36 



OCEANOGRAPHY: 



longshore current 



Figure 3.7 Shore currents. Oblique movements of waves toward the 

 shore and downslope movements of undertow water contribute to a 

 longshore current and movement of water-carried particles with the long- 

 shore current. 



are partly deposited along portions of the coast that 

 are in a cycle of shore deposition. Some of the de- 

 posits end as ocean sediments. The second source of 

 deposits is storm waves. These waves have more 

 energy than the normal ones striking a given area. 

 Therefore, storm waves frequently remove finer ma- 

 terials already on the shore, but deposit coarser rocks 

 and boulders. 



Any shore deposit tends to be sorted according to 

 size. The largest rocks are found farthest above the 

 water and there is a gradation of rock size downward 

 to ever smaller particles into the ocean. Any incom- 

 ing phase of surf has more energy, hence more carry- 

 ing capacity, than any phase of undertow. Therefore, 

 receding waters progressively lose load-carrying 

 capacity and deposit ever finer particles as they re- 

 turn into the sea. However, local characteristics of 

 oceans and shores cause individual beaches to be 

 essentially sandy, pebbly, or rocky. 



The subject of waves and wave action has barely 

 been outlined here. Later, the discussion in this 

 chapter will be used as a basis for further considera- 

 tion of the effects of waves upon the land. 



RIP CURRENTS 



Rip currents are formed ofTopen coasts with sandy 

 beaches. An open coast is one having no ofTshore 

 reefs, islands, kelp, or other features to interrupt the 

 full force of incoming waves. Therefore, on open 

 coasts strong waves strike the shore and strong under- 

 tows move seaward through the incoming surf. 



Rip currents are "the dangerous undertows" found 

 along certain sandy beaches. However, they are not 

 true undertows. They are related phenomena that 

 also involve the outward movement of water. For 

 analogy, one might consider a rip current as being a 

 collection of individual undertows that are organized 

 into feeder currents, a neck channel, and a head 

 region (Figure 3.8). The rip current is formed and 

 maintained by the feeder currents that converge from 

 up and down the shore into a narrow neck channel 

 and then spread into the head via eddy currents. In 

 the head, currents move straight seaward, counter- 

 clockwise to the left, and clockwise to the right. 



These fast currents (up to 2 miles per hour) pro- 

 duce marked effects upon the shore. The most obvi- 

 ous effects are due to the amount of force they pro- 

 duce. For example, the force develops the neck chan- 

 nel by easily removing the loose sand of the beach. 



shorefme ■ 



feeder 



Figure 3.8 Rip current. Diogrom of the structure of concentrated 

 water retreating from a shoreline os a rip current. 



