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Refaction/Defraction at Coastal Structures - Essay Example

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Water waves are created as a result of application of an external force, say a gust wind, working against the gravity and surface tension. These waves exhibit various phenomena like reflection, refraction, diffraction and shoaling. Phenomena like refraction and shoaling occur in shallow waters (Komar, P)…
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Refaction/Defraction at Coastal Structures
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--------------------------- --------------------------- --------------------------- --------------------------- Refaction/Defraction at Coastal Structures
Water waves are created as a result of application of an external force, say a gust wind, working against the gravity and surface tension. These waves exhibit various phenomena like reflection, refraction, diffraction and shoaling. Phenomena like refraction and shoaling occur in shallow waters (Komar, P). Diffraction takes place when the waves encounter an obstacle or protrusion; say an island, in its path. In the subsequent paragraphs, we will elaborate on refraction and diffraction of water waves.
Wave Parameters and Wave Propagation
As much like electromagnetic or optical waves, water waves can be described by the basic parameters - height, length and period. In addition to the above parameters depth of water over which the wave travels also assumes importance (Dean, R.G. and Dalrymple, R.A). Distance between two consecutive crests is called the wave length while the time that elapses for two crests to pass a point is the period of the wave. These waves are associated with energy which is proportional to square of its height. A traveling wave carries this energy efficiently and the speed of energy travel is related to the wave speed, given by the product of wave length and reciprocal of wave period. As the wave approaches the shore or coastal line, it encounters change in water depth, that is, the wave enters shallow waters. The water is said to be shallow when one half of the wave length exceeds the depth of water.
Refraction is the process by which the wave, while propagating in shallow water at an angle to the physical topographical contours, changes its direction so as to align to the contours (Wallingford, H.R.). As the wave propagates towards the shore, the depth of the water continuously decreases. As a result, the wave experiences varied depth. The speed of the wave is faster in deeper water. Thus, the parts of the wave travel at varying speeds. Part of the wave in the shallowest water is the slowest while the part in deepest water is the fastest. Thus, the wave bends with the variation of depth beneath the crest. Imagine a wave approaching a coast line at an angle. The wave crests nearer to the coast will slow down in comparison to the ones which are away. Thus the lagging wave crests will travel faster and catch up with the ones which are ahead. Thus, the wave crests will tend to become parallel to the coastline. Similarly, while approaching an island from one side, the crest will tend to wrap around the island reaching the beach close to parallel (Bascom, W.).
Wave diffraction is the process in which wave energy is transferred in lateral direction, perpendicular to the actual direction of propagation, from points of greater to lesser wave height along the crest (Mei, Chiag C). This happens whenever the waves encounter discontinuities in its field of travel. These discontinuities may be a surface-piercing obstacle, such as a breakwater or an island. To illustrate, consider a breakwater in the path of a water wave, propagating in the perpendicular direction to the breakwater. It may appear that the shadow region will remain disturbance free. However the wave disturbance gets transferred to shadow region also, by means of diffracting through finite length sheltered region (Dean, R.G. and Dalrymple, R.A).
Refraction and diffraction are the two phenomena involving change in wave direction. The latter effect is due to variations in depth of the water while the former is caused by the wave's interaction with the discontinuities like, breakwater or island.
Bascom, W. "Waves and beaches: the dynamics of the ocean surface." Garden City, NY: Doubleday & Co., Inc., 1964.
Dean, R.G. and Dalrymple, R.A., "Water wave mechanics for engineers and scientists." Singapore: World Scientific, 1991.
Mei, Chiag C. "The applied dynamics of ocean surface waves." United States of America: John Wiley & Sons, Inc, 1983.
Komar, P. "Beach processes and sedimentation." Englewood Cliffs, NJ: Prentice-Hall, Inc., 1976.
Wallingford, H.R. "A guide to managing coastal erosion in beach/dune system." Oct. 2000. Scottish Natural Heritage. 13 Feb. 2007 . Read More
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