Horitontal Sliori 



Siction 



•♦-Set Shoreword 

 Enough to Prevent 

 Flonking 



Set Seoward to 

 Desired Updrif, 

 Berm Widtti 



Noturol Berm 

 Elevation 



Inttrmtdioti 



Slopid Stction 



Slope with 

 Noturol Profile 



Outtr Stction 



Set According to Plonned 

 nterruption of 

 Longshore 

 Transport 



^ 



MLW or MLLW 

 Elevotion 



Figure 5-18. Summary of groin design. 



points in Figures 5-14, 5-15, and 5-17. Therefore, the position of u 

 becomes a design parameter which can be moved shoreward or seaward, depending 

 on the desired beach width. 



(3) Seawa rd Limit of Outer Section . The seaward extent of the OS 

 depends on the amount of longshore transport to be intercepted. Some guide- 

 lines on how to estimate this are discussed in Section VI, 2 of this chapter. 



d. Spacing of Groins . In the design of a groin system, the estimation of 

 the intermediate laeach alinement discussed in Section VI,3,b,(2) of this 

 chapter and shown in Figure 5-15 will usually determine the desired alongshore 

 spacing between groins. In the future, mathematical models will be used to 

 determine the groin spacing. However, if in the designer's opinion these 

 spacing values are unreasonable or indeterminable, the following general rule 

 is suggested: 



The spacing between groins should equal two to three times the groin 

 length from the berm crest to the seaward end. 



e. Groin System Transition . To avoid an abrupt change in the shore 

 alinement that may result in erosion of the downdrift beach, the use of 

 transitional groins (groins of gradually reduced lengths) is recommended. 

 A method for the design of a groin system transition that involves groin 

 shortening has been used by the U.S. Army Engineer District, Wilmington 

 (1973) (see Fig. 5-19). Kressner (1928) conducted model studies on groin 

 transitions, and more recently Bruun (1952) applied the principle of groin 

 shortening at the end of groin systems. Where there are reversals in the 

 direction of longshore littoral transport, transitions would be appropriate 

 for both ends of the system. Bruun (1952) indicated that in a long series of 

 groins, the shortening should possibly be carried out on both the updrif t side 

 and the downdrift side to ensure a smooth passage of littoral drift to the 

 uprotected coast. He further indicated that if the series consisted only of a 

 few groins, the shortening should start with the second groin from the updrif t 



5-45 



