problems involving littoral processes would not occur. A knowledge of 

 incident wave conditions is essential for coastal engineering planning, 

 design, and construction. 



Three important aspects of a study of waves on beaches are (1) the 

 theoretical description of wave motion, (2) the climatological data for waves 

 as they occur on a given segment of coast, and (3) the description of how 

 waves interact with the shore to move sand. 



The theoretical description of water-wave motion is useful in under- 

 standing the effect of waves on sediment transport, but currently the 

 prediction of vave-induced sediment motion for engineering purposes relies 

 heavily on empirical coefficients and judgment, as veil as on theory. 



Statistical distributions of vvave characteristics along a given shoreline 

 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 placed in motion; breaker 

 direction 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 transport the 

 sediment once motion is initiated. 



b. Currents . Water waves induce an orbital motion in the fluid in which 

 they travel (see Ch. 2, Sec. 11,3). The orbits are not closed, and the fluid 

 experiences a slight wave-induced drift or mass tvansport' The action of mass 

 transport, extended over a long period, can be important in carrying sediment 

 onshore or offshore, particularly 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 and 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 passage 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 zone is the principal 

 cause of longshore currents — currents that flow parallel to the shoreline 

 within the surf zone. These longshore currents are largely responsible for 

 the longshore sediment transport. 



There is some mean exchange between the water flowing in the surf zone and 

 the water seaward of the breaker zone. The most easily seen of these exchange 

 mechanisms are rip currents (Shepard and Inman, 1950), which are concentrated 

 jets of water flowing seaward through the breaker zone. 



c. Tides and Surges . In addition to wave-induced currents, there are 

 other currents affecting the shore that are caused by tides and storm 

 surges. Tide-induced currents can be impressed upon the prevailing wave- 

 induced circulations, especially near entrances to bays and lagoons and in 

 regions of large tidal range. (Notices to Mariners and the Coastal Pilot 



4-4 



