SPILLWAYS FOR RESERVOIRS AND CANALS. 



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They are installed as regulatory devices on numerous canal systems, 

 where they work perfectly, because in such capacity they are kept in 

 working condition by more or less frequent use of the operating 

 parts. On spillway crests, Avhere they have a limiting range of possi- 

 bilities and their operation is less frequent, tending to make them 

 less sensitive to the reaction of theoretically determined pressures, 

 they are not as serviceable, unless the mechanical equipment necessary 

 to operate them is more complicated and correspondingly costly. 

 The moving parts must be maintained in the same perfect condition 

 as that upon which their design was based if they are to function as 

 they are intended, and such ideal conditions are rarely approached 

 in actual spillway operation. Most types depend upon some system 

 of counterweights to effect adjustment after they have been tilted 

 by the action of the water pressure beyond a certain point. Or they 

 may be so arranged that the water pressure on the gate or flashboard 

 will cause them to react upon a cylindrical weight in such a manner 

 as to make it ascend an incline by means of energy transmitted to 

 the ends of the cylinder through ropes or cables; or others are ar- 

 ranged so that the tilting of the gate transmits power to the shaft 

 from which counterweights are hung, such transmission being 

 through chains or cables. Various methods are provided by which 

 the introduction of cams or eccentric gearing may result in the com- 

 pensation of the. accelerated movement which might cause the slam- 

 ming down of the gates or their too rapid rise. 



An automatic form of spillway or floodgate has been installed on 

 the Cedar River, near Nashua, Iowa. The design is one of the many 

 patented types, but seems to be of simple operation. Figure 3 in 

 section shows an outline of the structure, which is mounted on the top 

 of a masonry dam 17 feet high and used for the generation of electric 

 power. Each panel of the gate is 46 feet long and is so set that it 

 will store 7 feet of water above the crest of the dam. To one end 

 of a walking beam there is attached a reinforced concrete counter- 

 weight and a bar connects the other end with the floodgates. The 

 gate is hinged at the bottom so that when the water rises above a 

 predetermined elevation the gate is forced down and the counter- 

 weight is raised. As the height of water increases the pressure still 

 further lowers the gate. When the gate opens the leverage between 

 the counterweight and fulcrum increases, while it decreases between 

 the fulcrum and gate hinges, and in this way overcomes the increased 

 weight of water at every stage of gate opening. 1 



ROLLING DAMS OR BARRIERS. 



Rolling dams or barriers have had their term of popularity and a 

 number of them were installed on dams in this country. Possibly 



1 Description from Engineering News-Record, Aug. 9, 1917. 



