CHAPTER VI 

 STORAGE RESERVOIRS 1 



MANY watersheds have some natural storage features tending 

 to equalize the stream-flow as compared with the rainfall, while 

 with others surplus water in times of high flow can only be held 

 back for use in times of low flow by the construction of artificial 

 reservoirs. 



Storage and Pondage. The impounding and accumulation 

 of surplus water which may be utilized when needed is termed 

 either " storage " or " pondage.'* The former generally refers 

 to reservoirs located on a watershed at some distance from the 

 power-house, and where large quantities of water may be im- 

 pounded for use during the dry season. " Pondage," on the other 

 hand, refers to the storage for taking care of the daily fluctuation 

 in the load curve, otherwise canals, flumes and pipe lines will 

 have to carry the peak flow of water instead of the average. It is 

 often the case that the average demand for power during twelve 

 or fourteen hours of the day is twice as great as the demand for 

 the remaining ten or twelve hours. The small volume of power 

 required during a portion of the day permits an accumulation of 

 water at 'the power dam itself which can be used as a reserve force 

 to meet the higher demand during the other portions of the day. 

 Thus, a stream that during the twenty-four hours might develop 

 a continuous horse-power would, if relieved of half of the demand 

 for half of the day, be able, with small pondage, to supply con- 

 siderably more than the average during the remaining portion of 

 the day. 



The importance of pondage should, however, not be exag- 

 gerated, as it can only be utilized at the expense of operating head, 

 but to counteract this it is possible to provide temporary flash- 

 boards by which the normal level may be raised several feet. 



The storage is, however, of the greatest importance, as it will 

 usually greatly increase the earning capacity of any development. 



1 See also section on " Water Storage," 

 159 



