problems involving littoral processes would not occur. A knowledge of 

 incident waves, or of surf, is essential for coastal engineering planning, 

 design, and construction. 



Three important aspects of a study of waves on beaches are: the 

 theoretical description of wave motion; the climatological data for waves 

 as they occur on a given segment of coast; and the description of how 

 waves interact with the shore to move sand. 



The theoretical approach can provide a useful description of water 

 motion caused by waves when the limiting assumptions of the theory are 

 satisfied. Surprisingly, the small -amplitude theory (Section 2.23) and 

 aspects of solitary wave theory (Section 2.27) have proved useful beyond 

 the limits assumed in their derivations. 



The theoretical description of water-wave motion provides estimates 

 of water motion, longshore force, and energy flux due to waves. These 

 estimates are useful in understanding the effect of waves on sediment 

 transport, but currently (1973) the prediction of wave-induced sediment 

 motion for engineering purposes relies heavily on empirical coefficients 

 and judgement rather than on theory. 



Statistical distributions of wave characteristics along a given shore- 

 line provide a basis for describing the wave climate of a coastal segment. 

 Important wave characteristics affecting sediment transport near the beach 

 are height, period, and direction of breaking waves. Breaker height is 

 significant in determining the quantity of sand in motion; breaker direc- 

 tion is a major factor in determining longshore transport direction and 

 rate. Waves affect sediment motion in the littoral zone in two ways: 

 they initiate sediment movement, and they drive current systems that trans- 

 port the sediment once motion is initiated. 



4.122 Currents . Water waves induce an orbital motion in the fluid beneath 

 them. (See Section 2.23.) These are not closed orbits, and the fluid 

 experiences a slight wave-induced drift, or mass transport. Magnitude and 

 direction of mass transport are functions of elevation above bottom and 

 wave parameters (Equation 2-55), and are also influenced by wind and tem- 

 perature gradients. The action of mass transport, extended over a long 

 period, can be important in carrying sediment onshore or offshore, par- 

 ticularly seaward of the breaker position. 



As waves approach breaking, wave-induced bottom motion in the water 

 becomes more intense, and its effect on sediment becomes more pronounced. 

 Breaking waves create intense local currents (turbulence) that move sedi- 

 ment. As waves cross the surf zone after breaking, the accompanying fluid 

 motion is mostly uniform horizontal motion, except during the brief pass- 

 age of the breaker front where significant turbulence occurs. Since wave 

 crests at breaking are usually at a slight angle to the shoreline, there 

 is usually a longshore component of momentum in the fluid composing the 

 breaking waves. This longshore component of momentum entering the surf 



4-4 



