1.23 BARRIER BEACHES, LAGOONS AND INLETS 



In some areas, an additional natural protection for the mainland is 

 provided in the form of barrier beaches. (See Figure 1-4.) Nearly all 

 of the U.S. east coast from Long Island to Mexico is comprised of bar- 

 rier beaches. These are long narrow islands or spits lying parallel to 

 the shoreline. Barrier beaches generally enclose shallow lagoons that 

 separate the mainland from the ocean. During severe storms these barrier 

 beaches absorb the brunt of the wave attack. When barrier-beach dunes 

 are breached, the result may be the cutting of an inlet. The inlet per- 

 mits sand to enter the lagoon and settle to the bottom, removing sand 

 from the beach. 



1.24 STORM ATTACK 



Diiring storms, strong winds generate high waves. Storm surge and 

 waves may raise the water level near the shore. If storm surge does 

 occur, large waves can then pass over the offshore bar formation without 

 breaking. If the storm occurs at high tide, storm surge super-elevates 

 the water, and some waves may break on the beach or even at the base of 

 the dunes. After a storm or storm season, natural defenses may again be 

 re-formed by normal wave and wind action. 



lo25 ORIGIN AND MOVEMENT OF BEACH SANDS 



Most of the sands of the beaches and nearshore slopes are normally 

 small, resistant rock particles that have traveled many miles from in- 

 land mountains. When the sand reaches the shore, it is moved alongshore 

 by waves and littoral currents. This alongshore transport is a constant 

 process, and great volumes may be transported. In most coastal segments 

 the direction of movement changes as direction of wave attack changes. 



1.3 THE SEA IN MOTION 



1.31 TIDES AND WINDS 



The motions of the sea originate in the gravitational effects of the 

 sun, the moon, and earth; and from air movements or winds caused by dif- 

 ferential heating of the earth. 



The moon, and to a lesser extent the sun, creates ocean tides by 

 gravitational forces. These forces of attraction, and the fact that the 

 sun, moon, and earth are always in motion with relation to each other, 

 cause waters of ocean basins to be set in motion. These tidal motions of 

 water masses are a form of very long period wave motion, resulting in a 

 rise and fall of the water surface at a point. There are normally two 

 tides per day, but some localities have only one per day. 



1.32 WAVES 



The familiar waves of the ocean 'are wind waves generated by winds 

 blowing over water. They may vary in size from ripples on a pond to 



1-5 



