GROINS 



lltO, Definitions . - A groin is a shore protective structure devised 

 to provides build or widen a protective beach by trapping littoral drift 

 or to retard loss of an existing beach. It is usually perpendicular to 

 the shore, extending from a point landward of possible shore line recession 

 into the water a sufficient distance to stabilize the shore line at a 

 desired location. Groins are relatively narrow in width (measured parallel 

 to the shore line) and may vary in length from less than 100 feet to 

 several hundred feet. 



lUl. Groins may be classified as permeable or impermeable, high or 

 low, and fixed or adjustable. They may be constructed of timber, steel, 

 stone, concrete, or other materials, or combinations thereof. Impermeable 

 groins have a solid or nearly solid structure which prevents littoral 

 drift passing through the structure. Permeable groins have openings 

 through the structure of sufficient size to permit passage of appreciable 

 quantities of littoral drift. Some permeable stone groins may become 

 impermeable with heavy marine growth. A series of groins acting together 

 to protect a long section of shore line is commonly called a groin field. 



IU2, Groins differ from jetties in that jetties generally are larger 

 ■ftith more massive component parts, and are used primarily to direct and 

 confine the stream or tidal flow at the mouth of a river or entrance to 

 a bay and prevent littoral drift from shoaling the channel. In some 

 sections of the country groins are commonly referred to as jetties or 

 piers. 



IU3. Theory of Groin Operation . - The manner in which a groin operates 

 to modify the rate of littoral drift is approximately the same whether 

 it operates singly or as one of a field. As discussed under "Littoral 

 Drift", the beach material so referred to moves approximately parallel to 

 the shore line as a broad band extending from the limit of wave uprush to 

 some depth of water greater than 30 feet, A groin interposes a total or 

 partial barrier to littoral drift moving in that part of the band between 

 the seaward end of the groin and the limit of wave uprush. The extent 

 to which the littoral drift is so modified depends on the height, length, 

 and permeability of the groin. Groins are most effective, economically, 

 when the largest part of the littoral movement is close inshore. 



lUil. A typical groin is illustrated in Figure 60. In this figure, 

 the groin extends from some distance landward of the top of berm to the 

 6-foot depth contour. The net direction of wave attack, as typified by 

 the orthogonals shown, is such as to cause a net movement of littoral 

 drift in a downcoast direction. The crest of berm and 6-foot depth con- 

 tour are represented by e a i and £ £ h, respectively, occurring in a 

 state of nature prior to the construction of the groin a b c. Prior to 

 the construction of the groin the offshore beach slope had stabilized 

 between a and £ in a manner dependent on the median diameter of the 

 beach material and the type of wave attack. 



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