Fish Hatchery Management - fisheries & aquaculture

Appendi* DUse of Weirs to MeasureFlowThe discharge of water through a hatchery channel can be measured easilyif a Cippoletti or a rectangular weir (Figure O-t) 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 thecorresponding discharge in gallons per minute.Water-flow determinations will be inaccurate if the head is measured atthe wrong point or if the weir has not been constructed carefully. The followingconsiderations must be met if weir operation is to be successful.(l) The head must be measured at a point sufficiently far behind theweir. Near the weir, the water level drops as water begins its fall over theweir crest. The head never should be measured closer to the weir than2 j times the depth of water flowing over the crest. For example, if 2inches of water are flowing over the weir crest, the head should be measured5 inches or more behind the weir. A practical measuring technique isto drive a stake into the channel bottom so that its top is exactly level withthe weir crest. Then, the head can be measured with a thin ruler as thedepth of water over the stake. A ruler also can be mounted permanently onthe side of a vertical channel wall behind the weir, if such a wall has beenconstruc ted.(2) The weir crest must be exactly level and the weir faces exactly vertical,or the standard head-to-discharge calibrations will not apply.ll84