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 the rip currents (Shepard and Inman, 1950), 

 which are concentrated jets of water flowing seaward through the breaker 

 zone. 



4.123 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 often carry this information.) Tidal currents are particularly 

 important in transporting sand in shoals and sand waves around entrances 

 to bays and estuaries. 



Currents induced by storm surges (Murray, 1970) are less well known 

 because of the difficulty in measuring them, but their effects are un- 

 doubtedly significant. 



The change in water level by tides and surges is a significant factor 

 in sediment transport, since, with a higher water level, waves can then 

 attack a greater range of elevations on the beach profile. (See Figure 

 1-7.) The appropriate theory for predicting storm surge levels is dis- 

 cussed in Section 3.8. 



4.124 Winds . Winds act directly on beaches by blowing sand off the 

 beaches (deflation) and by depositing sand in dunes. (Savage and Wood- 

 house, 1968.) Deflation usually removes the finer material, leaving 

 behind coarser sediment and shell fragments. Sand blown seaward from 

 the beach usually falls in the surf zone; thus it is not lost, but is 

 introduced into the littoral transport system. Sand blown landward from 

 the beach may form dunes, add to existing dunes, or be deposited in 

 lagoons behind barrier islands. 



For dunes to form, a significant quantity of sand must be available 

 for transport by wind, as must features that act to trap the moving sand. 

 Topographic irregularities, the dunes themselves, and vegetation are the 

 principal features that trap sand. 



The most important dunes in littoral processes are foredunes - the 

 line of dunes immediately landward of the beach. They usually form be- 

 cause beachgrasses growing just landward of the beach will trap sand blown 

 landward off the beach. Foredunes act as a barrier to prevent waves and 

 high water from moving inland, and provide a reservoir of sand to replen- 

 ish the nearshore regime during severe shore erosion. 



The effect of winds in producing currents on the water surface is 

 well documented, both in the laboratory and in the field. (Bretschneider, 



4-5 



