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Tabby sea wall made of oyster shells
Tabby seawall protects this part of Bay Street in Beaufort from tidal erosion
     

Mud extends to the middle of the Beaufort River at low tide
Salt marsh at Pigeon Point (City of Beaufort)
at Low Tide
(see same scene at high tide)
Photograph by Dennis Adams
(August 7, 2002)

 

Tides:
Why They Happen

by
Dennis Adams
Information Services Coordinator

Why does Beaufort County have a 6-foot tide, while parts of Florida have only a 2-foot range between high and low tides?

First of all, let's turn to The Columbia Encyclopedia for a quick definition of "tide": the "alternate and regular rise and fall of sea level in oceans and other large bodies of water." We all know that the "pull" of the moon causes tides, but often forget that the gravitational attraction of the sun also plays a lesser part. These forces pull the earth out of a strictly spherical shape and create a long, vertical band of waves that "wraps" all around the surface of the planet. This "pull" accounts for about 70% of tidal forces in the oceans, and can also raise the continents as high as a foot during tides. The attraction of the moon and sun can make the earth's atmosphere bulge out for miles, while the effect on a small lake may amount to only a few hundredths of an inch. This force even affects the earth's core.

The moon revolves around the earth in the same direction that the earth rotates on its own axis (this rotation is another major cause of the tides). Because this earth-moon cycle takes about 24 hours and 50 minutes to complete, there is an average interval of 12 hours, 25 minutes between the daily high tides (direct and indirect).

So at any point in time, two high tides are happening on the earth's surface: the direct tide on the side facing the moon and the indirect tide on the opposite side of the planet. These two "tidal bulges" move around the globe as the earth rotates, lagging slightly behind the moon and sun as they move westward over the earth's surface.

 

At high tide, the mud has been completely covered by water
Salt marsh at Pigeon Point (City of Beaufort)
at High Tide
(see same scene at low tide)
Photograph by Dennis Adams
(August 7, 2002)

Along much of the Atlantic coast in North America and Europe, both of the daily high-water levels are fairly equal (these are "semidiurnal tides"). "Mixed" tides, however, may occur in narrow estuaries, shallow seas and as in the Pacific coast of North America, where two daily high/low tide levels differ significantly. The interval between mixed tides may also be unequal. Large and partly-enclosed basins (like the Gulf of Mexico) and shores at higher latitudes may see only one ("diurnal") high tide/low tide occurrence per day.

Though the worldwide average tidal range is 6 to 10 feet, there are wide variations from place to place. The Caribbean, Mediterranean and Baltic Seas experience almost no tidal changes at all. At the other extreme, high tides at the tip of South America (and in parts of Europe and Asia) can swell to over 50 feet above sea level!

Kris Freeman (Encyclopedia of Earth and Physical Sciences) wrote of "more than 400 different factors that cause and influence the movement of tides," with "extremely complex" interactions among these forces. The shape of the continents and friction with the ocean floor are major disruptions. Locally, the tide responds to the depth, shape and size of the ocean basins. Weather conditions like winds, lingering zones of high or low pressure, or sudden change in barometric pressure can also affect the tidal range.

According to Freeman, most of the "tidal waves are trapped in the tidal basins between continents" over deep waters. The World Almanac and Book of Facts said that "the actual range of tides in the open ocean is less than in the shoreline regions. However, as the ocean tide approaches shoal (shallow) waters and its effects are augmented, the tidal range may be greatly increased." The tides around Antarctica are relatively stable, because those waters are not blocked by continental landmasses. It is from Antarctica, in fact, that the earth's tidal wavelengths spring forth and multiply into the Atlantic, Pacific and Indian Oceans.

Here, from the World Almanac , are some averages of the rise and fall of tides from selected areas (these are diurnal ranges, within the timeframe of 24 hours and 50 minutes):

Location
Tide Range
Baltimore, MD 1 foot, 8 inches
Charleston, SC 5 feet, 10 inches
Eastport, ME 19 feet, 4 inches
Galveston, TX 1 foot, 5 inches
Key West, FL 1 foot, 10 inches
New London,
CT
3 feet, 1 inch
New York, NY 5 feet, 1 inch
Portland, ME 9 feet, 11 inches
San Diego, CA 5 feet, 9 inches
San Francisco, CA 5 feet, 10 inches
Vancouver, B.C 10 feet, 6 inches
 
Location
Tide Range
Boston, MA 10 feet, 4 inches
Cristobal, Panama 1 foot, 1 inch
Ft. Pulaski, GA 7 feet, 6 inches
Hampton Roads, VA 2 feet, 10 inches
Mobile, AL 1 foot, 6 inches
Newport, RI 3 feet, 11 inches
Philadelphia, PA 6 feet, 9 inches
St. Petersburg, FL 2 feet, 3 inches
Sandy Hook, NJ 5 feet, 2 inches
Seattle, WA 11 feet, 4 inches
Washington, DC 3 feet, 2 inches.

For your specific area, go to the WWW Tide and Current Predictor Web site (http://tbone.biol.sc.edu/tide/sitesel.html#useastlower).

The Mediterranean Sea is virtually without tides because the tidal waters do not get far past the Strait of Gibraltar before falling back out into the Atlantic Ocean. Because the Mediterranean was the only sea the ancient Greeks ever sailed, the concept of "tides" was unknown in classical Greece. One intrepid Hellene, Pytheas (around 300 B. C.) sailed along old Phoenician routes past Gibraltar and as far as Britain and Scandinavia. But Asimov's Chronology of Science & Discovery said that few of Pytheas' compatriots believed his accounts of the strange lands to the north. The Greeks rejected his descriptions of the harsh waves that lashed the shores twice a day. They would not accept the rising and falling of water at predictable intervals.

How strange our Lowcountry marshes would have seemed to those Greeks! And how Pytheas would have enjoyed a marshfront home right here in Beaufort County.


Note: "Diurnal" means "pertaining to or occurring in a day or each day" (The American Heritage Dictionary), derived from Late Latin diurnalis ("daily"), from diurnus ("day"). "Journal" came to English through French from the Latin, and "diurnal" is an archaic word for "newspaper."

 
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