(2) Sediment transport in the nearshore zone. Seaward of the 

 breakers, sand is set in motion by waves moving over ripples, either 

 rolling the sand as bed load, or carrying it up in vortices as suspended 

 load. The sand, once in motion, is transported by mean tidal and wind- 

 induced currents and by the mass transport velocity due to waves. The 

 magnitude and direction of the resulting sediment transport are uncer- 

 tain under normal circumstances, although mass transport due to waves 



is more than adequate to return sand lost from the beach during storms. 

 It appears that bottom mass transport acts to keep the sand close to the 

 shore, but that some material, probably finer sand, escapes offshore as 

 the result of the combined wind- and wave-induced bottom currents. 



(3) The shape and expectable changes in shape of nearshore and 

 beach profiles. Storms erode beaches to produce a simple concave-up 

 beach profile with deposition of the eroded material offshore. Rates of 

 erosion due to individual storms vary from a few cubic yards per foot to 

 10 's of cubic yards per foot of beach front. The destructiveness of the 

 storm in producing erosion depends on its intensity, duration, and orienta- 

 tion, especially as these factors affect the elevation of storm surge and 

 the wave height and direction. Immediately after a storm, waves begin to 

 return sediment to the eroded beach, either through the motion of bar-and- 

 trough (ridge-and-runnel) systems, or by berm building. The parameter, 



Fo = Ho/iVfT), given by Equation 4-20 determines whether the beach erodes 

 or accretes under given conditions. If V^ is above critical value 

 between 1 and 2 the beach erodes. (See Figures 4-29, and 4-30.) 



(4) The slope of the foreshore. There is a tendency for the fore- 

 shore to become steeper as grain size increases, and to become flatter 



as mean wave height increases. Data for this relation exhibit much scat- 

 ter and quantitative relationships are difficult to predict. 



4.833 Sediment Budget. Section 4.76 siommarizes material on the sediment 

 budget. Table 4-14 tabulates the elements of the sediment budget and in- 

 dicates the importance of each element. Table 4-13 classifies the ele- 

 ments of the sediment budget. 



A sediment budget carefully defines the littoral control volume, 

 identifies all elements transferring sediment to or from the littoral 

 control volume, ranks the elements by their magnitude, and provides an 

 estimate of unknown rates by the balancing of additions against losses 

 (Equation 4-46) . 



If prepared with sufficient data and experience, the budget permits 

 an estimate of how proposed improvements will affect neighboring segments 

 of the littoral zone. 



4-154 



