MOVABLE DAMS. 



211 



knowledge of construction but also a practical acquaintance with the operation and 

 effects of movable dams in all their bearings. 



Every such dam not operated by the natural forces of the water should fulfill, as 

 far as possible, the following conditions: 



(1) The head of water sustained should not be less than that advisable for a 

 stationary dam at the same point. 



(2) The dam should be capable of being operated by the regular employees and 

 appliartces, both in lowering and raising, under full head, in whole or in part, without 

 much risk to the operatives. 



(3) The crest should be submersible to a sufficient extent to regulate the flow at 

 ordinary stages. 



(4) The leakage should not exceed the discharge of the stream at any season. 



(5) The parts should be complete in themselves without the introduction of addi- 

 tional means for sustaining the water, even in low-water seasons. 



(6) The cost should not greatly exceed that of a fixed dam for the same location. 



In general, movable dams are constructed in two or more sections, one for navi- 

 gation, called the pass, and one or more for the passage of surplus water, called the 

 weir. The object of the latter is to provide a means for passing small rises without 

 having to handle the heavy appliances of the pass. On wide rivers with two or more 

 weirs to the dam they are usually made of different heights, affording more or less 

 easy maneuvering. When the flood has reached the full discharge capacity of the 

 weir, and is still rising, the pass must be lowered, and if the dam has been properly 

 designed there will then be enough depth of water on the pass sill to allow full-draught 

 boats to cross it in either di- 

 rection ; that is, the contracted 

 current must not be swift 

 enough to embarrass up-going 

 craft. 



The pass is always located 

 next the lock, so as to throw 

 the currents created by the 

 weir as far from the entrances 

 as possible. It is usually 

 placed opposite the lower end 

 of the chamber, but where the 

 walls are long it is sometimes 

 placed near the middle. 



The choice of type is 

 usually decided by the charac- 



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/ 



FIG. 130. 



teristics of the river, and the results of experience in America and in Europe seem to 

 show that for rivers of slow rises needles, gates, or curtains are well adapted, the first 



