(4) Beach Allnement for Reversing Direction of Longshore Transport . 

 Where there is a periodic reversal in the direction of longshore transport, an 

 area of accretion may form on both sides of a groin, as shovm in Figure 5-17. 

 The fillet between groins may actually oscillate from one end of the compart- 

 ment to the other, as shown by the dashlines, or may form a U-shaped shoreline 

 similar to the maximum recession alinement, depending on the rate of supply 

 of littoral material. With regular reversals in the direction of longshore 

 transport, the maximum line of recession would probably be somewhat as shown 

 by the solid line, with areas A and C about equal to area B. The extent 

 of probable beach recession must be considered in establishing the length of 

 the horizontal shore section of groin and in estimating the minimum width of 

 beach that may be built by the groin system. 



(5) Mathematical >fodels . Mathematical models are being developed 

 which will replace the above procedures. The models will allow the many 

 different spacing and length combinations to be quickly and inexpensively 

 tested to determine the optimum design. The Engineering Computer Program 

 Library Catalog, published by the U.S. Army Engineer Waterways Experiment 

 Station, should be consulted for an abstract of approved computer programs. 

 The use of other engineering computer programs is governed by ER 1110-1-10. 



Reversing Direction of 

 Longshore Tronsport 



Figure 5-17. 



Intermediate beach alinement with reversal of longshore trans- 

 port direction. 



c. Gr oin Dimension . Once the adjusted shoreline is estimated, a deter- 

 mination of the groin dimensions is possible. The discussion which follows is 

 illustrated by Figure 5-18. 



(1) Shoreward Limit of Hori zont al Shore Section. The primary design 

 objective in establishing the position of the shoreward end of the groin is 

 the prevention of flanking due to beach recession. This is done by conserva- 

 tively estimating the predicted recession position represented by the 

 r points in Figures 5-15 and 5-16. 



(2) Seaward Limit of Horizontal Sho re Section. The updrift berm 

 crest is expected to move to the seaward limit of the HSS, shown as the u 



5-44 



