interpreted as bounding the seaward edge of sediment moved by waves in sig- 

 nificant quantities. This band of finer material suggests that there is 

 little exchange between littoral and shelf sands in most places. 



c. Sand Budget Balancing . Onshore sand transport has been suggested 

 as a source of sand for several coastal localities that lack other obvious 

 sand sources. (Shepard, 1963, pp. 176-177; and Pierce, 1969.) For example. 

 Pierce suggests that the offshore must supply 440,000 cubic yards per year 

 of sand to the southern segment of the Outer Banks of North Carolina be- 

 cause other known sources do not balance the budget. 



d. Transport in Nearshore Zone . Although theoretical, experimental, 

 and field data show that waves move sand some of the time over most of the 

 Continental Shelf, most of the data suggest that sand from the shelf is not 

 a significant contributor to the sediment budget of the littoral zone. 



Sand transport from the nearshore zone is more likely. Surveys show 

 that sand in the nearshore zone does move, although it is difficult to meas- 

 ure direction of motion. The presence of shore-parallel contours along most 

 open shores is evidence that the waves actively mold the sand bottom in the 

 nearshore zone. However, the time scale of transport in the nearshore zone 

 may be relatively slow. 



In tests at Santa Barbara, California, and at Atlantic City and Long 

 Branch, New Jersey, dredged sands were dumped offshore in depths ranging 

 from 15 to 40 feet, but no measurable onshore migration of the sand re- 

 sulted for times of about 1 year. (Hall and Herron, 1950.) Radioactive 

 tracers have shown that gravel moves slowly landward in 30 feet of water 

 at a rate of about 0.5 cubic yard per year per foot of beach. (Crickmore 

 and Waters, 1972.) 



At shallower depths in the nearshore zone, onshore sand transport 

 following storms is well documented. Transport of sediment suspended 

 over ripples by the mass transport velocity is more than adequate to 

 return sand eroded from the beach or to transport sand eroded from the 

 nearshore bottom to the beach. 



e. Summary on Seaward Limit . The deepest shore-parallel contour 

 appears to be a usable estimate of the maximum depth where significant 

 sand transport can be expected. This depth varies from 15 feet to per- 

 haps 60 feet or more along U.S. coasts. This choice may be modified for 

 specific wave conditions using Figure 4-23 to find the depth where maxi- 

 mum wave-induced velocity first exceeds 0.5 foot per second. If this 

 depth is less than the depth of the deepest shore-parallel contour, it 

 should be used as the seaward limit for the given wave conditions, 



4.524 Beach Erosion and Recovery . 



a. Beach Erosion . Beach profiles change frequently in response to 

 winds, waves, and tides. Profiles are also affected by events in the long- 

 shore direction that influence the longshore transport of sand. The most 



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