a well established net littoral transport in one direction, 



151. Purpose . - Under some conditions and subject to definite 

 limitations imposed on their use, groins maj be used to: 



a. Stabilize a beach subject to intermittent pericds of advance 

 ejid recession; 



b. Provide upland protection through prevention of removal of 

 a protective beach; 



c» Reduce the rate of littoral drift out of an area by re- 

 orienting a section of the shore line to an alinement more parallel to 

 the wave crests; 



d. Build or lolden a beach by trapping littoral material; 



e. Prevent loss of material out of an area by compartmenting 

 the beach; 



f . Prevent advance of a downdrift area by acting as a littoral 

 barrier; and 



g. Stabilize a specific area by reducing the rate of loss from 

 the area. 



All of the foregoing ends are attained through modification of the rate 

 of littoral di'ift. Also, all are forms of shore protection, 



152. Limitations On The Use of Groins . - Because of their limitations, 

 the use of the groins as a major protective feature should be decided upon 

 only after careful consideration of the problem and the many factors involved. 

 Principal liraitations on the use of groins include: 



a. Because groins function by modifying the rate of littoral 

 drift, they serve no useful purpose in an area of no Vjave action, or where 

 the waves are parallel to the beach because there will be little or no 

 littoral drift; 



b. Even where the wave action is such as to set up a strong 

 littoral current, there must be a continuing supply of littoral material 

 from updrift of the groin or groin field under consideration for it to 

 function properly; 



G. V/here the quantity of m.aterial in the littoral stream is 

 small and there is a delicate balance between stability and erosion, the 

 interruption of this drift may cause damaging erosion to the shore line 

 downdrif t of the groin; 



d> Caution must be used that a single groin, or the doTfjncoast 

 groin of a field of groins, is securely anchored at the shore end, so 



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