32 BULLETIN 831, U. S. DEPARTMENT OF AGRICULTURE. 



where the old overflow spillway required a head of from 4 to 8 feet to 

 discharge the freshets, and, as is usually the case where the overfall 

 spillway is used, quantities of valuable water ran to waste over the 

 spillway after each storm. A battery of 10 siphons, each with an in- 

 ternal cross section (fig. 6) 6 feet 4 inches square and a working head 

 of 18 feet 11 inches has a calculated capacity of 5,250 second-feet. At 

 the same time the sill of the spillway was elevated 3 feet, producing 

 additional storage lot about 240 acre-feet. This additional ca- 

 pacity is a very important advantage in places where water is 

 valuable. It was, moreover, proved that the siphon gave a discharge 

 of 525 second- feet when the water rose to 4 inches over the lip, which 

 was a rise of 14 inches less than with the old spillway, thus reduc- 

 ing the water pressure against the dam correspondingly. The ad- 

 ditional storage referred to above represented an income of at least 

 $3,000 per year, and if capitalized at 5 per cent would produce a 

 total of $60,000, or at least six times the cost of the new spillways. 

 There are numerous examples of such possibilities in this country 

 known to the writer where the siphon spillway should be considered. 



In one of the large hydroelectric developments in the western part 

 of the United States there was constructed a dam over 100 feet in 

 height with a spillway section close to 600 feet in length. The 

 maximum head on the crest with a discharge of 100,000 second-feet 

 is 14 feet, with a freeboard of 4 feet. In other words, it was neces- 

 sary to design the structure so as to limit the range of rise in the 

 reservoir to 14 feet above the crest of the spillway and at the same 

 time take care of the maximum inflow. With a battery of spillways 

 of the siphon type such as was installed in the Sweetwater dam or 

 the Huntington Lake dam, in California, at least one-half of the 

 spillway area could have been saved with an additional storage of 

 about 8 feet of water in the top of the reservoir, where each foot 

 means an enormous amount of stored energy. In addition, much 

 closer regulation of the pond level would have been provided. 



In Italy the structure has been more fully developed and used, and 

 it is stated that there are more than 100 siphon spillways in use in 

 connection with Italian dams and canals. 



HUNTINGTON LAKE SIPHON SPILLWAY. 



Huntington Lake is located about 50 miles northeast of Fresno, 

 Calif., and is formed by the construction of three dams impounding 

 the waters of Big Creek, a tributary of the San Joaquin River. It 

 has a watershed area of 80 square miles and an average annual pre- 

 cipitation of about 31 inches. The dams are of concrete, of the 

 gravity arch type, two of them 100 feet in height and one over 150 feet 



