The speed of wave propagation is primarily a function of wave period 

 in deep water (where the wave speed in feet per second is approximately 

 5 times the wave period in seconds : C = 5 . 12T) . As waves move into 

 water depths shallower than one-half of the wavelength, the propagation 

 speed decreases with decreasing depth. The period of an individual wave 

 remains constant as the wave approaches the beach. Therefore, the 

 reduction in speed leads to a reduction in wavelength; the energy per 

 unit length of wave crest remains reasonably constant for a given wave. 

 As the wavelength is reduced in shallow water, the energy density is 

 increased and the wave height increases. This process is known as 

 shoati-ng . 



5. Longshore Current Generation . 



If water depth varies along a wave crest (usually the case when a wave 

 approaches the shoreline at an angle) that part of the wave in deeper 

 water will travel faster. The result of this process, kno^^/n as refraction, 

 is that the wave front will aline itself so that it is nearly parallel to 

 the bottom contours. If bottom contours are parallel to the shore, 

 refraction will cause a decrease in wave energy density and a decrease in 

 wave height. The combination of refraction and shoaling can lead to 

 either a decrease or increase of wave height near a shore. In a shoal 

 area, with deeper water on both sides, wave crests will be bent to cause 

 a convergence of energy over the shoal. The net result of shoaling and 

 refraction will increase the wave height in the shallow region and 

 decrease the wave height in the deeper water between shoal areas. As 

 waves move closer to shore, the effect of shoaling in increasing wave 

 height usually overrides any effect of refraction in decreasing wave 

 height. Therefore, waves eventually reach a point where the fluid motion 

 under the crests becomes unstable, and then the waves break. During 

 breaking, a part of the wave energy is dissipated, and if the waves break 

 at an angle with the shoreline, a part of the wave energy generates a 

 current parallel with the shoreline. This is termed a longshore current. 

 The speed of the current is determined by both the direction and height 

 of the waves. Thus, both direction and speed of the longshore currents 

 are highly variable. Quantitative mathematical and empirical discussions 

 of the generation of longshore currents by waves are given by Bowen (1969) 

 and Longuet-Higgins (1970). In the Great Lakes, waves are generally small 

 between storms and longshore currents will be small. During storms when 

 waves near shore may be 10 to 12 feet in height, the magnitude of the 

 longshore current becomes significant. Theory and empirical data both 

 indicate that wave- induced longshore currents provide the primary motive 

 forces in the swash and surf zones where maximum sand transport occurs. 

 In comparison, currents generated in the deeper pa:rts of a lake are 

 relatively insignificant. 



6. Water Motions Generated by Sharp Changes in Atmospheric Pressure . 



Sharp changes in atmospheric pressure associated with squall lines 

 occasionally produce long wave disturbances which can be regarded as 



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