Section 4.531 describes four methods for estimating longshore trans- 

 port rate, and Sections 4.532 through 4.535 describe in detail how to use 

 two of these four methods. (See Methods 3 and 4.) 



One approach to estimating longshore transport rate is to adopt a 

 proven estimate from a nearby locality, after making allowances for local 

 conditions. (See Method 1.) It requires considerable engineering judge- 

 ment to determine whether the rate given for the nearby locality is a 

 reliable estimate, and, if reliable, how the rate needs to be adjusted 

 to meet the changed conditions at the new locality. 



Method 2 is an analysis of historical data. Such data may be found 

 in charts, maps, aerial photography, dredging records, beach fill records, 

 and related information. Section 4.811 describes some of these sources. 



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 suc- 

 cessful 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 lit- 

 toral 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 

 4-50 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 Section 4.532 

 with a worked example in Section 4.533. Method 4 (the empirical pre- 

 diction of gross longshore transport rate) is described in Section 4.534 

 with a worked example in Section 4.535. 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 4.3. 



4.832 Onshore-Offshore Motion . Typical problems requiring knowledge of 

 onshore-offshore sediment transport are described in Section 4.511. Four 

 classes of problems are treated: 



(1) The seaward limit of significant sediment transport. Avail- 

 able field data and theory suggest that waves are able to move sand dur- 

 ing 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 15 to 60 feet. 



4-153 



