Figure 19. Stone revetment at Cape Henry, Virginia (Source: Reference 51) 



R.prop (1000 lbs to 6000 lbs. - 

 averaging 4000 lbs l.chmked 

 writh one mon stooe 



y-v 



4.4.4 Groins 



Groins are structures built at right angles to the beach to interrupt 

 and trap longshore sand movement (littoral drift) in order to stabilize 

 or widen a beach. Groins are constructed of timber, steel, concrete, or 

 rock [62]. The trapping of sand by a groin is done at the expense of the 

 adjacent downdrift shoreline. 



Groins are effective (1) when there, is a significant volume of 

 littoral drift, (2) when the drift carries coarse materials (greater than 

 0.2 mm), and (3) when the beach downstream from the groin can be sacri- 

 ficed (the sand gained at one place is denied to another) [63]. A row of 

 parallel groins tends to force the littoral drift of sand offshore because 

 much of the sand moves from tip to tip of the groin instead of moving 

 along close to the beach, thereby causing sand starvation of the whole 

 length of the beach (Figure 20) [62]. When a jetty system is constructed 

 in a coastal area, fill between the structures should be required to assure 

 natural beach equilibrium. 



4.4.5 Jetties 



Jetties are structures developed to modify or control sand movement 

 at inlets. They are constructed of steel, concrete, or rock, depending 

 on foundation conditions, wave climate, and cost. Although jetties are 

 longer and larger and are generally found at inlets or major physio- 

 graphical breaks in the shoreline. 



108 



