SPILLWAYS FOR RESERVOIRS AND CANALS. 27 



spillway 230 feet long and through two sluiceways into the canal of 

 still another plant of a hydroelectric company. Obligations fixing 

 the disposal of the water over the spillway provided that the head 

 must not exceed 2 inches, and since on the one hand the discharge was 

 variable with 160 second-feet as a maximum, the demand of the power 

 plant for water was also variable, so that close regulation and con- 

 tinuous attention were imperative. The power company had built 

 a 20-foot spillway, but the water often rose 10 inches above the 

 permissible stage and thereby interf erred with the operation of the 

 plant above. To eliminate the disadvantage to all concerned, the 

 twine mill built a siphon for a computed discharge of 70.6 second- 

 feet and the power company built one for a computed discharge 

 of 88.3 second- feet. The latter was built on the old spillway, which 

 was broken through and covered with a reinforced concrete hood. 

 The head between pond and -tail water was only 4.92 feet, but the 

 completed structure gave a test discharge of 91.8 second-feet. The 

 cross-section of the siphon was uniform and of an area of 9.47 square 

 feet, giving an actual velocity of 9.7 feet per second and correspond- 

 ing value of 0.55 as a velocity coefficient. This case is described in 

 detail to illustrate the adaptability of the structure and to emphasize 

 its value in just such cases of conflicting regulatory requirements, of 

 which there are many. 1 



TENNESSEE POWER COMPANY'S SIPHONS ON OCOEE RIVER. 



Where the canal of the Tennessee Power Company crossed a ravine 

 it was necessary to design a structure capable of spilling irom 1,300 to 

 1,400 second-feet. The structure must also take care of a rise of 1 

 foot in water elevation in a period of 8 seconds in case the power 

 plant just below it should be suddenly shut down or in case of 

 stoppage of flow due to slides or other obstruction just above the 

 f orebay. The 1 foot referred to was the limit of freeboard in the canal 

 at the point. An overflow spillway capable of satisfying the condi- 

 tions would have been 400 feet long, and since the available space 

 in which the structure had to be placed was insufficient, a battery 

 of 8 siphons, each having a throat cross-section of 8 square feet, 

 (1 by 8 feet), was installed, with the addition of a sand gate. The 

 operating head on four of the siphons was 27.2 feet and on the re- 

 maining four 19.2 feet and the draft resulting produced an increased 

 velocity corresponding to the difference in elevation between the 

 water surface in the forebay and the center line of the siphon outlets, 

 minus the usual losses due to entry, pends, friction, etc. The 

 vertical draft tubes change gradually from the throat cross-section 

 at its upper end to 4 by 2 feet at the lower end, where it connects 



1 Data taken from a paper by Herr J. Huber and published in Engineering News, 

 May 3, 1913. 



