Normal Spacing 



>i — *- 



-* — ^2 — >U ■S3-»4 



I I Origin 



Gl Shoreline 



Adjusted Shoreline '^°''^°' length 



Figure 5-19. Schematic of groin-shortening procedure. 



end. This would result in the entire groin system becoming a transitional 

 section. 



Kressner (1928) found in model tests that only three or four groins need 

 to be shortened at the downdrift end of the system (see Fig. 5-19). He also 

 found that the transition is most effective if a line connecting the seaward 

 ends of the shortened groins and the last full-length groin meets the natural 

 shore alinement at an angle of about 6° , as shown in Figure 5-19. Bruun 

 (1952) indicates that a 6° angle has been successfully used, ^e length of a 

 groin, a , is measured from the orest of the beach berm to the seaward 

 end. (The actual groin length extends shoreward of the berm.) The limit of 

 the shortening is a judgment decision of the designer; however, in the case of 

 coastal tidal areas, it is suggested that the last transitional groin extend 

 no farther than the MLLW line. With y being the shortening, I the normal 

 groin length, I the length of the first shortened groin, l^ the length 

 of the second shortened groin, £„ the length of the third shortened groin, 

 etc., and s the spacing between groins, then 



and 



y = s. tan 6 



(5-5) 



I, = i, - y 



1 n ^ 



(5-6) 



or 



I, = I 



s. tan 6 



then 



and 



l^ = I, - 



s„ tan 6 



(5-7) 



1^ = 1^- 



s- tan 6 



(5-8) 



5-46 



