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CHAPTER IX.
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MEASUREMENT OF FLOWING WATER.*
Weirs.—The
direct measurement of flowing water in a stream or channel can be made
in various ways. Occasionally gauge wheels are used, but the method is
expensive. Gauging by rectangular overfalls (weirs) of certain
dimensions and under certain circumstances gives results within one per
cent, of absolute exactitude (Francis' formula).
In
employing this method the height above the crest of the surface of
still water, some little distance back from the weir, must be carefully
measured. It is also desirable that there should be no considerable
current to the water at the place of measurement.
Orifices.—Flowing
water is measured also by its discharge under pressure through an
aperture of regular section. Though it is not theoretically correct,
there will be no practical error in assuming the average head to be
from the centre of the aperture when the width is considerably less
than the height of the water above the top of the opening.
Open Channels.—The
measurement of the surface velocity of water passing through a flume or
canal of uniform size can be used to determine its discharge, and in
some cases the simple calculation of discharge made by
*
For details on the subject of the measurement of water see " The
Mechanics of Engineering," by Julius Weisbach, translated by E. B.
Coxe ; Francis' " Lowell Hydraulics " ; "Report Mississippi River," by
Humphreys and Abbot; "Hydraulic Manual," by Louis D'A. Jackson ; u
The New Formulae for the Mean Velocity of Discharge of Rivers and
Canals," by W. R. Kutter ; "Hydraulic Tables," by Thos. Higham ; "A
Treatise on Water-Supply Engineering," by J. T. Fanning ; " Experiments
on the Flow of Water," by A. Fteley and E. P. Stearns, vol. xii. "
Transactions of the American Society of Civil Engineers."
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