permeable or impermeable, and fixed or adjustable, according to their design 

 and construction. A high groin, extending through the surf zone for ordinary 

 or moderate storm waves, initially entraps nearly all of the longshore moving 

 sand within that intercepted area until the accumulated sand fills the entrap- 

 ment area and the sand passes around the seaward end of the groin to the down- 

 drift beach. Low groins (top profile no higher than that of desired beach 

 dimensions or natural beach elevation) trap sand like high groins. However, 

 some of the sand also passes over the top of the structure. Permeable groins 

 permit some of the wave energy and movement of sand through the structure. 



Jettves are structures used at inlets to stabilize the position of the 

 navigation channel, to shield vessels from wave forces, and to control the 

 movement of sand along the adjacent beaches so as to minimize the movement of 

 sand into the channel. The sand transported into an inlet will interfere with 

 navigation and will usually necessitate more frequent dredging to maintain the 

 navigation depth. Because of the longshore transport reversals common at many 

 sites, jetties are often required on both sides of the inlet to achieve com- 

 plete channel protection. Jetties are built from a variety of materials, 

 e.g., timber, steel, concrete, and quarrystone. Most of the larger structures 

 are of rubble-mound construction with quarrystone armor and a core of less 

 permeable material to prevent sand passing through. It is the impoundment of 

 sand at the updrift jetty which creates the major impact. When fully devel- 

 oped, the fillet of impounded sand extends well updrift on the beach and 

 outward toward the tip of the jetty. 



Like the groin, the jetty's major adverse impact is the erosion of the 

 downdrift beach. Before the installation of a jetty, nature supplies sand by 

 intermittently transporting it across the inlet along the outer bar. The 

 reduction or cessation of this sand transport due to the presence of a jetty 

 leaves the downdrift beach with an inadequate natural supply of sand to 

 replace that carried away by littoral currents. 



To minimize the downdrift erosion, some projects provide for dredging the 

 sand impounded by the updrift jetty and pumping it through a pipeline (by- 

 passing the inlet) to the downdrift eroding beach. This provides for 

 nourishment of the downdrift beach and may also reduce shoaling of the 

 entrance channel. If the sand impounded at the updrift jetty extends to the 

 head or seaward end of the jetty, it will move around the jetty and into the 

 channel causing a navigation hazard. Therefore, the updrift impounded sand 

 should be bypassed to the downcoast beach, not only to reduce downdrift 

 erosion but also to help maintain a safe navigation channel. 



A more recent development for sand bypassing provides a low section or weir 

 in the updrift jetty over which sand moves into a sheltered predredged, 

 deposition basin. By dredging the basin periodically, channel shoaling is 

 reduced or eliminated. The dredged material is normally pumped across the 

 navigation channel (inlet) to provide nourishment for the downdrift shore. 

 A we-lv jetty of this type at Murrells Inlet, South Carolina, is shown in 

 Figure 1-13. 



1-24 



