Until such data are available to develop a functional design of permeable 

 groins similar to that in Sections 5.65 through 5.68 for impermeable 

 groins, evaluation and design of permeable groins will be inexact. In 

 general, the desired degree of sand passing the groin can be achieved as 

 effectively and economically by appropriate design of groin height and 

 length, or by notching or lowering the groin on the shore end. Permeable 

 groins are not normally used to retain fill placed to restore or widen a 

 beach. Permeable groins are used in rich-drift areas to widen or prevent 

 recession of specific beach areas and to reduce scalloping (saw-tooth 

 shape) of the shoreline. 



5.642 High and Low Groins . The amount of sand passing a groin partly 

 depends on the height of the groin. Groins based where it is unnecessary 

 or undesirable to maintain a sand supply downdrift of the groin, may be 

 built high enough to completely block sand moving in the zone influenced 

 by the groin. Such groins are called terminal groins. Where it is neces- 

 sary to maintain a sand supply downdrift, the groin may be built low 

 enough to allow overtopping by storm waves, or by waves at high tide. 

 Such low groins serve a purpose similar to that of permeable groins. 



5.643 Adjustable Groins . Nearly all groins are permanent, fixed struc- 

 tures. However, in England and Florida, adjustable groins have been used. 

 These groins consist of removable panels between piles. These panels can 

 be added or removed to maintain the groin at a specific height (usually 



1 to 2 feet) above the beach level, thus allowing a part of the sand to 

 pass over the groin and maintain the downdrift beach. However, if these 

 structures undergo even slight movement and distortion, removal or addi- 

 tion of panels becomes difficult or even impossible. 



5.65 GROIN OPERATION 



A groin is a barrier to sand moving in the zone between its seaward 

 end and the limit of uprush. Height, length, and permeability of the groin 

 determine its effect on longshore transport. The way a groin modifies the 

 littoral transport rate is about the same whether the groin operates singly 

 or as one of a system, provided spacing between adjacent groins is adequate. 

 However a single groin or the updrift groin of a system, may impound less 

 than the other individual groins of a system. 



The typical groin, illustrated in Figure 5-7, extends from a point 

 landward of the top of the berm to the normal breaker zone (for instance, 

 the 6-foot depth contour on the Atlantic coast). The predominant direction 

 of wave attack shown by the orthogonals will cause a predominant movement 

 of littoral drift. 



The groin acts as a partial dam that intercepts a part of the normal 

 longshore transport. As material accumulates on the updrift side, supply 

 to the downdrift shore is reduced, and the downdrift shore recedes. This 

 results in a progressively steepening slope on the updrift side and a 

 flattening slope on the downdrift side, since both slopes reach a common 

 elevation near the end of the groin. Since the grain size of the beach 



5-33 



