Waves and Shorelines

I. Waves

• Waves are caused by the frictional drag of wind over the water -

• Waves of oscillation - water particles move in near-circular orbits which decrease in diameter with depth.

• Waves of translation - water particles actually move forward.

• Description of waves -

• Wave crest - The highest part of a wave.

• Wave trough - The lowest part of a wave.

• Wave height - vertical distance between the trough and the crest of a wave.

• Wavelength - horizontal distance between the successive crests of a wave.

Source for Diagram: http://www.geology.uiowa.edu/~12_003/eh&r98/webversion/shorelines/sld004.htm

• Wave period - time interval between the passage of successive crests at a stationary point.

• Wave base - maximum depth to which waves move the water; equals about half of the wave length.

Source for Diagram: http://www.geology.uiowa.edu/~12_003/eh&r98/webversion/shorelines/sld006.htm

II. Effect of approaching shoreline on waves

• Moving water particles in a wave of oscillation cannot move up and down when they run into the bottom. This interference of the bottom with the moving water particles forces the wave to slow down.

• Slowing of the wave causes shortening of the wavelength and a corresponding increase in the wave height.

• Finally, as the water continues to shallow, the upper part of the wave runs away from the lower part. The wave breaks, turns into a wave of translation and washes up onto the shore.

Source for Diagram: http://www.geology.uiowa.edu/~12_003/eh&r98/webversion/shorelines/sld008.htm

III. Wave refraction around headlands

• Waves reach shallow water sooner in front of headlands.

• These waves slow down and the wavelength shortens in front of the headlands, but waves in the bays continue at the same speed and wavelength because they have not yet come into the shallow water.

• Result is that the waves bend (refract) around the headlands

Source for Diagram: http://www.geology.uiowa.edu/~12_003/eh&r98/webversion/shorelines/sld013.htm

IV. Longshore currents and beach drift

• Oscillation waves that are breaking at an angle to the shoreline produce translation waves that move diagonally up and down the beach. This diagonal swash and backwash on the beach creates a longshore current just offshore that moves parallel to the beach and also produces beach drift, which is transport of sand along the beach.

Source for Diagram: http://www.geology.uiowa.edu/~12_003/eh&r98/webversion/shorelines/sld010.htm

• When two longshore currents converge, a rip current is formed. Swim parallel to the shore to escape a rip current.

Source for Diagram: http://www.lifeguard-csla.org/surfbear/rips.htm

V. Wave erosion

• Waves erode and transport materials just like running water in a river does; dissolved, suspended, and bed loads are all transported in the surf zone.

• Clay and silt in the suspended load are washed out to sea and are deposited offshore in the deeper parts of the ocean.

• Sand and gravel in the bed load are transported down the beach in the direction of the beach drift.

• Wave erosion in the surf zone is by the process of abrasion, which is most active along headlands where wave energy is concentrated.

• Features produced by wave erosion -

• Wavecut cliff - A steep cliff along the shoreline.

• Wavecut platform - A flat, nearly horizontal surface eroded by the waves at the low tide level.

• Sea stack - An isolated, resistant rock island just offshore from the cliff or beach.

• Sea arch - A tunnel eroded by the waves through an otherwise resistant rock mass.

SEE HISTORY OF A SEA STACK

VI. Wave deposition

• When the velocity of the wave currents or the longshore currents decreases, the transported material will be deposited.

• Wave deposition is most active in bays where the wave energy is dissipated.

• Features produced by wave or longshore current deposition -

• Beach - A sand deposit along the shoreline.

• Baymouth bar - A linear sand deposit that is continuous across the mouth of a bay.

• Spit - A linear sand deposit that is connected only at one end to the shore.

Source for Diagram: http://www.geology.uiowa.edu/~12_003/eh&r98/webversion/shorelines/sld017.htm

• Tombolo - A sand deposit that connects a stack or island to the shore.

• Barrier island - A sand deposit that is offshore and parallel to the shore.

Source for Diagram: http://www.cnmoc.navy.mil/educate/neptune/quest/ports/beaches.htm

VII. Development of irregular coastlines

• Wave energy is concentrated on headlands due to wave refraction; erosion is maximum.

• Longshore currents and beach drift diverge from the headlands due to wave refraction.

• Wave energy is dispersed in the bays; deposition is maximum.

• Headland cliffs are cut back by wave erosion and the bays are filled with sand deposits until the coastline becomes straight.

VIII. Implications for man

• Beaches will erode away when their sand supply is cut off by a dam on a river and sand no longer enters the beach drift.

• Construction of a breakwater protects the beach from erosion and allows sand to deposit in the protected harbor.

• Construction of jetties will allow sand to deposit on the updrift side and cause beach erosion on the downdrift side.

• Trying to stop wave erosion of sea cliffs and shoreline features is economically unfeasible.

IX. Possible essay questions

• Define and illustrate the following terms: wave crest, wave trough, wave height, wavelength, and wave base.

• Describe and illustrate what happens to a wave as it approaches a shoreline.

• Illustrate and explain why it is that waves refract (bend) around headlands.

• Describe and illustrate the origin of longshore currents and beach drift.

• Describe and illustrate the origin of a wavecut cliff.

• Describe and illustrate the origin of a wavecut platform.

• Describe and illustrate the origin of a baymouth bar.

• Describe and illustrate the origin of a spit.