144 



SLOWLY VARIED FLOW 



but now (5i — ^Oi • At) and (52 + ^02 • At) are functions of F and E 

 instead of E only. A direct solution may be obtained by use of the 

 sliding charts illustrated in Fig. 1104. 



The index at the left end of the upper slide is set in line with the inflow 

 for the step, read on the scale at the left of the lower slide. The point 

 corresponding to the value of the stage and tributary flow at the begin- 

 ning of the step is located on the lower slide. Assume that this point is 

 the point A shown in the figure. If the tributary flow does not change 



Value of S+ iio-M at End of Step 



Fig. 1104. Slide Chart for Flood Routing When Discharge from Reservoir is 

 Affected by Backwater from a Downstream Tributary. 



The upper slide is drawn in black on thin celluloid, and the lower slide in red on white board. The 

 two parts are then fixed so that they will slide with their base lines in coincidence. The dotted lines 

 and figures on the upper slide may be omitted since their position will be directly abov he same lines 

 and figures on the lower scale. 



appreciably during the step, the stage at the end of the step may be read 

 on the upper slide, directly above point A. If the tributary flow does 

 change appreciably during the step, the stage at the end of the step is 

 determined approximately by the location of point B, in line with A, 

 but at the correct value of the tributary flow at the end of the step. 

 This procedure should not be depended upon to give good results if the 



