VI. GROINS 



1. Definition , 



A groin is a shore protection structure designed to trap longshore drift 

 for building a protective beach, retarding erosion of an existing beach, or 

 preventing longshore drift from reaching some downdrift point, such as a 

 harbor or inlet. Groins are narrow structures of varying lengths and heights 

 and are usually constructed perpendicular to the shoreline. 



2. Groin Operation . 



The interaction between the coastal processes and a groin or groin system 

 is complicated and poorly understood. However, there are a few basic prin- 

 ciples which can be applied to the design of groins. These principles are 

 discussed below and summarized in the form of several concise rules of groin 

 design. 



RULE 1: Groins can only be used to interrupt longshore transport. 



Groins do not interrupt onshore-offshore transport. They do not attract 

 to an area any sand which would not otherwise have passed. 



RULE 2: The beaah adjustment near groins will depend on the magnitude 

 and direction of the longshore transport. 



The longshore drift builds up on the updrift side of a groin, thereby 

 creating a fillet. The downdrift side is deprived of this sediment and 

 usually erodes. Figure 5-9 illustrates the single groin process and Figure 

 5-10 the groin system process. Note the direction of the net longshore trans- 

 port. This direction depends on the predominant angle of wave approach. If 

 the wave approach is normal to the shoreline, or if the shoreline adjusts 

 itself normal to the wave approach through the process of fillet formation, 

 then the longshore transport rate will be zero. Thus, a second way that 

 groins will reduce the longshore transport rate is by allowing the shoreline 

 to approach an orientation normal to the wave approach. The wave climate 

 controls the longshore transport rate and is therefore an important aspect of 

 coastal groin design. 



RULE S: The groin-induced accumulation of longshore drift an the fore- 

 shore will modify the beach profile, which will then try to 

 reestablish its natural shape. 



The shore-normal profile of a beach, from the highest limit of wave uprush 

 to the seaward limit of sediment movement, is the transient result of sand 

 particle movement as dictated by waves, currents, sand size, and beach slope 

 (through the action of gravity). When one of these controlling factors is 

 changed, the profile will also change through sand movement. The accumulation 

 of sand in the foreshore zone by groins changes the beach profile at its 

 shoreward end. The reaction to this change will be erosion of the foreshore, 

 accretion of the nearshore, or both, in the profile's attempt to reestablish 

 its balance. These effects may cause differential settlement of graded beach 

 material along the beach profile. This reestablishment can be accomplished in 

 a number of ways. For example, the natural onshore movement of sand by swell 



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