CHAPTER V. 



MOVABLE DAMS. 



History. A movable dam differs from a fixed dam in that the dam proper is sc 

 designed that it can be lowered or raised as may be needed, its principal object being 

 to provide slack-water in times of low water, without forming an obstruction to navi- 

 gation in moderate stages, or to floods. The last is an advantage of great importance 

 on rivers of low banks, as a movable dam does not increase the dangers of inundation. 



With the exception of the bear-trap dam, which is an American type, the inven- 

 tion of movable dams is due to the genius of the French engineers. Prior to 1830 the 

 fixed or stationary dam was the only one used for navigation purposes. These had 

 been in use on the Lot since the thirteenth century, and, with the introduction of locks 

 in the fifteenth century, had been constructed on many rivers, but they were open to 

 the same objection that exists to-day the principle is in part unfavorable to navi- 

 gation. Dams with small navigable passes had been used, but the ascent of the pass 

 was always very laborious. These openings or passes were closed either by beams 

 lying one upon another, supported against piles or piers at the ends, or by planks 

 resting against a sill in the river-bed at the bottom, and against a beam spanning the 

 opening at the top. When it was desired to open the passage the beams or planks 

 were removed, either one by one, or simultaneously, and the water rushed through 

 with great violence. Sometimes they were used for the purpose of producing arti- 

 ficial floods, by damming up the whole river until the level of the pool above the dam 

 had been raised to the desired height, when, by their sudden removal, the water escaped 

 and carried rafts or boats over the shallow places below. The operation of letting out 

 the water was called "flushing" or "flashing"; in this country on log streams it is 

 called "splashing." 



On the Yonne and one or two other rivers in France this system of navigation was 

 still in use recently, the flashes being commenced at prearranged times at the upper 

 end of the river and continued down stream successively as the boats neared the dams. 

 The passes were 25 to 40 feet wide. 



Falling gates or shutters, supported by props when upright, were built across a 

 fixed dam on the river Orb, in France, in the eighteenth century, forming the first 

 attempt at placing movable weirs on fixed dams. 



NOTB. Part of the matter in the following chapters, with some of the illustrations, is taken from a paper on 

 "Movable Dams," by B. F. Thomas, published in the Transactions of the American Society of Civil Engineers, 

 June, 1898, and republished by permission of that Society, and others of the illustrations are republished by 

 permission from "The Design and Construction of Dams," by Edward Wegmann, C.E. 



