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MAINE EMERGENCY MANAGEMENT AGENCY
Hurricane Awareness: the Secrets of Storm Surge
July 10, 2008
The greatest potential for loss of life related to a hurricane is from the storm surge.
Storm surge is simply water that is pushed toward the shore by the force of the winds swirling around the storm. This advancing surge combines with the normal tides to create the hurricane storm tide. This can increase the mean water level 15 feet or more. In addition, wind driven waves are superimposed on the storm tide. This rise in water level can cause severe flooding in coastal areas, particularly when the storm tide coincides with the normal high tides. For areas of the coast less than 10 feet above mean sea level, the danger from storm surge can be tremendous.
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. Storm tides, waves, and currents in confined harbors severely damage ships, marinas and pleasure boats.
In general, the more intense the storm, and the closer a community is to the right-front quadrant, the larger the area that must evacuate. The problem is always the uncertainty about how intense the storm will be when it finally makes landfall. Emergency managers and local officials balance that uncertainty with the human and economic risks to their community. This is why a rule of thumb for emergency managers is to plan for a storm one category higher than what is forecast. This is a reasonable precaution to help minimize the loss of life from hurricanes.
In northern New England, the greatest factor in determining the effects of a storm surge is the timing of the surge with respect to the astronomical tides. If the storm surge hits at the time of low tide, little if any coastal flooding will occur. If, however, the surge hits at high tide, considerable coastal flooding, beach erosion, and other damage is possible. Unfortunately, the exact timing of landfall in northern New England is often difficult to predict very far in advance, so plans should be made based on the possibility the surge could strike at high tide.
Wave and current action associated with the tide also 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. Waves generated from distant or approaching storms can also present a hazard to those who are near the ocean. Strong rip currents can carry even strong swimmers out to sea, and unexpected large waves can wash people from rocks.
The currents created by the tide combine with the action of the waves to severely erode beaches and coastal highways. Many buildings withstand hurricane force winds until their foundations, undermined by erosion, are weakened and fail.
Along the New Hampshire and Maine coast, the greatest threat of damage from storm surge lies in the beach areas south of Portland and in the Penobscot Bay. In Bangor, computer model estimates indicate that the funneling effect of the Penobscot Bay and River could lead to a 23 foot tide for a Category 3 hurricane moving north at 40 mph.
(This information prepared by the National Weather Service Forecast Offices in Gray and Caribou, Maine.)
For additional information about hurricanes and hurricane safety, visit the National Hurricane Center's web site at: http://www.nhc.noaa.gov/
Contact:Beth Barton or Lynette Miller
Last update: 07/20/10
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