For the more information about the geologic resources of the National Park Service, please visit http://www.nature.nps.gov/geology/.
Storms, Rip Currents, and Tsunamis
During the passage of major storms, strong winds, heavy rains, large waves, and storm surges frequently impact coastal parks. When these large storms originate near the equator, they are called tropical cyclones, and when their wind speed exceeds 74 mph (119 km/h), they are called hurricanes. When these large storms originate over North America or over the near-continent portions of the Atlantic Ocean, they are called extra-tropical cyclones or nor'easters.
Hurricanes usually pass over the eastern coast of North America in one or two days, most often during the period from June to November. Nor'easters, on the other hand, can last for two to four days and occur most frequently from October through April.
Large storms regularly impact National Park units, sometimes causing significant damage. For example, Hurricane Isabel made landfall in Cape Hatteras National Seashore in September 2003 with winds of 105 mph (169 km/h). The storm surge washed out a 2,000 ft (610 m) portion of Hatteras Island and created a new, temporary inlet, isolating the Hatteras community by road for two months.
A tsunami is a series of water waves generated by any large displacement of the sea surface. Sea floor uplift from an earthquake is the most common cause of a tsunami, but volcanic eruptions, underwater landslides, or meteorite impacts can also generate tsunami waves. In the open ocean these waves may only be a few inches high, but can travel at speeds in excess of 500 mph (805 km/h)! In contrast, when tsunami waves reach shallow water, they slow down considerably and may reach large heights.
Tsunamis have caused great destruction and loss of life. In 2009 the 8.1 Samoa earthquake generated tsunami waves that reached up to 40 ft (12 m) in height and flooded areas more than a third of a mile (600 m) inland. The tsunami caused widespread damage and destroyed the National Park of American Samoa headquarters and visitor center. A webinar is available for those interested in learning more about the tsunami impacts to the park (Windows Media File - 166MB).
Rip currents are powerful, narrow channels of water that flow away from the shore. Rip currents occur along the East, Gulf, and West coasts of the United States, and along the shores of the Great Lakes. These currents (often incorrectly called rip tides) can flow up to 8 ft per second (2.4 m/s) , which is faster than the average person can swim.
Rip currents are created due to set-up near the shoreline. Set-up is a slight increase in water levels compared to those found seaward of the surf zone. Set-up creates unstable conditions that are eventually relieved through the formation of rip currents. These dangerous currents generally form at a low point in a sandbar. Before going into the ocean, check for these signs of a rip current:
- A channel of churning, choppy water
- An area having a notable difference in water color
- A line of foam, seaweed, or debris moving steadily seaward
- A break in the incoming wave pattern
Because rip currents tend to be narrow, swimmers caught in a rip current should swim parallel to the shore until they escape the current, then swim at an angle away from the current toward shore. Swimmers should be especially cautious during storm events, which may increase the frequency and strength of rip currents.
Last Updated: August 15, 2011