STORAGE RESERVOIRS 



161 



maps, the topography being taken by means of transits and stadia, 

 and the contours plotted as in Fig. 83. The area is found by 

 planimeters and the volume by multiplying the vertical distance 

 between the contour levels with the mean area of the sections. A 

 certain dead space must be allowed at the bottom of the reservoir 

 as it is not advisable to draw off the water from the bottom level 

 on account of the silt and mud which accumulates there. The 

 following table gives the capacity of the reservoir site outlined 

 in the above figure: 



TABLE XXXV 

 RESERVOIR CAPACITY 



The unit measure of stored water is generally the " acre-foot," 

 representing 43,560 cubic feet, and the curves in Figs. 84 and 85, 

 show the kilowatt-hours for different acre-feet storage on various 

 heads, and vice versa, the over-all hydro-electric efficiency being 

 assumed to be 65 per cent. 



It has also been proposed to adopt the " square-mile foot " 

 as a unit for expressing large quantities of stored water. This' 

 is equivalent to 27,878,400 cubic feet or, 640 acre-feet. 



The building of storage reservoirs involves many engineering 

 problems, the most important being the dam construction, which 

 was treated in Chapter IV. Spillways must be provided for dis- 

 charging excess flood waters, and with earthen dams or masonry 

 dams of considerable height, outlets, in the form of tunnels or 

 otherwise, are generally provided some distance from the dam to 

 prevent any possibility of damage to the same. Provision must 

 also be made for outlets at the bottom of the reservoir, so that 

 excess accumulation of silt and mud may be sluiced away. 



