164 STORAGE RESERVOIRS 



and erosive effect is reduced, while, on the other hand, there is 

 less danger of a shut-down in case there were only one gate open- 

 ing at the bottom, which would be liable to be clogged up by silt 

 and mud. 



Such intakes are often built in the form of towers, a typical 

 design being shown in Fig. 86. There are four square intake 

 openings placed from 18 to 21 feet apart vertically and at angles 

 60 to each other in the plan. The openings are provided with 

 screens and sliding steel gates which are controlled from the oper- 

 ating floor. There is also a secondary intake placed entirely inside 

 the tower, consisting of a standpipe 42 inches in diameter, built up 

 in four separate sections. Each section has a conical seat at the 

 upper and lower ends, and is seated on the one next below, the 

 bottom section seating on a heavy cast-iron elbow which connects 

 with the intake pipe. The water entering the intake openings 

 in the tower wall must, therefore, pass through the top of the 

 vertical standpipe and in this manner any silt or mud is pre- 

 vented from being carried along. As the water level goes down, 

 sections of the standpipe are removed. This is readily accom- 

 plished by means of a lifting gear, the pipe sections being closely 

 guided. 



Seepage and Evaporation. Consideration must also be given 

 to seepage and extreme care should always be taken to insure 

 imperviousness of the reservoir bottom. It may thus be neces- 

 sary to strip the top soil until impervious strata are reached, while 

 fissures may have to be closed. 



Evaporation must necessarily be taken into account when 

 determining the reservoir capacity. This loss can, however, 

 not be regulated, although a deeper and narrower reservoir will 

 have a less evaporation loss than a wider and shallower, 



