Refraction/Diffraction at Coastal Structures - Essay Example

Wave Parameters and Wave PropagationAs 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 wavelength 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 the 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 wavelength and reciprocal of wave period. As the wave approaches the shore or coastal line, it encounters a change in water depth, that is, the wave enters shallow waters. The water is said to be shallow when one-half of the wavelength 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 the deepest water is the fastest. Thus, the wave bends with the variation of depth beneath the crest. Imagine a wave approaching a coastline 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.). DiffractionWave diffraction is the process in which wave energy is transferred in the lateral direction, perpendicular to the actual direction of propagation, from points of greater to lesser wave height along the crest (Mei, Chiang C).

This happens whenever the waves encounter discontinuities in their 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 the shadow region also, by means of diffracting through a finite length sheltered region (Dean, R.G. and Dalrymple, R.A). ConclusionRefraction and diffraction are the two phenomena involving a 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.