Groins may be classified as permeable or impermeable, high or low, 

 long or short, and fixed or adjustable. They are constructed of timber, 

 steel, stone, concrete, or other materials. Impermeable groins have a 

 solid or nearly solid structure which prevents littoral drift from passing 

 through the structure. Permeable groins have openings through the struc- 

 ture big enough to permit passage of significant quantities of littoral 

 drifto Some permeable stone groins are made impermeable by heavy marine 

 growth. A series of groins acting together to protect a long section of 

 shoreline is called a groin system or groin field. 



Groins differ from jetties structurally and functionally. Jetties 

 are larger with more massive components, and are used primarily to direct 

 and confine the stream or tidal flow at the mouth of a river or inlet, 

 and to prevent littoral drift from shoaling the channel. 



5o63 PURPOSE 



The purpose of groins is to provide or maintain a protective or 

 recreational beach. Groins may be used to: 



(a) Build or widen a beach by trapping littoral drift; 



(b) Stabilize a beach, subject to excessive storms or seasonal 

 periods of advance and recession, by reducing the rate of loss; 



(c) Reduce the rate of longshore transport out of an area by re- 

 orienting a section of the shoreline to an alignment more nearly perpen- 

 dicular to the predominant wave direction; 



(d) Reduce losses of material out of an area by compartmenting 

 the beach, usually a relatively short section of beach artificially 

 filled seaward of adjacent shores; and 



(e) Prevent accretion in a downdrift area by acting as a littoral 

 barrier. 



These ends are attained by reducing the longshore transport rate 

 which decreases the quantity of drift reaching downdrift shores,, This 

 can lead to the need for downdrift extension of the system or for arti- 

 ficially nourishing the downdrift shore, unless the system is artificially 

 filled initially and suitably renourished, 



5.64 TYPES OF GROINS 



5.641 Permeable Groins . Permeability helps avoid. the abrupt offset in 

 shore alignment found at impermeable groins. Part of the littoral forces 

 and materials pass through the groin, and induce sand deposition on both 

 sides of the groin. Many types of permeable groins have been employed. 

 The degree of permeability above the ground line affects the pattern and 

 amount of deposition within the limits of the groin's influence. Insuffi- 

 cient empirical data have been compiled to establish quantitative relation- 

 ships between littoral forces, permeability, and resulting shore behavior. 



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