b. For high <^roins extending tc a depth of U to 10 feet belovj 

 mean low water (or raean !^oxirer low water), or for lovj t^roxne extending-- to 

 a depth greater than 10 feet, use 75 per cent of the total littoral 

 drift; 



bo For high groins extending from mean low water to h feet below 

 mean low water (or inean lower low water), or for low groins extending to 

 depth less than 10 feet below raean low water, use 50 per cent of the total 

 annual rate of littoral drift. 



170. The following steps can now be used to determine the position 

 of the downdriit ground line. 



a. Lay out the wave diagram as f or ' the updrift side. See 

 Figure 6?; 



b. Determine the time required for the groin t^o fill in years 

 or fractions of a year; 



c. Draw receded shore fl.ine, "do" . noirm^l to the orthogonals 

 such that area dec in square 'Teef.is equal to the portion of the annual 

 rate of littoral drift in cubic yards (reduced according to groin type) 



determined by the time for the groin to fill; 



d. If more than one year is reciuired, the recession during- the 

 second year is shown in the same way such that area dfge is equal to 



the adjusted annual ra^^e of littoral drift; 



e. Plot the original bottom profile and show the groin on 

 this profile as in Figure 66. Plot cd_ as the maximum recession expected 

 and proceed as in para'gmph l66. 



171. The foregoing assumes an erodible bottom and backshore. 

 Naturally, wherever a non-erodible substance is encountered, recession 

 would halt at that point. However, all other places would continue to 

 recede to the ultimate plane predicted. This would also be true where 

 the groins are tied to a seawall or bulkhead. In this case the ex- 

 pected ground line would be determined as if the seawall were not there or 

 in a similar manner as for seawalls. The position of ihe ground line 

 where it intersects the seawall would determine the approximate scour to 

 be expected in front of the wall. The deficiency in material would tejid 

 to be made up by recession of the shore line at the downdrift end of the 

 seawall. 



172. Alinement of Groins . - For most effective operation with least 

 tendency toward localized erosion^, groins should be constructed parallel 

 to the direction of resaaltant wave action; in other words,*, parallel to 

 the direction of approach of the wave which will produce the predominant 

 direction and net rate of littoral drift. This direction may be determined 

 through the use of refraction diagrams combined with a statistical wave 

 studj' over an extended period of time, as discussed previously under wave 

 analysis (para, 76). However the accuracy of the determination 



