AMOUNT OF ENERGY AVAILABLE 675 



AMOUNT OF ENERGY AVAILABLE 



The two principal factors which enter into the determination 

 of the available energy of a stream are the fall or head and the 

 quantity of water flowing. 



The head is usually limited by the cost of the overflowed lands, 

 and the fall may be either naturally concentrated at one point in a 

 cascade or it may be artificially concentrated, for the purpose of 

 development, by combining the fall of several cascades or a series 

 of rapids. This may be accomplished by either of two methods: 

 First, by building a dam at the downstream end of the rapids to 

 impound the water so that the entire fall is concentrated at the 

 dam; or, second, by building a dam at the upstream end of the 

 rapids and conducting the water through a closed pipe to the lower 

 end of the rapids, where the resulting head and pressure will be 

 exactly the same as in the first instance. A variation in the latter 

 method consists in diverting the water from the natural channel 

 at the head of the rapids and carrying it to a canal or flume, on a 

 slight down grade, along the side of a hill to a suitable point, and 

 there erect a forebay from which the water is turned into pen- 

 stocks which run directly down the slope to the stream, where the 

 power-house may be located. The latter method, involving the 

 construction of an open canal or flume, is open to the objection 

 that trouble may be experienced from the accumulation of ice 

 in the winter time. The first two methods described are the 

 most common. 



The second quantity to be determined was the water flowing 

 in the stream per unit of time, usually expressed in cubic feet per 

 second, but for low-head developments the two factors of head- 

 and stream-flow are, as a rule, inseparable, as the head fluctuates 

 considerably with the different stages of the stream. 



To be of value the stream-flow data should extend over a 

 period of several years (fifteen to twenty) in order that the min- 

 imum as well as the maximum flows which may be expected, and 

 their duration, may be known, and while the average flow charac- 

 teristics are of interest they are not of very great value. 



The United States Geological Survey and various states have, 

 for many years, carried on a systematic stream-flow measure- 

 ments, and data are now available for streams in nearly all sections 

 of the country. There are, however, a large number of streams, 



