Geography Reference
In-Depth Information
Since it takes the moon a little more than a day to orbit the earth, there are two cycles of tides in roughly
every twenty-five hours. From the low point in the tidal cycle, coastal waters rise gradually in the flood
tide , lasting a little more than six hours. Maximum water level, or high tide , is reached and the water be-
gins to drop or move away from the coast in the ebb tide. After about six hours, the minimum water level,
or low tide , is reached and the process begins all over again. There are also regular fluctuations in these
cycles. The highest tides, called spring tides (even though they have nothing to do with the season), occur
twice a month when the sun and moon are in a straight line with the earth. The smallest tides, called neap
tides , occur when the moon is at a right angle to the sun.
The difference between low and high tide—called the tidal range —is different in locations all over the
world. In the open ocean, tidal range is insignificant, often no more than three feet. But in coastal shallows,
the range is much greater. And in places like bays or channels, where the incoming ocean water is funneled
into a narrow inlet, the tidal range is greatly exaggerated. The greatest tidal range occurs in Canada's Bay
of Fundy, which is located between the provinces of New Brunswick and Nova Scotia. There, the tides of
the Atlantic Ocean are funneled into a narrow channel to produce a difference between low and high tide of
as much as fifty feet (15 meters) twice a day. This dramatic tidal action is now viewed as a potential source
of clean, inexhaustible energy. A power station that uses the ebb and flow of tidal water to drive turbines
which generate electricity has been in operation for more than thirty years at the mouth of the Rance River
in the Gulf of Saint-Malo in France.
Another phenomenon of tides is the tidal bore , which, unlike a colossal bore, goes away. On the other
hand, like many bores, it comes back. Regularly. A tidal bore is a high wave that travels up an estuary (an
inlet of the sea where the mouth of a freshwater river meets the incoming salty tide). The most remarkable
tidal bore is on China's Qiantang River, which flows into the Bay of Hangzhou. During spring tides, the
bore is over twenty-four feet high and travels at almost fifteen miles per hour. The rushing noise of the
waters as they move upstream—a sort of reverse waterfall—can be heard fifteen miles away.
What Do Tides Have to Do with Tidal Waves?
In a word, nothing. Until fairly recently, the phrase “tidal wave” was widely used, or, more precisely, mis-
used. “Tidal wave” is a misnomer, an obsolete term inaccurately used to describe either a tsunami (Japan-
ese for “overflowing wave”), a fast-moving sea wave caused by an underwater earthquake or volcanic
eruption, or a storm surge , an abnormal rise in the sea level caused by high winds like those in a tropical
hurricane. Whatever you call them, they can be terribly destructive to low-lying coastal areas when they
hit. When a cyclone blew out of the Bay of Bengal and hit Bangladesh in 1991, the storm surge and subse-
quent flooding killed hundreds of thousands of people.
But in two extraordinary recent occurrences, the world witnessed the incredible power of a tsunami,
captured live and broadcast around the world. The first was in Indonesia, where an earthquake off the coast
of Sumatra on December 26, 2004, created a tsunami wave as tall as 30 meters (98 feet). The most widely
recorded tsunami, it resulted in some 230,000 deaths.
The second instance came in March 2011 and struck the North Pacific coast of Japan. Spawned by a
magnitiude 9 earthquake, it sent 9- to 10-meter (30 feet) waves sweeping over the east coast of Japan,
killing more than 24,000 people and knocking out a nuclear power plant that began leaking radioactive
steam.
According to the Australian Geographic , the other most deadly tsunamis were:
Ise Bay, Japan (1586)
 
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