feet at Gulfport Beach, MS. 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. Because much of the United States' densely populated Atlantic and Gulf Coast coastlines lie less than 10 feet above mean sea level, the danger from storm surge is tremendous. As these recent events have shown hurricane and tsunami storm surges seem to wipe out 1st floors, whereas the higher structures above the storm surges are much less damaged. However, an open first floor does not assure safety unless it is tall enough to be above the storm surge:
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