shelter for a floating dredge to pump the impounded material across the 

 navigation opening back onto the downdrift beach. This method is used 

 at Channel Island Harbor near Port Hueneme, California. 



1.56 GROINS 



The groin is a barrier-type structure that extends from the backshore 

 into the littoral zone. The basic purposes of a groin are to interrupt 

 longshore sand movement, to accumulate sand on the shore, or to retard 

 sand losses. Trapping of sand by a groin is done at the expense of the 

 adjacent downdrift shore unless the groin or groin system is artificially 

 filled with sand to its entrapment capacity from other sources. To reduce 

 the potential for damage to property downdrift of a groin, some limitation 

 must be imposed on the amount of sand permitted to be impounded on the 

 updrift side. Since more and more shores are being protected, and less 

 and less sand is available as natural supply, it is now desirable, and 

 frequently necessary, to place sand artificially to fill the area between 

 the groins, thereby ensuring a more or less uninterrupted passage of the 

 sand to the downdrift shores. 



Groins have been constructed in various configurations using timber, 

 steel, concrete or rock. Groins can be classified as high or low, long 

 or short, permeable or impermeable, and fixed or adjustable. 



A high groin, extending through the breaking zone for ordinary or 

 moderate storm waves, initially entraps nearly all of the longshore 

 moving sand within that intercepted area until the areal pattern or sur- 

 face profile of the accumulated sand mass allows sand to pass around the 

 seaward end of the groin to downdrift shores. Low groins (top profile no 

 higher than that of desired beach dimensions) function like high groins, 

 except that sand also passes over the top of the structure. Permeable 

 groins permit some of the wave energy and moving sand to pass through the 

 structure. 



1.57 JETTIES 



Jetties are generally employed at inlets in connection with naviga- 

 tion improvements. When sand being transported along the coast by waves 

 and currents arrives at an inlet, it flows inward on the flood tide to 

 form an inner bar, and outward on the ebb tide to form an outer bar. 

 Both formations are harmful to navigation through the inlet, and must be 

 controlled to maintain an adequate navigation channel. The jetty is sim- 

 ilar to the groin in that it traps sand moving along the beach. Jetties 

 are usually constructed of steel, concrete, or rock. The jetty type 

 depends on foundation conditions, wave climate, and economic considera- 

 tions. Jetties are much larger than groins, since jetties sometimes ex- 

 tend from the shoreline seaward to a depth equivalent to the channel 

 depth desired for navigation purposes. To be efficient in maintaining 

 the channel, the jetty must be high enough to completely obstruct sand 

 movement . 



1-17 



