278 



mason. SHORE-CONTROL PROBLEMS 



[Ch. 15 



stabilization in position of the inlet channel to serve the interests of 

 navigation. The engineer recognizes that the natural conditions repre- 

 sent an equilibrium situation involving, among other factors, the rate 

 of transport of material alongshore and the tidal or flow characteristics 

 of the inlet. Equilibrium will be destroyed by dredging of a chan- 



FiG. 2. Cold Spring Harbor, New Jersey. A wide beach has been formed by 



material trapped by jetties (right). The downdrift beach (left) is starved and 



subject to serious erosion. April 1940. (Photo by Aero Service Corp.) 



nel, and natural forces will tend to re-establish equilibrium, in part at 

 least, by filling the dredged channel. Jetties, or structures parallel to 

 the channel axis and extending from shore seaward to the end of the 

 dredged channel, are usually provided to eliminate refilling of the 

 dredged channel. These structures, however, also inhibit or prevent 

 the natural movement of material alongshore, thus establishing a new 

 regimen which probably will involve starvation of the downdrift 

 beaches due to interruption of the natural littoral supply (Fig. 2). 

 The engineer's problem then is so to design the navigation works as 

 to interfere as little as possible with the natural littoral transport of 

 material, yet protect the navigation channel against filling, and to pro- 



