

IMPROVEMENT OF RIVER OUTLETS. 129 



tidal variation (14 inches normal), an average increase of depth to 12 available 

 feet.* The cost was about one-third that of double jetties. Another jetty of this type, 

 of small dimensions, was built in 1901 across a dry bar at Longport, New Jersey, for 

 the use of the Pennsylvania Railroad ferries, and is reported to have produced in 18 

 months a channel from 8 to 12 feet deep. 



A curved jetty concave to the channel was also built at the harbor of Swinemuende 

 on the Baltic Sea, many years ago, supplemented by a convex one on the opposite side. 

 It is stated that a considerable improvement resulted in the depth. In this instance 

 dredging is resorted to in order to secure the necessary width. 



Mississippi Jetties. Although a number of river outlets have been improved by 

 the jetty system in this country, the leading example has always been, and will con- 

 tinue to be, the works at the mouth of the Mississippi River. As in some of the more 

 prominent rivers abroad, an attempt was at first made to secure increased depth by 

 harrowing up the bottom so the current might carry away the deposit. This was 

 beneficial to some extent, but not entirely satisfactory, and an attempt was made to 

 combine this method with that of jetties, but it resulted in failure, owing to the lack of 

 stability of the structures and to their meager length. Dredging was next resorted to, 

 and this also proved ineffective for permanent results, and lastly the jetty system was 

 adopted for one of the outlets, the South Pass. This has given fairly satisfactory results 

 up to the present time, but the great increase of commerce and of the draught of ships 

 have shown the necessity for increased facilities, and it is now proposed to open up one 

 of the other outlets, with a channel of greater depth (35 feet). A very complete and 

 interesting history of the various attempts at improving the mouth of this river will be 

 found in the report of the Chief of Engineers for 1899, page 1914. This report shows 

 that not only stirring, jetties, and dredging had been tried, but also that a board had 

 made a favorable report upon the construction of a canal to the Gulf at a cost of ten 

 millions of dollars. This was almost immediately followed by the proposition of 

 James B. Eads (1874), to improve the entrance by double jetties, which, after numerous 

 modifications and delays, was adopted, the South Pass being selected, and the work 

 was completed in 1879. This contemplated 30 feet of depth of channel in the center, 

 with 26 feet of depth for 200 feet in width, the total width between jetties being 1000 

 feet. It is claimed that the channel required had been maintained between 1879 an( i 

 1899 with the exception of about five hundred days. The jetties were made slightly 

 curved in plan, and placed about 1000 feet apart, reduced later where necessary by 

 spur-dikes and inner jetties to about 600 feet. Recent legislation (1902) authorized the 

 improvement of the South-west Pass in order to secure a navigable channel of suitable 

 width with a depth of 35 feet at mean low water. Those having the matter in charge 

 decided upon 1000 feet for the width. This pass is about 15 miles in length, about 13 

 of which already have a central depth of not less than 40 feet and a width between the 



* Transactions Am. Soc. C. E., vol. xlii., p. 485 et seq. 



