Figure 1-5. Developed barrier island, Atlantic City, New Jersey. 



IV. DYNAMIC BEACH RESPONSE TO THE SEA 



The beach constantly adjusts its profile to provide the most efficient 

 means of dissipating incoming wave energy. This adjustment is the beach's 

 natural dynamic response to the sea. Although an equilibrium is sometimes 

 reached between the beach and the sea, the "peace" is short-lived and the 

 "battle" soon begins anew. 



There are two general types of dynamic beach response to wave motion: 

 response to normal conditions and response to storm conditions. Normal con- 

 ditions prevail most of the time, and the wave energy is easily dissipated by 

 the beach's natural defense mechanisms. However, when storm conditions gener- 

 ate waves containing increased amounts of energy, the coast must respond with 

 extraordinary measures, such as sacrificing large sections of beach and 

 dune. In time the beach may recover, but often not without a permanent loss. 



1. Normal Beach Response . 



As a wave moves toward shore, it encounters the first beach defense in the 

 form of the sloping nearshore bottom. When the wave reaches a water depth 

 equal to about 1.3 times the wave height, the wave collapses or breaks. Thus 

 a wave 0.9 meter (3 feet) high will break in a depth of about 1.2 meters (4 

 feet). Breakers are classified as four types — plunging, spilling, surging, or 

 collapsing. The form of breakers is controlled by wave steepness and 

 nearshore bottom slope. Breaking results in a dissipation of wave energy by 

 the generation of turbulence in the water and by the transport of sediment 

 lifted off the bottom and tossed around by the turbulent water. Broken waves 

 often re-form to break again, losing additional energy. Finally, the water 

 travels forward as a foaming, turbulent mass and expends most of its remaining 

 energy in a rush up the beach slope. If there is an increase in the incoming 



1-9 



