3. Sinks for Littoral Materials . 



a. Inlets and Lagoons . Barrier islands are interrupted locally by inlets 

 which may be kept open by tidal flow. A part of the sediment moved alongshore 

 by vave action is moved into these inlets by tidal flow. Once inside the 

 inlet, the sediment may deposit where it cannot be moved seaward by the ebb 

 flow (Brown, 1928) . The middleground shoals common to many inlets are such 

 depositional features. Such deposition may be reduced when the ebb currents 

 are stronger than the flood currents (Johnson, 1956) . Also, particularly 

 during times of strong ebb tidal flow, sand is jetted sufficiently far 

 offshore to be deposited outside the control volume and removed from the 

 littoral zone. 



It is evident from aerial photography (e.g., of Drum Inlet, N.C., Fig. 

 4-45) that inlets do trap significant quantities of sand. Caldwell's (1966) 

 estimate of the sand budget for New Jersey calculates that 23 percent of the 

 local gross longshore transport is trapped by the seven inlets in southern New 

 Jersey, or about 191,000 cubic meters (250,000 cubic yards) per year for each 

 inlet. In a study of the south shore of Long Island, McCormick (1971) 

 estimated from the growth of the floodtide delta of Shinnecock Inlet (shown by 

 aerial photos taken in 1955 and 1969) that this inlet trapped 16,000 cubic 

 meters (60,000 cubic yards) per year. This amounts to about 20 percent of the 

 net longshore transport (Taney, 1961, p. 46) and probably less than 10 percent 

 of the gross transport (Shinnecock Inlet is a relatively small inlet). It 

 appears that the rate at which an inlet traps sediment is higher immediately 

 after the inlet opens than it is later in its history. 



b. Overwash. On low barrier islands, sand may be removed from the beach 

 and dune area by overwashing during storms. Such rates may average locally up 

 to 0.5 cubic meter per year per meter (1 cubic yard per year per foot). Data 

 presented by Pierce (1969) suggest that for over half of the shoreline between 

 Cape Hatteras and Cape Lookout, North Carolina, the short-term loss due to 

 overwash was 1.5 cubic meters per year per meter (0.6 cubic yard per year per 

 foot) of beach front. Figure 4-46 is an aerial view of overwash in the region 

 studied by Pierce (1969). Overwash does not occur on all barrier islands, but 

 if it does, it may function as a source for the beach on the lagoon side. 



c. Backshore and Dune Storage . Sand can be temporarily withdrawn from 

 transport in the littoral zone as backshore deposits and dune areas along the 

 shore. Depending on the frequency of severe storms, such sand may remain in 

 storage for intervals ranging from months to years. Backshore deposition can 

 occur in hours or days by the action of waves after storms. Dune deposits 

 require longer to form — months or years — because wind transport usually moves 

 material at a lesser rate than wave transport. If the immediate beach area is 

 the control volume of interest and budget calculations are made based on data 

 taken just after a severe storm, allowance should be made in budget cal- 

 culations for sand that will be stored in berms through natural wave action. 



d. Offshore Slopes . The offshore area is potentially an important sink 

 for littoral material. Transport to the offshore is favored by (1) storm 

 waves which stir up sand, particularly when onshore winds create a seaward 

 return flow, (2) turbulent mixing along the sediment concentration gradient 

 which exists between the sediment-water mixture of the surf zone and the clear 



4-120 



