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 artificially placing a protective beach and subsequently 

 stabilized by artificial nourishment, that is artificial placement of sand 

 to make up the deficiency in rate of supply, or by artificial nourishment 

 supplemented by structures (groins) to reduce the rate of loss. Justifi- 

 cation of groins must be based on the relative costs of the two methods of 

 shore stabilization. 



On long straight beaches, making up the deficiency of 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 nourishment of the shore down- 

 drift of the reach is not required. A method sometimes used for economic 

 determination of 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 this annual cost is less 

 than the estimated annual cost of stabilizing by artificial nourishment 

 alone. No firm guide is available on the reduction in nourishment require- 

 ments 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. After filling, no nourishment will be required if the littoral 

 transport rate has not been reduced. The volume required to fill the 

 groin system is easily estimated; the volume required to fill the off- 

 shore area, which is equally important, is difficult to estimate. There- 

 fore, the time needed for complete filling is difficult to estimate, but 

 it may take several years for long groins: During this long time, the 

 downdrift shore will erode lonless 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 suffered before groin 

 construction. After complete filling and shore realignment at the groin 

 system, the littoral transport rate will probab.ly be reduced from that 

 required during the filling period and the downdrift shores may require 

 more nourishment. 



Another approach for economic determination 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 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 hreak-even point. A judgment is then made as to whether the 

 groin system will actually reduce annual nourishment requirements below 

 the break-even point. The groin system 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 1 mile or less, 

 it becomes impracticable to maintain the increased width by artificial 

 nourishment of that beach alone. The nourishment material would rapidly 



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