Appendix 



D 



Use of Weirs to Measure 

 Flow 



The discharge of water through a hatchery channel can be measured easily 

 if a Cippoletti or a rectangular weir (Figure D-l) is built into the channel. 

 The only measurement needed is that of the water head behind the weir; 

 the head is the height the water surface above the crest of the weir itself. 

 Reference of this head to a calibration chart (Table D-l) gives the 

 corresponding discharge in gallons per minute. 



Water-flow determinations will be inaccurate if the head is measured at 

 the wrong point or if the weir has not been constructed carefully. The fol- 

 lowing considerations must be met if weir operation is to be successful. 



(1) The head must be measured at a point sufficiently far behind the 

 weir. Near the weir, the water level drops as water begins its fall over the 

 weir crest. The head never should be measured closer to the weir than 

 2 2 times the depth of water flowing over the crest. For example, if 2 

 inches of water are flowing over the weir crest, the head should be meas- 

 ured 5 inches or more behind the weir. A practical measuring technique is 

 to drive a stake into the channel bottom so that its top is exactly level with 

 the weir crest. Then, the head can be measured with a thin ruler as the 

 depth of water over the stake. A ruler also can be mounted permanently on 

 the side of a vertical channel wall behind the weir, if such a wall has been 

 constructed. 



(2) The weir crest must be exactly level and the weir faces exactly verti- 

 cal, or the standard head-to-discharge calibrations will not apply. 



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