which are difficult to describe in form or motion, and which may be com- 

 prised of several components are termed complex waves. Sinusoidal or 

 simple harmonic waves are examples of simple waves since their surface 

 profile can be described by a single sine or cosine function. A wave is 

 periodic if its motion and surface profile recur in equal intervals of 

 time. A wave form which moves relative to a fluid is called a progressive 

 wave; the direction in which it moves is termed the direction of wave 

 propagation. If a wave form merely moves up and down at a fixed position, 

 it is called a complete standing wave or a clapotis. A progressive wave 

 is said to be a wave of permanent form if it is propagated without experi- 

 encing any changes in free surface configuration. 



Water waves are considered osoiltatory or nearly oscillatory if the 

 water particle motion is described by orbits, which are closed or nearly- 

 closed for each wave period. 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 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 wave 

 length), or by the reciprocal of T, the wave frequency f. One illustra- 

 tion of classification by period or frequency is given by Kinsman (1965) 

 and shown in Figure 2-1. The figure shows the relative amount of energy 

 contained in ocean waves having a particular frequency. Of primary concern 

 are those waves referred to as gravity waves in Figure 2-1, having 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; that is, the force due to gravity attempts to 

 bring the fluid back 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 confused 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. 



2-4 



