INTAKES 101 



water passages through racks, and in the design allowance should 

 be made for the accumulation of trash as a factor in the restriction 

 of water passage. 



Low-head Installations. With low-head plants the intake 

 generally forms a part of the dam or power-house, as shown in 

 Fig. 26 under section " Low-head Developments." The up- 

 stream bay comprises the gate room, and by thus installing the 

 gates and screens indoors, there is less danger from ice forming 

 therein during cold weather. In certain stations arrangements 

 are also made whereby the heated air from the generators can be 

 led to the gatehouse for preventing the formation of ice. 



The water from the forebay enters the gatehouse through 

 arches in the front wall, and by submerging these below the low- 

 water level certain floating material will be prevented from 

 entering. 



High-head Installations. For high-head plants the intakes 

 are often built as independent structures, and where overflow 

 diversion dams are used, they should preferably be located at a 

 right angle to the dam. This arrangement has several advantages, 

 among which are the ease with which logs, trees and other float- 

 ing debris can be cleared away by simply opening one or two of 

 the nearest flashboards. 



The intake shown in Figs. 51 and 52 represents a typical 

 installation of the latest design. It is a caisson-like, self-contained 

 structure, divided by partitions into five sections in order to resist 

 the stresses on the outside walls due to the hydrostatic pressure 

 when the intake is empty and the water in the pond is at its 

 maximum elevation. At the rear of each division wall there is an 

 opening which allows the water to pass to the tunnel entrance 

 located at the center and bottom of the rear wall. 



There are two sets of racks, a coarse set consisting of f-inch 

 round iron rods spaced 4 inches apart, being placed in front of the 

 head gates to prevent large debris from interfering with their 

 operation. In addition, there is a fine set mounted in an inclined 

 position in each of the intake chambers. These racks are made 

 of 4 X f-inch flat iron bars, spaced 1 inches apart. They are 

 provided with a rack cleaner, each rack section being cleaned by 

 three rakes placed in a staggered position and operated at a 

 speed of 3 feet per minute by means of link chains from a motor- 

 driven countershaft located on top of the structure. At the 



