137 



zone is for the protection and maintenance of navigation. Almost 

 every harbor area in the United States requires some form of dredging 

 maintenance to maintain access and berthing space. This may take 

 the form of a channel six feet deep or one forty deep, depending upon 

 the ship traffic. Table IV.2.9 shows the amount of dredging required 

 by the Corps of Engineers to maintain the harbors of U.S. Ports. 



Jetties are a less common item on the coastal scene. These structures 

 are generally placed where it is necessary to protect a channel and are 

 usually built only where narrow harbor entrances are subjected to 

 shoaling and wave action. On the west coast of the United States jet- 

 ties are often used to form harbor enclosures as in Los Angeles Harbor 

 and Half moon Bay (figure IV.2.25) . 



Groins are not too frequently used in the estuarine environment. 

 Normally they are built along sandy coastal beaches to help control 

 beach erosion. The groins effectively interfere with the littoral trans- 

 port phenomena by trapping materials that would be carried away ; 

 they are used extensively along the east coast and in southern Cali- 

 fornia. 



TABLE IV.2.9.— ANNUAL HARBOR AND CHANNEL DREDGING AND MAINTENANCE COSTS 



Volume Number of 



dredged Cost, years of 



Biophysical region (cubic yards) dollars record 



North Atlantic _ 751,000 



Middle Atlantic 5,241,000 



Chesapeake Bay.. __ 6,123,000 



South Atlantic ,. 5,668,000 



Caribbean 123,000 



Gulf of Mexico 30,880,000 



Pacific Southwest 166,200 



Pacific Northwest 992,000 



Alaska... 6,900 



Pacific Islands 74,200 



Reference: The National Estuarine Inventory. 

 Source: U.S. Army Corps of Engineers. 



Utilizing barriers to protect the land from the fury of storms at sea 

 is a procedure that has been frequently proposed but little used. There 

 are two examples of hurricane barriers along the east coast, in New 

 Bedford, Mass., and Providence, R.I. Schemes have been developed 

 for other hurricane barriers in Narragansett Bay and Tampa Bay but 

 have not materialized. Feasibility investigations of a tsunami barrier 

 for Hilo Bay in Hawaii were conducted by the Corps of Engineers but 

 no construction has taken place. 



Major modifications of estuarine areas by land fill or marsh and wet- 

 land reclamation have occurred throughout the Nation. The area re- 

 claimed is generally the highly productive tidal marsh which is so 

 important to estuarine ecology. As an example, 80 percent of the 

 300 square miles of wetlands that originally surrounded San Fran- 

 cisco Bay have been filled. San Francisco Bay is not unique. Table 

 IV.2.10 lists areas of basic marsh and wetland habitat filled in the 

 past 20 years (figure IV.2.26) . Expanding residential and commercial 

 needs for more shoreline land and navigation spoil disposal require- 

 ments are the major causes of dredging and filling operations. 



Two-thirds of the total marsh and wetland areas are important fish 

 and wildlife habitat. Since the late 1940's, 7 percent of the im- 



