10. Cost E ffectiveness of Groi n Constru ction . 



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 remain stable if the rate of loss from an area 

 does not exceed the rate of supply to that area. If the rate of supply is 

 less than the rate of loss, erosion and recession of the beach will occur. An 

 eroding beach can be restored by the placement of an artificial protective 

 beach and subsequently stabilized by artificial nourishment, i.e., the arti- 

 ficial placement of sand to make up the deficiency in rate of supply or the 

 artificial nourishment supplemented by structures (groins) to reduce the rate 

 of loss. The choice of groins over the artificial nourishment alternative 

 should be based on the relative costs of the two methods of shore 

 stabilization. 



On long straight beaches, making up the deficiency of sand supply presum- 

 ably affects and stabilizes much of the entire reach of shore. A groin system 

 for such a long reach is obviously expensive, but requires less artificial 

 nourishment, especially where the nourishment of the shore downdrift of the 

 reach is not required. A method sometimes used to estimate the comparative 

 life cycle cost for such a groin system is to estimate the annual cost of the 

 system, including the annual cost of artificially nourishing the reach with 

 groins and the downdrift shore, to find if the annual cost will be less than 

 the estimated annual cost of stabilizing by artificial nourishment alone. No 

 firm guidance is available on the reduction in nourishment requirements where 

 a complete groin system is built. 



Where the littoral transport rate is high, a groin system will not require 

 artificial nourishment while the groins and offshore area are filling. If 

 the littoral transport rate has not been reduced, no nourishment will be 

 required after filling. The volume required to fill the groin system is 

 easily estimated; the volume required to fill the offshore area, which is 

 equally important, is difficult to estimate. Therefore, the time needed for 

 complete filling is difficult to estimate. It may take several years for long 

 groins and during this long time, the downdrift shore will erode unless it is 

 artificially nourished. This nourishment volume will be equal to the volume 

 impounded by the groin system and its offshore area plus any deficiency suf- 

 fered before groin construction. After complete filling and shore realinement 

 at the groin system, the littoral transport rate will probably be reduced from 

 that required during the filling period and the downdrift shores may require 

 more nourishment. 



Another approach to estimate the comparative life cycle cost of a groin 

 system for a long reach of shore is to estimate the annual cost, as before, 

 and convert this cost to the equivalent quantity of sand that could be 

 artificially placed annually at the estimated cost of sand over the life of 

 the project. This will indicate how much the groins must reduce annual 

 nourishment requirements to be at the "break-even" point. A judgment can then 

 be made as to whether the groin system will actually reduce annual nourishment 

 requirements below the break-even point. The choice of a groin system over 

 artificial nourishment would be justified only if its costs (including reduced 

 nourishment costs) are less than the costs of artificial nourishment alone. 



Where it is necessary to widen a short beach, perhaps 2 kilometers or 



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