Most of the sediment carried to the coast by rivers is deposited in 

 comparatively small areas, often in estuaries where the sediment is trapped 

 before it reaches the coast (Strakhov, 1967). The small fraction of sand in 

 the total material brought to the coast and the local estuarine and deltaic 

 depositional sites of this sediment suggest that rivers are not the immediate 

 source of sediment on beaches for much of the world's coastline. Sand-sized 

 sediment is not supplied to the coasts by rivers on most segments of the U.S. 

 Atlantic and gulf coasts. Therefore, other sediment sources must be impor- 

 tant. 



b. Erosion of Shores and Cliffs . Erosion of the nearshore bottom, the 

 beach, and the seaward edge of dunes, cliffs, and mainland results in a sand 

 loss. In many areas, erosion from cliffs of one area is the principal source 

 of sand for downdrift beaches. Kuenen (1950) estimates that beach and cliff 

 erosion along all coasts of the world totals about 0.12 cubic kilometer (0.03 

 cubic mile) or 120 million cubic meters (160 million cubic yards) per year. 

 Although this amount is only about 1 percent of the total solid material 

 carried by rivers, it is a major source in terms of sand delivered to the 

 beaches. Shore erosion is an especially significant source where older 

 coastal deposits are being eroded, since these usually contain a large 

 fraction of sand. 



If an eroding shore maintains approximately the same profile above the 

 seaward limit of significant transport while it erodes, then the erosion 

 volume per meter of beach front is the vertical distance from dune base or 

 berm crest to the depth of the seaward limit h , multiplied by the horizontal 

 retreat of the profile Ax (see Fig. 4-44). 



Figure 4-44 shows three equivalent volumes, all indicating a net erosion 

 of hAx . To the right in Figure 4-44 is a typical beach profile (the dashed 

 line profile below is the same as the solid line profile) . The horizontal 

 distance between solid and dashed profiles is Ax , the horizontal retreat of 

 the profile due to (assumed) uniform erosion. The unit volume loss, hAx 

 between dune base and depth to seaward limit is equivalent to the unit volume 

 indicated by the slanted parallelogram in the middle of Figure 4-44. The unit 

 volume of this parallelogram, hZtoc , is equivalent to the shaded rectangle on 

 the left of Figure 4-44. If the vertical distance h is 10 meters and Ax = 

 1 meter of horizontal erosion, then the unit volume lost is 10 cubic meters 

 per meter of beach front. 



c. Transport from Offshore Slope . An uncertain but possibly significant 

 source in the sediment budget is the contribution from the offshore slope. 

 Hovever, hydrography, sediment size distribution, and related evidence 

 discussed in Section V,2,c indicate that contributions from the continental 

 shelf to the littoral zone are probably negligible in many areas. Most 

 shoreward-moving sediment appears to originate in areas fairly close to 

 shore. Significant onshore-offshore transport takes place within the littoral 

 zone due to seasonal and storm-induced profile changes and to erosion of the 

 nearshore bottom and beaches, but in the control volume defined, this trans- 

 port takes place within the control volume. Transport from the offshore has 

 been treated as a line source. 



In some places, offshore islands or shoals may act as point sources of 



4-117 



