Shorelines and Coastal Processes

Pamela J. W. Gore
Georgia Perimeter College

Outline

Water Movements

  1. Waves
  2. Tides
  3. Currents

Sea Level Changes

Waves

 

Know the following terms:

Wavelength is the horizontal distance between two adjacent wave crests

Note that waves bend to become more parallel with the coast.

Longshore drift (or transport); also called beach drift

Sediment is transported along the beach by the waves.

Waves rush onto the beach at a slight angle, but they rush straight back out to sea because of gravity. Because of this, sediment in the surf zone is transported along the beach in a zig-zag pattern. It is referred to as a longshore current.

Hard structures built perpendicular to the beach (i.e., sticking out into the water) will tend to trap sand on the up-current side. On the down-current side, erosion occurs.


Jetties along Lake Huron near Grand Bend (as seen from between Detroit, Michigan and Toronto, Canada).  43.313001   -81.762829
Can you tell the direction of longshore drift?

See movie: The Beach - A River of Sand for more details on longshore drift. A classic paper in Scientific American has aerial photos of many of the sites mentioned in the movie, and you may be able to locate it.

Bascom, Willard, 1960, Beaches, August 1960, Scientific American.
I located this paper in a Scientific American book called Ocean Science, which is a compilation of articles.

Note that a tsunami is a seismic sea wave. It is caused by earthquakes under the sea, and has nothing to do with tides, although it is commonly (and erroneously) called a tidal wave.


Erosional features formed by waves:


Sea Arch, Kilauea Volcano, Big Island of Hawaii

 


Coastal erosion at the north end of Jekyll Island, Georgia is cutting into a forest.
Note downed trees in the surf zone.


Dune erosion on the Outer Banks of North Carolina near Cape Hatteras.

 

Note that man-made structures along the coastline may enhance coastal erosion (seawalls for example, or down-current from groins and jetties).


Sea wall attempting to prevent coastal erosion, south end of St. Simons Island, GA
These are the Johnson Rocks, placed following a hurricane when Lyndon Johnson was President
.

See the movie Portrait of a Coast to learn about the differences between winter and summer waves, and the problems of coastal erosion from winter storms, and attempts to prevent coastal erosion.


Snow fences on the beach at Tybee Island, Georgia, used to slow wind and cause sand deposition from wind. Note the dune building that has occurred between the fences.


Vegetation stabilizes coastal dunes.
Signs ask visitors not to walk on the dunes to protect the vegetation.
Tybee Island, Georgia

Depositional features formed by waves:

Barrier Islands


Air view of Outer Banks of North Carolina near Kitty Hawk. Note Wright Brothers Memorial (white column on green hill) on right.


A narrower part of the barrier island near Cape Hatteras, NC. Note fences meant to stabilize dunes on right.

 


Tides

There is a rise and fall in sea level of approximately a meter or more, once or twice per day.

Cause of tides?

The moon's gravitational pull.

A bulge in the water on the Earth's surface occurs on both the side facing the moon and the side away from the moon.

The highest tides in the world occur in the Bay of Fundy in Nova Scotia, Canada. The tidal range is 15-16 meters (approximately 45-50 feet). Because there are 2 high tides and 2 low tides in each day (roughly a 24 hour period), then the tide must come in within about a 6 hour period. As a rough estimate, the tide rises about 8 feet (or 96 inches) an hour (48 feet divided by 6 hours). This translates to a tide which rises at more than one inch per minute. If you have walked down a beach with a steep cliff along side (which is common there), be sure to watch the tides. If you walk for about an hour and then notice that the tide is coming in, the water will be over your head before you get back to where you started!


High tide and low tide at Five Islands Provincial Park, Bay of Fundy, Nova Scotia, Canada


High tide and low tide at Five Islands Provincial Park, Bay of Fundy, Nova Scotia, Canada

Areas that are alternately submerged and exposed by rising and falling tides are called tidal flats.

