Hurricane Awareness; Storm Surge, Waves and Marine Safety
July 19, 2012
Hurricanes bring severe hazards to coastal and marine interests. For Maine, because of our densely populated coastline, fisheries and ports, these hazards need to be well understood.
Storm surge is a large dome of water that is pushed toward the shore by the force of the winds circulating around the storm. This advancing surge combines with the normal tides to create the hurricane storm tide, which can increase the mean water level 15 feet or more.
The greatest storm surge generally occurs just to the right of the storm track, where the strongest winds of the storm are blowing onshore, perpendicular to the coast. In addition to the surge, wind-driven waves in that area can also cause considerable damage to structures. This rise in water level due to the surge can result in severe flooding in coastal areas, particularly when the storm surge 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.
DID YOU KNOW?The location with the greatest potential for storm surge along the northern New England coast is the Penobscot River near Bangor, Maine. 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.
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 Penobscot Bay.
In addition to the speed and intensity of the hurricane, the level of surge in a particular area is also determined by the slope of the underwater topography and the shape of the coast line. A shallow slope and the funneling effects of a bay will contribute to a greater surge. Areas with a steeper continental shelf will not see as much surge, although large breaking waves can still present major problems. Storm tides, waves, and currents in confined harbors can severely damage ships, marinas, and pleasure boats.
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 1700 pounds per cubic yard, and 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.
Hurricanes have been the cause of many maritime disasters. And, unfortunately, there is no single rule of thumb that can be used by mariners to ensure safe separation from a hurricane at sea. In order to minimize risk, mariners should allow for a large margin of error in the hurricane track and intensity forecasts. Today, even as our understanding of and ability to forecast hurricanes increases, there is still considerable error in forecasting the movement and intensity of these systems.
Average errors in the hurricane track forecast increase considerably as the forecast projection increases. The following list gives average errors of hurricane forecasts for the 5-year period from 2007 to 2011. Note that errors for storms in northern New England are likely greater than these "average" values due to the acceleration that often occurs south of New England and due to the comparatively fast movement of the storms in New England waters.
|FORECAST PROJECTION||AVERAGE ERROR|
|12 HOURS||30 n mi|
|24 HOURS||48 n mi|
|48 HOURS||83 n mi|
|72 HOURS||124 n mi|
|96 HOURS||167 n mi|
|120 HOURS||213 n mi|
For those with boats
It's important to plan ahead. Know exactly what you need to do and how long it will take you to accomplish the necessary tasks. Keep in mind that others will also be taking preparatory actions too, so leave yourself additional time.
If you plan to leave your boat in the water, consider the possible effects of the storm tide and waves. Make sure your anchor is sufficient to hold the boat, and have enough anchor line to account for the storm tide. Secure or remove all non-permanent equipment from the deck. Never try to ride out the storm on your boat. You will endanger your life and possibly the lives of rescuers.
If you are able to put your boat on a trailer, get it out of the water early. If you wait too long, you may be in a long line. If possible, store your boat inside a garage. However, if you leave your boat outside, put it in a sheltered location, and secure it to sturdy objects such as large trees.
Question of the Day: What was the greatest loss of life from storm surge in the United States?
The National Weather Service has declared the week of July 16th through the 20th to be Hurricane Awareness Week in New England. (This information was prepared by the National Weather Service, Gray, Maine.
For additional information about hurricanes and hurricane safety, visit the National Hurricane Center's web site