the type of groin constructed. Percentage of net longshore transport 

 considered conservative for computing downdrift losses due to certain 

 groin types based on the normal breaker zone occurring at the 6-foot 

 depth contour (Atlantic coast) is given as follows: 



(a) For high groins extending to a depth of water 10 feet or 

 more, use 100 percent of the total longshore transport. 



(b) For high groins extending to a depth of 4 to 10 feet below 

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



a depth greater than 10 feet, use 75 percent of the total longshore trans- 

 port. 



(c) For high groins extending from mean low water to 4 feet 

 below mean low water (or mean lower low water), or for low groins extend- 

 ing to a depth less than 10 feet below mean low water, use 50 percent of 

 the total annual rate of longshore transport. 



The following steps can now be used to determine the position of the 

 downdrift shoreline or berm crest line: 



(a) Estimate the time required for the updrift side of the groin 

 to fill. 



(b) Draw receded shoreline, d e (Figure 5-14) with an align- 

 ment determined for the updrift fillet such that area d e a in square 

 feet is equal to the volume of littoral material in cubic yards (reduced 

 according to groin type) determined by the time for the groin to fill. 



(c) Plot the original bottom profile, and show the groin on this 

 profile as in Figure 5-13. Plot ad as the maximum recession to be 

 expected. 



This method assumes an erodible bottom and- backshore. Wherever a 

 nonerodible substance is encountered, recession would halt at that point. 

 This would also be true where the groins are tied to a seawall or bulk- 

 head. In this case the expected profile seaward of the seawall would be 

 determined as if the seawall were not there or in a similar manner as for 

 scour at a seawall. The position of the bottom where it intersects the 

 seawall would determine the approximate scour to be expected in front of 

 the wall. The deficiency in material would tend to be made up by reces- 

 sion of the shoreline beyond the downdrift end of the seawall. 



5o67 ALIGNMENT OF GROINS 



Examples may be found of almost every conceivable groin alignment, 

 and advantages are claimed by proponents of each type. Based on the theory 

 of groin operation, which establishes the depth to which the groin extends 

 as the critical factor affecting its impounding capacity, maximum economy 

 in cost is achieved with a straight groin perpendicular to the shoreline. 

 Various modifications such as a T- or L-head are usually designed with 



5-4 



