About 20 percent of the water energy goes into surface drift currents 

 and the rest into surface waves. Ordinary wind-driven waves are 

 classified as capillary waves if surface tension controls the forma- 

 tion, or gravity waves if gravity controls wave structure. 



The shape of deepwater waves depends to large extent on their 

 length. Gravity waves that are longer than about 1.73 cm are controlled 

 by gravitational and inertial forces. Wave shapes range from a nearly 

 sinusoidal form to a sharply peaked crest and rounded trough which can 

 be approximated by a trochoid (fig. 6-1). A trochoid is the curve 

 generated by a point on the radius of a circle as the circle rolls 

 uniformly on a line. Water particles execute small circular orbits in 

 response to the passing wave, the size of the orbits diminishing with 

 increasing depth below surface (fig. 6-2). Surface waves shorter than 

 1.73 cm in length are generally controlled by forces associated with 

 surface tension, causing the short capillary waves to have rounded 

 crests and v-shaped troughs. Figure 6-3 shows the shape of capillary 

 waves in terms of the amplitude-to-wavelength parameter C^/K) . 



Ocean waves are dispersive, since wave speed depends on wave- 

 length or frequency (fig. 6-4). Gravity leaves show normal dispersion, 

 since the wave speed increases with increase in wavelength. Capillary 

 waves show anomalous dispersion, since their wave speed increases with 

 decreasing wavelength. The minimum wave speed for combined gravity 

 and capillary waves is 23 cm/s, corresponding to a wavelength of 1.73 cm. 

 The wave frequency co , is related to wave number k, for gravity waves 

 through the expression: w -gk where g is the gravitational constant. 

 For capillary waves the relationship is : w = r'k-'/p, where: p is the 

 water density and r is the air-water surface tension coefficient, 

 equal to 73 dyn/cm at 20°C. 



Wind effects on the sea surface are not simple, because turbu- 

 lence and viscosity in both sea and air introduce complex nonlinear 

 effects. Wind turbulence creates a moving pattern of minute fluctuations 

 in air pressure over water, which can generate the initial tiny ripples 

 that eventually become fully developed waves. It is thought that much 

 of the grip that wind has on water to produce waves and currents is 

 provided through those very small wavelets, since they are very numerous 

 and move slowly before the wind. Gravity waves speed up with growth 

 to the point where they can eventually move at the same speed as the 

 wind and can extract no further energy from it. Swell is the name 

 given to those gravity waves that move away from the generating area 

 faster than the wind. 



Ordinary gravity waves normally observed at sea are composed of 

 two varieties: swell, which is a long and relatively symmetrical wave, 

 having a period in the order of 10 seconds, produced by wind and 

 storms at some distance from the point of observation, and sea, which 

 is the local wind-generated wave motion, usually of shorter period, 

 with unsymmetrical slopes and steep or white-capped crests depending 

 upon the wind speed and fetch. Fetch is the length of sea surface over 

 which the wind blows without appreciable change of direction. 



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