transport direction can vary from season to season, day to day or hour 

 to hour. These reversals of transport direction are quite common for 

 most United States shores. Direction may vary at random, but in most 

 areas the net effect is seasonal. 



The rate of longshore transport is dependent on both angle of wave 

 approach, and wave energy. Thus, high storm waves will generally move 

 more material per unit time than low waves. However, if low waves 

 exist for a much longer time than do high waves, the low waves may be 

 more significant in moving sand than the high waves. 



Because reversals in transport direction occur, and because different 

 types of waves transport material at different rates, two components of 

 the longshore transport rate become important. The first is the net rate, 

 the net amount of material passing a particular point in the predominant 

 direction during an average year. The second component is the gross rate, 

 the total of all material moving past a given point in a year regardless 

 of direction. Most shores consistently have a net annual longshore trans- 

 port in one direction. Determining the direction and average net and 

 gross annual amount of longshore transport is important in developing 

 shore protection plans. 



In landlocked water of limited extent, such as the Great Lakes, a 

 longshore transport rate in one direction can normally be expected to be 

 no more than about 150,000 cubic yards per year. For open ocean coasts, 

 the net rate of transport may vary from 100,000 to more than 2 million 

 cubic yards per year. The rate depends on the local shore conditions 

 and shore alignment as well as the energy and direction of wave action. 



1.46 EFFECT OF INLETS ON BARRIER BEACHES 



Inlets may have significant effects on adjacent shores by interrupt- 

 ing the longshore transport and trapping onshore- offshore moving sand. 

 On ebb current, sand moved to the inlet by waves is carried a short dis- 

 tance out to sea and deposited on an outer bar. When this bar becomes 

 large enough, the waves begin to break on it, and sand again begins to 

 move over the bar back toward the beach. On the flood tide, when water 

 flows through the inlet into the lagoon, sand in the inlet is carried a 

 short distance into the lagoon and deposited. This process creates shoals 

 in the landward end of the inlet known as middlegroimd shoals or inner 

 bars. Later, ebb flows may bring some of the material in these shoals 

 back to the ocean, but some is always lost from the stream of littoral 

 drift and thus from the downdrift beaches. In this way, tidal inlets 

 may store sand and reduce the supply of sand to adjacent shorelines. 



1.47 IMPACT OF STOM/IS 



Hurricanes or severe storms moving over the ocean near the shore may 

 greatly change beaches. Strong winds of a storm often create a storm surge. 

 This surge raises the water level and exposes to wave attack higher parts 

 of the beach not ordinarily vulnerable to waves. Such storms also generate 



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