the primary purpose of limiting recession on the downdrift side of a 

 groin. While these may achieve the intended purpose, the zone of maxi- 

 mum recession is often simply shifted downdrift from the groin, and 

 benefits are thus limited. 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 pur- 

 poses as the T- or L-head head types. They also invite excessive scour, 

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

 lar groin. Where the adjusted shore alignment expected to result from a 

 groin system will differ greatly from the alignment at the time of con- 

 struction, it may be desirable to align the groins normal to the adjusted 

 shore alignment 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. 



5.68 ORDER OF GROIN CONSTRUCTION 



At sites where a groin system is under consideration, two condi- 

 tions arise: 



(a) The groin system will be filled artificially, and it is desired 

 to stabilize the new beach in its advanced position; and 



(b) Littoral transport is depended upon to make the fill, and it 

 is desired to stabilize the existing beach or build additional beach with 

 a minimum of detrimental effect on do\>mdrift areas. 



With artificial fill, the only interruption of longshore transport 

 will be between the time the groin system is constructed and the time the 

 artificial fill is made. For economy, the fill is normally placed in one 

 continuous operation, especially if it is being accomplished by hydraulic 

 dredge„ Accordingly, to reduce the time between groin construction and 

 deposition of fill, all groins should preferably be constructed concur- 

 rentlyo Deposition of fill should commence as soon as the stage of groin 

 construction will permit. 



When depending on littoral transport no groin can fill until all of 

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

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

 system to fill and the material to resume its unrestricted movement down- 

 drift may be so long that severe damage will result Accordingly, to 

 reduce downdrift damage, only the groin or group of groins at the down- 

 drift 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 practicable guide to spacing of groins to 

 verify the design spacing. 



5-43 



