Water waves are considered oscillatory or nearly oeaillatory if the water 

 particle motion is described by orbits that are closed or nearly closed for 

 each wave period. The linear, or Airy, theory describes pure oscillatory 

 waves. Most finite-amplitude wave theories describe nearly oscillatory waves 

 since the fluid is moved a small amount in the direction of wave advance by 

 each successive wave. This motion is termed mass transport of the waves. 

 When water particles advance with the wave and do not return to their original 

 position, the wave is called a wave of translation, A solitary wave is an 

 example of a wave of translation. 



It is important to distinguish between the various types of water waves 

 that may be generated and propagated. One way to classify waves is by wave 

 period T (the time for a wave to travel a distance of one wavelength) , or by 

 the reciprocal of T, the wave frequency f. Figure 2-1 is an illustration 

 of classification by period or frequency given by Kinsman (1965). The figure 

 shows the relative amount of energy contained in ocean waves having a partic- 

 ular frequency. Of primary concern are the waves referred to in Figure 2-1 as 

 gravity waves, which have periods from 1 to 30 seconds. A narrower range of 

 wave periods, from 5 to 15 seconds, is usually more important in coastal 

 engineering problems. Waves in this range are referred to as gravity waves 

 since gravity is the principal restoring force; i.e., the force due to gravity 

 attempts to return the fluid to its equilibrium position. Figure 2-1 also 

 shows that a large amount of the total wave energy is associated with waves 

 classified as gravity waves; hence, gravity waves are extremely important in 

 dealing with the design of coastal and offshore structures. 



Gravity waves can be further separated into two states: 



(a) Seas, when the waves are under the influence of wind in a 

 generating area, and 



(b) swell, when the waves move out of the generating area and 

 are no longer subjected to significant wind action. 



Seas are usually made up of steeper waves with shorter periods and 

 lengths, and the surface appears much more disturbed than for swell. Swell 

 behaves much like a free wave (i.e., free from the disturbing force that 

 caused it), while seas consist to some extent of forced waves (i.e., waves on 

 which the disturbing force is applied continuously) . 



Ocean waves are complex. Many aspects of the fluid mechanics necessary 

 for a complete discussion have only a minor influence on solving most coastal 

 engineering problems. Thus, a simplified theory that omits most of the com- 

 plicating factors is useful. The assumptions made in developing the simple 

 theory should be understood, because not all the assumptions are justified in 

 all problems. When an assumption is not valid in a particular problem, a more 

 complete theory should be employed. 



The most restrictive of common assumptions is that waves are small pertur- 

 bations on the surface of a fluid which is otherwise at rest. This leads to a 

 wave theory that is variously called small-amplitude theory, linear theory, or 

 Airy theory. The small-amplitude theory provides insight for all periodic 

 wave behavior and a description of the periodic flow that is adequate for most 

 practical problems. This theory cannot account for mass transport due to 



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