Storm surge, caused by very low atmospheric pressure, is a volume of oceanic water driven by the wind toward the shore where it "builds up" along the coast producing a localized increase in sea level.
Such low atmospheric pressure occurs during cyclonic storms, called typhoons in the Pacific region and hurricanes along the Atlantic seaboard. During these storms, a dome of water forms in the area of low pressure that moves across the ocean as the winds drive the storm. Upon reaching shallow coastal water, winds blowing toward the shore move over the domed water, and pile water along the coast, producing an elevated sea level.
Storm surges are the deadliest element of cyclonic storms. These storms form in areas with warm surface water temperatures in the zone 8° north and 15° south of the equator. Winds may reach 328 MPH (100 km/h) and move in a counterclockwise spiral around a calm center. Winds of nearly 200 MPH (60 km/h) associated with cyclonic storms can extend the storm's diameter to more than 2,000 mi (640k). Hurricane Hugo, the most expensive cyclonic storm in U.S. history had a storm surge of 7.9 ft (2.4 m) above normal tide levels. However, smaller storms can produce storm surge when they coincide with high tides.
Tides, the rhythmic rise and fall of the ocean's surface caused by the gravitational attraction of the sun and moon, can amplify the effect of storm surges. In most areas, two high tides and two low tides occur daily. Tides may rise and fall a few inches to more than 49 ft (15 m).
Normal tides are driven by three astronomical occurrences. First, when the moon is nearest the earth, due to its oval shaped orbit, the moon's gravitational attractions is at its greatest; thus tides occurring during this period are greater than normal tides. Second, when the sun, moon and earth are aligned, "spring" tides occur. These are 20% greater than normal tides. When the sun, moon and earth are at 90°, to one another "neap" tides, which are 20% weaker than normal, occur. Finally, twice a month the moon crosses the earth's equatorial plane resulting in higher than normal tides.
In addition to tidal influences, the shape of ocean basins, inlets and bays, and sea level rise may cause higher than normal storm surges. In February 1976, Bucksport, Maine experienced a 5.8-ft (1.76-m) storm surge and Bangor, located 15 mi (24 km) inland from Bucksport, suffered a storm surge of nearly 11 ft (3.35 m). The difference in storm surges was due to the funnel shape of the bay. The great amount of water in the surge that was forced through a relatively narrow channel caused water levels to rise rapidly in Bangor. Damages of more than $2 million occurred.
London historically experienced recurrent tidal flooding from storm surges until the Thames barrier was constructed in 1986. Ten steel gates reaching 1,640 mi (500 m) across the river now protect the city from storm surge impacts.