sediment size, and beach slope. In general, high steep waves move material 



offshore, and low waves of long period (low steepness waves) move material 



onshore. The onshore-offshore process associated with storm waves is 

 illustrated in Figure 1-8. 



Longshore transport results from the stirring up of sediment by the break- 

 ing wave and the movement of this sediment by both the component of the wave 

 energy in an alongshore direction and the longshore current generated by the 

 breaking wave. The direction of longshore transport is directly related to 

 the direction of wave approach and the angle of the wave (crest) to the 

 shore. Thus, due to the variability of wave approach, longshore transport 

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

 Reversals of transport direction are quite common for most U.S. coasts. 

 Direction may vary at random, but in most areas the net effect is seasonal. 



The rate of longshore transport is dependent on the angle of wave approach, 

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

 material per unit time than that moved by low waves. However, if low waves 

 exist longer than 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 transport in one direc- 

 tion. Determining the direction and average net and gross annual amount of 

 longshore transport is important in developing shore protection plans. In 

 inland seas, such as the Great Lakes, a longshore transport rate in one direc- 

 tion can normally be expected to be no more than about 115, UUU cubic meters 

 (150,000 cubic yards) per year. For open ocean coasts, the net rate of trans- 

 port may vary from 75,000 to more than 1.5 million cubic meters (100,000 to 2 

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

 and shore alinement, as well as the energy and direction of wave approach. 



5. Effect of Inlets on Barrier Beaches . 



Inlets may have significant effects on adjacent shores by interrupting the 

 longshore transport and trapping onshore-offshore moving sand. During ebb- 

 tide, sand transported to the inlet by waves is carried seaward a short dis- 

 tance and deposited on an outer bar. When this bar becomes large enough, 

 the waves begin to break on it, moving the sand over the bar back toward the 

 beach. During floodtide, 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 middle- 

 ground shoals or inner bars. Later, ebb flows may return some of the material 

 in these shoals to the ocean, but some is always lost from the littoral system 

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

 and reduce the supply of sand to adjacent shores. Estimates of the amount of 

 material deposited in the middleground shoals range from 100,000 to 160,000 

 cubic meters (130,000 to 210,000 cubic yards) per year for inlets on the east 



1-14 



