To apply method 2, it is necessary to know or assume the transport rate 

 across one end of the littoral zone being considered. The most successful 

 applications of method 2 have been where the littoral zone is bounded on one 

 end by a littoral barrier which is assumed to completely block, all longshore 

 transport. The existence of such a complete littoral barrier implies that the 

 longshore transport rate is zero across the barrier, and this satisfies the 

 requirement that the rate be known across the end of the littoral zone being 

 considered. Examples of complete littoral barriers include large jetties 

 immediately after construction, or spits building into deep, quiet water. 



Data on shoreline changes permit estimates of rates of erosion and 



accretion that may give limits to the longshore transport rate. Figure 4-51 



is a shoreline change map which was used to obtain the rate of transport at 

 Sandy Hook, New Jersey (Caldwell, 1966). 



Method 3 (the energy flux method) is described in Sections V,3,b and V,3,c 

 with a worked example in Section V,3,d. Method 4 (the empirical prediction of 

 gross longshore transport rate) is described in Section V,3,e, with a worked 

 example in Section V,3,f. The essential factor in methods 3 and 4, and often 

 in method 1, is the availability of wave data. Wave data applicable to 

 studies of littoral processes are discussed in detail in Section III. 



b. Onshore-Offshore Motion . Typical problems requiring knowledge of 

 onshore-offshore sediment transport are described in Section V,l,a. Four 

 classes of problems are treated: 



(1) The seaward limit of significant sediment transport. Available 

 field data and theory suggest that waves are able to move sand during some 

 days of the year over most of the continental shelf. However, field evidence 

 from bathymetry and sediment size distribution suggest that the zone of 

 significant sediment transport is confined close to shore where bathymetric 

 contours approximately parallel the shoreline. The depth to the deepest shore- 

 parallel contour tends to increase with average wave height, and typically 

 varies from 5 to 18 meters (15 to 60 feet). 



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

 ers, 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 uncertain 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 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 meters per meter to 10' s of cubic 

 meters per meter of beach front. The destructiveness of the storm in 

 producing erosion depends on its intensity, duration, and orientation, 

 especially as these factors affect the elevation of storm surge and the wave 



4-147 



