Wave Erosion and Marine Geology

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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Waves

Ocean waves are driven by the winds. The distance over which the wind blows in a straight line, the fetch, determines the size of waves. Long weather fronts in the North Pacific result in huge waves.

The water in waves does not move but merely oscillates. When waves reach shore they "feel bottom" and begin to pile up. 
Spilling waves simply roll over themselves

Plunging waves slow down more abruptly and the top overshoots the bottom.

When waves break simultaneously over a long distance, they trap a tube of compressed air that eventually bursts upward in a jet of spray.

 

Rips

When waves break directly on a straight coast, there is no place for water to return to sea. The water tends to return in narrow currents called rips.

There is no such thing as a "rip tide," nor do they pull swimmers under. They carry swimmers rapidly beyond their swimming range, however.

Obviously, the thing to do, recommended by all survival manuals, is not to fight the rip but swim parallel to the coast, then use the waves to help return to shore.

Storm Waves: Galveston, Texas, 1900

The worst disaster in U.S. history took place September 8, 1900 when a hurricane struck Galveston, Texas, killing an estimated 6,000-8,000 people

GAlveston 1900 Storm waves flooded the Gulf shore of Galveston almost 16 feet deep, and surged into Galveston Bay, flooding the "sheltered" shore ten feet deep. The waves demolished everything, pushing it inland until the debris wall was thick enough to resist the waves.
GAlveston 1900 After the disaster, a seawall was built along the Gulf shore, and virtually the entire city was raised anywhere from six inches to 17 feet. A canal was dug behind the seawall. Dredges excavated sand from the bay, sailed in along the canal and pumped sand into low-lying areas. Buildings were raised on pilings and people got around on board walks looking, as one author put it, "like something from Doctor Seuss." It typically took several weeks to fill any given area.
GAlveston 1900 The design of the seawall was unusually well thought-out. Instead of attempting to resist waves by brute force, the seawall has a concave face to direct waves upward. A long gentle slope behind the wall allows for the inevitable wave overflow to dissipate itself and run back. The seawall has resisted several storms that survivors thought were actually worse than the 1900 storm.

Property Values and Erosion

As long as erosion is moderate, it's "location, location, location."

Once about half the lot is gone, prospective buyers realize that soon there won't be a location and the value plummets.

In the long run, nothing is as futile as trying to resist shoreline change. Change can be resisted for a while, but when the water wants something badly enough, it will come in and take it.

Freak Waves

Long rumored to exist by seamen, freak waves are a reality. They occur when waves of several frequencies superimpose. Typically, a ship encounters a steep trough often likened to a "hole in the ocean" followed by a steep crest. These events can occur quickly enough to sink a ship before any distress call can be sent. During World War II, a freak wave struck the ocean liner Queen Mary in the North Atlantic, rolling it on its side to within a hair of capsizing. Had it capsized, it would have been by far the worst marine disaster ever. There were 15,000 soldiers aboard bound for Europe - it would have been ten times worse than the Titanic disaster. Computer modeling indicates that freak waves up to almost 200 feet high can occur in the Gulf of Alaska.

Freak WAves Here, waves of three different frequencies combine to produce very high crests and very deep troughs (bottom)

What was the worst marine disaster ever?

Marine Erosion

Longshore and Beach Drift

Most Beach Sand Is Created by Weathering and Carried to Coasts by Rivers

Beach Sand Moves along the Coast by Longshore and Beach Drift



Types of Coast

Wave Refraction

When waves come ashore, they tend to bend or refract. This process concentrates energy on protrusions and spreads it out in bays, so that coastlines tend to become straighter.

Degree of Modification

Primary - Not Modified Much by Wave Action
Secondary - Highly Modified by Wave Action

History

Emergent - Land Rises or Water Level Falls
Submergent - Land Sinks or Water Level Rises

Dominant Process

Erosional
Depositional

Secondary Coasts Are Modified by Marine Erosion or Deposition

secondary coasts

End state in both situations is a regular coastline with gentle curves and cusps.

Erosion

Once the sea cliffs retreat a few hundred meters, wave energy is largely dissipated crossing the wave-cut terrace. Meanwhile, the cliffs become higher, meaning more material to move. Erosion rate becomes very slow.

Deposition

Primary Coastlines Are Very Common

Sea Level Has Risen 100-200m (300-600 Ft) since the Pleistocene

submerged river valleys Submerged river systems can still be recognized on the sea floor of Indonesia. This shallow sea bottom was dry land during the Pleistocene.

Other Ways Primary Coasts Can Form

Cape Hatteras The North Carolina coast exhibits both primary and secondary coastlines. The inner shore is a series of estuaries that resulted from flooding of river valleys during the post-Pleistocene rise in sea level. It is a primary coastline. The Outer Banks, the narrow offshore islands, are a secondary coastline created by wave action.

Reefs Are a Major Type of Coast in Tropical Areas

Many Volcanic Islands in the Pacific have sunk, producing a sequence of reef types

reefs

Reefs Also Form along the Edges of Large Islands and Continents

These Are Barrier Reefs

Deep Ocean Phenomena

Turbidity Flows

turbidity flow A powerful earthquake off Newfoundland in 1929 caused a submarine landslide on the edge of the continental shelf. Submarine cables in the slump area broke immediately but cables downslope broke up to several hours later. Apparently a dense current of suspended sediment traveled several hundred kilometers across the sea floor.

 


The Worst Marine Disaster Ever

The Titanic wasn't even the worst "natural" marine disaster ever, that is, without the ship being deliberately sunk (few marine disasters are wholly natural). The French cruiser Provence sank in the Mediterranean on February 26, 1916 with the loss of 3,100 lives. A collision between the Philippine ferry Dona Paz and the tanker Victor killed 4,341 on December 20, 1987. The worst sinking ever took place in the closing months of World War II. A German ship, the Wilhelm Gustloff, sailed from Danzig filled to overflowing with German refugees fleeing the advancing Russians. It was torpedoed by a Russian submarine in the Baltic. About 900 survivors were rescued and an estimated 8,000-9,000 people died.

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Created 21 May 1997, Last Update 4 May 2000

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