sio THE IMPROVEMENT OF RIVERS. 



In the needle dam the water is dammed up by vertical planks, called needles, 

 resting against bars connecting the trestles at the top. and against a sill in the river- 

 bed at the bottom. The trestles are spaced from 3 to 8 feet apart, and when not in 

 use lie down across the stream, being protected from injury by the sill. A walkway 

 connects them when standing. 



The Chanoine wicket is an upright shutter, hinged near its middle to a horse con- 

 nected to the floor, and also to a prop which rests against a shoe on the floor. The 

 displacement of the end of the prop permits the wicket to fall with the current. 



The Bould gate and Camer6 curtain replace the needles by small gates and by cur- 

 tains respectively, resting directly against the trestles or against uprights leaning on 

 the trestles. 



In the overhead bridge dam, where the closure is always composed of such gates 

 or curtains, the supports are all drawn up to the bridge when not in use, while their 

 bottoms rest against a sill in the river-bed when in use. 



These are the principal types of dams, and practically the only ones employed. 

 Many modifications of each one have been suggested, but rarely, if ever, put into 

 practice, and will therefore not be described.* 



Height of Lift. The lift of movable dams has, until recent years, been very 

 moderate. It was the idea of the earlier engineers, who did not possess the appliances 

 of the present day, that all parts must be small enough to be maneuvered by hand, 

 and in consequence the head of water had to be small. Thus on the first needle dams 

 the head was limited to about 4 feet. As experience was gained, however, the lifts 

 were gradually increased, until we find examples to-day of needle dams with lifts of more 

 than 10 feet, and a curtain dam at Poses, on the lower Seine, with a lift of nearly 14 

 feet. Several Boul6 dams are also in existence with lifts of 10 to 1 2 feet. 



The lift of a Chanoine dam, however, has rarely exceeded 8 feet. No recent 

 examples of this type are to be found in Europe, where other methods of closure have 

 come into use, and in consequence the Chanoine wicket has never received its proper 

 development. There is no reason, however, why it cannot be used for much greater 

 lifts than heretofore, since it simply means the application of heavier parts to hold 

 back the water, and heavier machinery with which to perform the operations, than 

 are now used. 



The greatest drawback of movable dams has been that their cost was far too great 

 for the amount of river made navigable thereby ; in other words, the lift attained was too 

 small to justify the expense, and their success cannot be considered complete until 

 they have been applied to lifts at least equal to those which would have been given to 

 fixed dams at the same points, and at not much greater cost. 



General Design. The designing of a movable dam, in order to secure the best 

 results, is one of the difficult problems of engineering, and requires not only a thorough 



* Most of these modifications, such as the Stickney Lifting Dam, the Marshall Bear-traps, the Janicki Fold- 

 ing Dam, the Vrnable Tunnel Dam, etc., will be found described in the pacer on "Movable Dams," Transactions 

 of the American Society of Civil Engineers, June, 1898. 