Tides at certain times of the month are unusually lower or higher than at others. The reason for this has to do with the position of both the sun and the moon relative to the Earth. If all three are lined up in a straight line, the tides will have a higher tidal range. They are called spring tides. But if the sun and moon are at right angles to one another, the tidal range will be lower. These tides are called neap tides.

Tides are used in Nova Scotia (at Annapolis Royal) to generate electrical power (and also in France, Russia, and China). See chapter on Energy and Mineral Resources in your textbook.


Currents

Currents are the unidirectional flow of water.

You have already learned about turbidity currents, longshore currents, flood currents, and ebb currents.

You also need to know about rip currents (sometimes incorrectly called rip tides). When waves approach the coast at an angle, and the water rushes straight back into the sea, along some coastlines, the waters that rush back are concentrated into relatively narrow zones (perhaps up to 25 m or 82 feet wide), which can reach speeds of several miles per hour. These rip currents are responsible for drowning deaths at beaches in the US and around the world each year. You will likely remember hearing about several drownings from these currents during the past summer. Typically people who are wading or swimming accidentally venture into the rip currents, and they are pulled out to sea. It is impossible to swim or walk against these currents (if you try, you will end up swimming or walking backwards out to deep water). It is best to try to move out of the current by going parallel to the beach; if you are lucky you will get out of the current. Knowing this may save your life some day.

There are also other currents in the open ocean which you might study in an oceanography course. (For example, the Gulf Stream is a current.) These currents are often controlled by winds (the trade winds, for example) or by uneven solar heating of the earth. Currents (and winds) are acted upon by the Coriolis effect. The Coriolis effect is the tendency for currents (or other moving things) in the northern hemisphere to turn to the right, and for currents in the southern hemisphere to turn to the left. This is due to the Earth's rotation, and its influence on currents was first noted in 1835 by Gaspar de Coriolis.

Surface water currents in the world's oceans tend to form large circular patterns or gyres. In the northern hemisphere, these gyres rotate clockwise (turning to the right), and in the southern hemisphere they rotate counterclockwise (turning to the left). In fact, the aforementioned Gulf Stream, flowing northeastward along the east coast of the US, is part of one of these gyres.

So far, we have mainly discussed surface currents, but there are other currents in the oceans, acting at great depths. For instance, cold waters flowing from the Antarctic region are dense (because they are so cold), and they sink to the ocean floor and flow northward. (Note that cold water can hold more oxygen than warmer water, so bottom waters in the world's oceans tend to be oxygenated (but as it moves away from Antarctica, it gradually becomes depleted in oxygen).


Sea Level Changes

Sea level is caused by a combination of things. The total volume of water in the oceans at any given time. Note the effects of glaciation causing sea level to drop (and melting of the glaciers causing sea level to rise), and the displacement of sea water onto the land by increased rates of seafloor spreading and undersea volcanism (with the submarine lava displacing the water) as two examples. Sea level may also change locally due to localized subsidence or uplift of the coastline.

Sea level may also appear to change as a result of the deposition of sediment.

Because of sea level increasing or decreasing, shorelines may be described as emergent coasts (coastal land is rising relative to sea level) or submergent coasts (coastal land is sinking relative to sea level; sea level is rising).

Global warming seems to be causing sea level to rise.

Sea level has been rising at a rate of 10-15 cm over the past 100 years. That is equal to 1 to 11/2 cm of sea level rise in 10 years. That seems almost too slow to notice, but the rate seems to be accelerating.

Storm Erosion


Storm damage along the Mississippi coast from Hurricane Katrina, August 2005. Photo credit: U.S. Geological Survey.


Storm damage from Hurricane Katrina, August 2005. Dauphin Island, south of Mobile, AL, more than 110 km from where the eye of the storm came onshore. In the lower left corner of the second photo , you can see an oil rig that broke loose and washed ashore.

USGS Hurricane Katrina Impact Studies


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Copyright 1998, 2009 Pamela J.W. Gore

Page created by Pamela J. W. Gore
Georgia Perimeter College,
Clarkston, GA

November 23, 1998
Modified July 17, 1999
Modified June 2000
Modified April 25, 2003
Modified April 28, 2009