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Wave Erosion. As wind moves over the ocean, it produces waves and currents that erode the bordering land. Waves can also be produced by underwater earthquakes and the effect of the moon in producing tides.
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As wind moves over the ocean, it produces waves and currents that erode the bordering land. • Waves can also be produced by underwater earthquakes and the effect of the moon in producing tides.
Water is not carried along with the wave. Each particle moves in place in a circular motion. • This motion occurs below the surface as well, in smaller and smaller circles. • This motion ceases at a depth equal to one half of the waves wavelength.
Wave height is the distance from the crest of a wave to its trough.
When waves enter shallow water near the shore, they drag against the bottom. • The waves then become unstable and water rushes toward the shore as breaking waves or surf. • The wavelength is the distance from wave crest to wave crest. • On average the wavelength for ocean waves is 20-30 times its height.
The height of a wind created wave depends on the length of time the wind blows and the Fetch. • Fetch is the length of open water over which the wind blows. • Normal winds rarely produce ocean waves higher than 15m.
Strong winds create waves with a long wavelength. • Choppy seas are created when the wind is gusty and changes direction.
Spilling Breakers • Spilling breakers are a familiar sight on most beaches. • They arise from long waves breaking on gently sloping beaches. • There are several rows of breakers. • Photos Van Dorn, 1974
Plunging Breakers • Plunging breakers can occur on steeply sloping beaches. • There is only one row of breakers. • Photos Van Dorn, 1974
Surging Breakers • Surging breakers surge over steeply sloping (but not vertical) beaches or rocks. • Waves break one at a time. • Photos Van Dorn, 1974
Long-Shore Current: • The direction that a wave approaches the shore determines how it will move sand grains. • Most waves approach the beach at a slight angle and retreat in a direction that is more perpendicular to the shore. • As a result, individual sand grains are moved by waves in a zig-zag motion. • The general movement of sand along the beach is in the direction in which the waves strike the shore.
Waves moving at an angle to the shoreline push water along the shore creating a longshore current. • A longshore current is a movement of water parallel to and near the shoreline. • Longshore currents transport sand in a direction parallel to the shoreline.
Sometimes it is scary Two hours later !
Wave movement toward the shore often builds up a strip of sediment at the coastline called a beach. • Beaches are composed of whatever sediment is available, but most beaches are composed of sand.
Sand moving along a relatively straight shore keeps moving until the shoreline changes direction. This happens at bays and headlands. Here the longshore current slows and sand is deposited at the near end of the headland. A long narrow deposit of sand connected at one end to the shore is called a spit.
During storms waves can be much more powerful than normal. • During these times the beach tends to erode more and the sand is carried back into the ocean. • There it is usually deposited as underwater bars parallel to the shore. • If these sand bars rise above the average sea level winds will help to pile up sediment. • When vegetation begins to grow and stabilize the offshore sediment pile, a barrier island is created.
When a shoreline has a natural or man-made projection out into the ocean sand is deposited on the side of the projection that faces the longshore current. • The side of the projection facing away from the longshore current usually get eroded because it is not protected by sand deposits.
