THE IMPROVEMENT OF RIVERS. 



35 feet contours of not less than 1000 feet. It is proposed to secure the necessary chan- 

 iK-1 by dredging most of the matt-rial, and then to maintain it by two linos <>f jetties 

 which will not only increase the current sufficiently to prevent silting up, but also pro- 

 tect the channel from unobstructed action of the waves. The jetties will extend to 

 the 20-foot contour on the outer slope of the bar and no attempt will be made to h. 

 them parallel, except near the outer end. Their distances apart between centers varies 

 from about 7000 feet to a minimum of 3000 feet. Owing to the fact that the bar will 

 be removed by dredging, it will be possible to maintain the channel with jetties so 

 widely separated, but if the walls were relied upon to do the burden of the work it 

 would be necessary to bring them nearer together. This wide distance apart will per- 

 mit the placing of the dredged material along the inner faces and thus prevent under- 

 mining. The jetties are to be carried only to the height of mean high tide. 



In their construction it is proposed to lay flexible foundation mattresses about 

 two feet thick and 150 to 200 feet in width, surmounted by timber grillages two 

 courses high, sunk with rock in the usual manner, throughout the full length of the 

 jetties and for 1000 feet beyond, before any part of the superstructure is started. The 

 latter will consist of a second mattress covered with stone in shallow water, while in 

 depths greater than three feet it will be cf concrete blocks or large stones or of con- 

 crete laid in place in large bags, possibly containing as much as one hundred tons each. 

 Another method suggested is to build the walls directly upon the grillage in quiet 

 water in monoliths having the full cross-section of the jetty and of a length conveniently 

 handled 20 to 60 feet and then to sink the grillage on to the mattress foundation, 

 the grillage being held up between barges until the monoliths are completed. The 

 dimensions proposed are a width at the crown equal to the depth of the water at mean 

 high tide, not, however, exceeding nine feet. The side slopes would be made two vertical 

 to one horizontal, and the depth of the block would be four and one-half feet more than 

 the depth of the water at mean high tide, to allow for settlement, less the thickness of 

 the grillage necessary to reduce the weight per square foot on the foundation to the 

 prescribed limit, 300 Ibs. The Board having the matter in charge conclude their report * 

 as follows: 



" In seeking the solution of the problem before it the Board has given great weight 

 to two important favorable factors, which have not heretofore in similar cases consti- 

 tuted such vital elements of such a problem, namely: The extraordinary progress re- 

 cently made in the improvement of dredging machinery and the fact that the appli- 

 cation of the jetty system to the improvement of the mouth of a very large sediment- 

 bearing stream differs radically from the more familiar case of the application of the 

 same system to the entrance of the ocean harbor obstructed by a sand bar. The first 

 of these favorable factors makes it possible to obtain the desired channel at the South- 

 West Pass by dredging, thereby reducing the function of the jetties to that of merely 

 assisting in the maintenance of the channel. The second factor referred to has made 



* Annual Report, Chief of Engineers, U. S. Army, 1900, p. 2297. 



