5„69 LIMITATIONS ON THE USE OF GROINS 



Because of its limitations, a groin should be used as a major 

 protective feature only after careful consideration of the many factors 

 involved. Principal factors to be considered are: 



(a) The adequacy of natural sand supply to ensure that groins will 

 function as desired. 



(b) Where the supply of littoral drift is insufficient to permit 

 the withdrawal from the littoral stream of enough material to fill the 

 groin or groin system without damage to downdrift areas, artificial place- 

 ment of fill may be required to fill the groin or groin system and thus 

 minimize the reduction of the natural littoral drift to downdrift shores. 

 As previously mentioned, any groin system will reduce the rate of long- 

 shore transport to some degree. 



(c) The adequacy of shore anchorage of the groins to prevent flank- 

 ing as a result of downdrift erosion. 



(d) The extent to which the downdrift beach will be damaged by a 

 reduction of material supply if groins are used. 



(e) The economic justification for groins in comparison with stabi- 

 lization by nourishment alone. 



Groins are usually considered in areas where the supply of littoral 

 drift is less than the capacity of the littoral transport forces. In 

 these areas, a shoreline adjustment resulting from the installation of a 

 groin or groin system may not reduce the actual transport rate but result 

 only in a reduction of the expected additional losses from the beach fill 

 within the groin system. However, for this to occur, the groins must 

 extend to the surf zone thereby diverting some of the littoral material 

 to the offshore zone resulting in adverse affects to downdrift beaches. 



Where littoral drift supply satisfies the capacity of transporting 

 forces, the adjustment in the shore alinement resulting from a groin 

 system may result in a reduction in capacity of longshore transport 

 forces at the groined site. Thus, less material is transported along- 

 shore than was the case prior to the construction of the groins, and a 

 permanent adverse effect to the downdrift shore would result. Adverse 

 effects on adjacent shores described above are not necessarily a measure 

 of the effectiveness of the groin or groin system since these groins 

 might well have diverted some of the longshore transport to deep water 

 which in turn has deprived the downdrift beach from receiving a full 

 amount of longshore transport and produced the adverse effect (erosion). 



5.610 ECONOMIC DETERJ^INATION OF GROIN CONSTRUCTION Beaches exposed to 

 wave action constantly change due to variation in wave direction and wave 

 characteristics. In spite of the constant movement of beach materials, a 

 beach will be stable if the rate of loss from an area does not exceed the 



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