Jetties aid navigation by reducing movement of sand into the channel, 

 by stabilizing the location of the channel, and by shielding vessels from 

 waves. Sand is impounded at the updrift jetty, and the supply of sand to 

 the shore downdrift from the inlet is reduced, thus causing erosion of 

 that shore. Before the installation of a jetty, nature supplied sand by 

 transporting it across the inlet intermittently along the outer bar to the 

 downdrift shore. 



To eliminate undesirable downdrift erosion, some projects provide for 

 dredging the sand impounded by the updrift jetty and pumping it through a 

 pipeline (bypassing the inlet) to the eroding beach. This provides an 

 intermittent flow of sand to nourish the downdrift beach, and also prevents 

 shoaling of the entrance 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 pre- 

 dredged, deposition basin. By dredging the basin periodically, deposition 

 in the channel is reduced or eliminated. The dredged material is normally 

 pumped across the inlet to provide nourishment for the downdrift shore. 

 A weir jetty of this type at Masonboro Inlet, North Carolina, is shown in 

 Figure 1-9. 



1.58 BEACH RESTORATION AND NOURISHMENT 



As previously stated, beaches are very effective in dissipating wave 

 energy. When maintained to adequate dimensions, they afford protection 

 for the adjoining backshore. Therefore, a protective beach is classed as 

 a shore-protection structure. When studying an erosion problem, it is gen- 

 erally advisable to investigate the feasibility of mechanically or hydrau- 

 lically placing borrow material on the shore to form and maintain an ade- 

 quate protective beach. The method of placing beach fill to ensure sand 

 supply at the required replenishment rate is important. Where stabiliza- 

 tion of an eroding beach is the problem, suitable beach material may be 

 stockpiled at the updrift sector of the problem area. The establishment 

 and periodic replenishment of such a stockpile is termed artificial 

 beaoh nourishment. To restore an eroded beach and stabilize it at the 

 restored position, fill is placed directly along the eroded sector, and 

 then the beach is artificially nourished by the stockpiling method. 



When conditions are suitable for artificial nourishment, long reaches 

 of shore may be protected by this method at a relatively low cost per 

 linear foot of protected shore. An equally important advantage is that 

 artificial nourishment directly remedies the basic cause of most erosion 

 problems - a deficiency in natural sand supply - and benefits rather than 

 damages the adjacent shore. An added consideration is that the widened 

 beach has value as a recreation feature. A project for beach restoration 

 with an artificial dune for protection against hurricane wave action, com- 

 pleted in 1965 at Wrightsville Beach, North Carolina, is shown in Figure 



l-lOo 



1-18 



