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, 19670 The small frac- 

 tion 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. Many sources of evidence indicate that 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 important. 



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

 the beach, and the seaward edge of dunes, cliffs, and mainland (Fig. 4-44) 

 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) esti- 

 mates that beach and cliff erosion along all coasts of the world totals 

 about 0.03 cubic mile or 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, 

 since the sand fraction in the river sediments is usually small, and is 

 usually trapped before it reaches the littoral zone. Shore erosion is an 

 especially significant source where older coastal deposits are being ero- 

 ded, 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 ero- 

 sion volume per foot 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 Figure 4-44.) 



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

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

 The dashed line profile below it 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 on 

 the middle of Figure 4-44. The unit volume of this parallelogram, hAx, 

 is equivalent to the shaded rectangle on the left of Figure 4-44. If the 

 vertical distance, h is 40 feet, and Ax = 1 foot of horizontal erosion, 

 then the unit volume lost is 40/27, or 1.5 cubic yards per foot of beach 

 front . 



4.723 Transport from Offshore Slope . An uncertain and potentially signifi- 

 cant source in the sediment budget is the contribution from the offshore 

 slopCo However, hydrography, sediment size distribution, and related evi- 

 dence discussed in Section 4.523 indicate that contributions from the con- 

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

 Most shoreward moving sediment appears to originate in areas fairly close 

 to shoreo Significant onshore-offshore transport takes place within the 

 littoral zone due to seasonal and storm-induced profile changes and to 



4-121 



