4 a THE IMPROVEMENT OF RIVERS. 



The principal of these devices are as follows: 



Gates. M. Sorrel and M. Collin * applied upon the Garonne and the Loire, respec- 

 tively, more than a half century ago, movable gates which were so placed as to contract 

 the current over a bar and produce scour by the increased velocity. Thus a passage 

 for boats was created by a simple displacement of material. The gates were reset 

 as often as required. 



M. Fouache used a trapezoidal leaf having teeth along its lower edge to deepen 

 the Somme Canal. It was supported at the desired height by boats, so as to confine 

 the stream and thus increase the water-level above and reduce it below. The head 

 thus produced moved the gate along the canal, and with it the material torn up by the 

 teeth as the leaf traveled. The depth of the cutting-edge was regulated by windlasses 

 on the boats. 



Engineer Masquelez, in 1811, used a similar device in building the canals at the 

 mouth of the Charente. It was placed astern of a suitable boat, the bottom being 

 loosened with a hook. Two movable wings served to close the canal behind the boat 

 and thus produced the head necessary for movement, the bottom of the canal being 

 cut as with a plane and pushed along toward the outlet. 



Screws. " About the earliest application of this principle on the Mississippi was 

 in 1867, when it was decided to improve Pass a 1'Outre by means of excavating and 

 stirring up the alluvial material deposited from the heavily laden waters of the river.t 

 In this work a double-ended dredge-boat, having an excavating-screw with four blades 

 14 feet in diameter, was used. This screw was similar to an ordinary propeller-wheel 

 and was similarly mounted. It was turned by means of a double engine at the rate 

 of 60 revolutions per minute, and reached a depth 2 feet below the keel. The work 

 of the screw was made more effective by auxiliary scrapers attached to the up-stream 

 end of the boat, on each side of the keel. The boat was moved down stream over the 

 bar with the screw operating and the scrapers in position. In this way some of the bar 

 material was again brought into suspension ' and carried off into deep water by the 

 current. 



" During the first month's work with this dredge the depth was not materially 

 improved. Later, better success was realized, and in a little less than two months 

 the depth was increased from n to 17 feet. The chief difficulty seemed to be in weak 

 propeller-blades, which were frequently broken and could only be renewed by docking 

 the vessel. This device was intended to cut out and maintain a 20-foot channel through 

 the bar at the mouth of the river. 



" At the Southwest Pass the same result was expected from the use of conical screws 

 attached to the bow of a suitable boat. These cones were 20 feet long and 5 feet in 

 diameter at their bases. They were set so that their points came together at the boat's 

 stem, and their bases were separated so as to cover a width of 20 feet from out to out. 



* Annales des Fonts et Chauss6es, 1835. 



f Trans. Am. Soc. C. E., vol. xl, pp. 223-226, J. A. Ockerson. 



