142 



SLOWLY VARIED FLOW 



the reading on the inflow scale of the rule of Fig. 1102 should equal the 

 reading of the outflow scales at the point where the two equal values 

 are lined up. The reading on the inflow scale of the rule of Fig. 1101 

 should equal the outflow corresponding to the lined-up values of ele- 

 vation. This property of the rule could be used to lessen the work of 

 plotting. It seems better, however, to plot both scales and then use it 

 as a check. 



PROBLEM 



llOL In estimating the cost of a fixed-outlet flood control reservoir, it is necessary 

 to determine the outflow from the reservoir for a flood having the inflow hydrograph 

 shown in the table below. The area-elevation data for the reservoir site are also 

 shown in the table. The discharge from the outlet conduit may be represented by 

 the equation o = 500£^'^ acre-feet per hour. Construct a slide rule and route the 

 flood through the reservoir. Use 4 hours as the length of step. In plotting the 

 scales, let 1 inch equal 10,000 acre-feet. This will require two pieces of " parting 

 stop " each 2 feet long. Select clear, straight pieces. 



Level-pool routing with variable stage-discharge relationship. The 



routing problem becomes more complicated when the discharge through 

 the outlets is not a single-valued function of the stage in the reservoir. 

 The discharge frequently depends upon other factors, such as draft for 

 waterpower, number of outlet gates open, or decrease in discharge 

 capacity of the outlets from submergence by backwater due to a down- 

 stream tributary. Many problems of this type can be set up for direct 

 solution by adaptation of the slide-rule method. 



