that it will not be flanked by doinincoast erosionj 



e. In providing protection by groins 3 it raust be remembered that 

 the alinement of the shore adjacent to a groin is dependent upon the direction 

 of wave attack and will vary. If minor fluctuations of the shore cannot 



be allowed a solution other than the use of groins must be found; and 



f . Where the supply of littoral material is insufficient to 

 permit the vjithdrawal from the littoral s tream of enough material to 

 fill the groin or groin field without damage to downdrift areas, 

 artificial fill may be required to fill the groin or groin field and thus 

 permit the natural littoral supply to pass without interruption, 



1^3. Permeability of Groins . - One criticism of the use of groins 

 is that in withdrawing material from the littoral stream to build an 

 accretion of sand on the updrift side of a groin, this amo'unt of sand is 

 lost from the downdrift side, often causing severe erosion. Many attempts 

 have been made to design a groin with the proper degree of permeability 

 to pass sufficient sand through the groin to maintain the alinement of 

 the downdrift shore line or lessen the erosion, at the same time reducing 

 the scouring effect around the end of the groin. The generally accepted 

 method of designing a permeable groin is to increase the degree of permeability 

 from the bottom to the top and from the shore to the seaward end of the 

 groin. No agreement has been reached as to the amount of permeability 

 needed. Although permeable groins have been built on the Great Lakes, at 

 Rockaway Beach, New York, in Florida, and at other locations, their effective- 

 ness as compared to impermeable groins has not been proven. At many 

 locations they have failed to trap sandj at some other locations where 

 accretions have occurred^ it is problematical whether or not the accretions 

 would not have occurred without the groins. 



l5i).. High and Low Groins, - The amount of sand passing a groin 

 depends to some extent on the height of the groin. Groins based on a 

 head]_and or reef, or at the entrance to a bay or inlet where it may be 

 either unnecessary or undesirable to maintain a sand supply downdrift of 

 the groin^ may be built to such a height as to completely block the passage 

 of all material moving in that part of the littoral zone covered by the 

 gi-oins. Where it is necessary to maintain a sand supply downdrift of a 

 groin, it may be built to such a height as to allow overtopping by storm 

 waves., or by waves at high tide. Such low groins serve the same purpose 

 as that intended by designers of permeable groins, and are more positive 

 in operation, 



155. Adjustable and Fixed Groins , - The great majority of groins are 

 fixed or permanently built structures. In England, the Case and Du-Plat- 

 Taylor adjustable groins have been used with reported success. These 

 groins are essentially adjustable batter boards between piles, with a 

 raising and]Dwering device so that the groin can be maintained at a fixed 

 height (usually one to two feet) above the sand level, allowing a con- 

 siderable part of the sand to pass over the groin and maintain the downdj:"ift 



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