downdrift side of a groin or discourage the development of rip currents. 

 While these modifications may achieve the intended purpose, the zone of max- 

 imum recession is often simply shifted downdrift from the groin, limiting the 

 benefits. Storm waves will normally produce greater scour at the seaward 

 extremities of the T- or L-head structures than at the end of a straight groin 

 perpendicular to the shore, delaying the return to normal profile after storm 

 conditions have abated. Curved, hooked, or angle groins have been employed 

 for the same purposes as the T- or L-head types. They also cause excessive 

 scour and are more costly to build and maintain than the straight, perpen- 

 dicular groin. Where the adjusted shore alinement expected to result from a 

 groin system differs greatly from the alinement at the time of construction, 

 it may be desirable to aline the groins normal to the adjusted shore alinement 

 to avoid angular wave attack on the structures after the shore has stabilized. 

 This condition is most likely to be encountered in the vicinity of inlets and 

 along the sides of bays. 



8 . Order of G roin Const ructi on. 



At sites where a groin system is under consideration, two possibilities 

 arise: either the groin system is to be filled artificially or longshore 

 transport is to be depended on to produce the fill. With artificial fill, the 

 only interruption of longshore transport will be the period between the time 

 the groin system is constructed and the time the artificial fill is made. For 

 economical reasons, the fill is normally placed in one continuous operation, 

 especially if it is being accomplished by hydraulic dredge. Accordingly, to 

 reduce the time period between the groin construction and the deposition of 

 the fill, all groins should preferably be constructed concurrently. Deposi- 

 tion of the fill should commence as soon as the stage of groin construction 

 permits. When depending on longshore transport, no groin will fill until all 

 the preceding updrift groins have been filled. This natural filling will 

 reduce the supply to downdrift beaches. The time period required for the 

 entire system to fill naturally and the material to resume its unrestricted 

 movement downdrift may be so long that severe downdrift damage may result. 

 Accordingly, to reduce this damage, only the groin or group of groins at the 

 downdrift end should be constructed initially. The second groin, or group, 

 should not be started until the first has filled and material passing around 

 or over the groins has again stabilized the downdrift beach. Although this 

 method may increase costs, it will not only aid in reducing damage, but will 

 also provide a practical guide to the spacing of groins. 



9. Guidanc e from Existing Projects . 



The guidelines presented here, in addition to knowledge of the study area 

 and experience with groins, should provide a strong basis for the proper 

 consideration and design of a groin system. Reports which summarize existing 

 groin fields are also helpful. For example, DeWall (1979), Everts (1979), and 

 Nordstrom, Allen, and Gares (1979) describe the effects of groin fields at 

 Westhampton Beach, New York; Sea Isle City and Cape May, New Jersey; and Sandy 

 Hook, New Jersey, respectively. The more similar an existing groin field is 

 to the study area in terms of the physical environment, the more applicable 

 its behavior and design will be to the study area. 



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