Text Measurement of flow in open channels

The flow in large natural channels is usually measured with some types of meters. The flow in small streams and man-made channels may be more conveniently measured by other methods. Among the most common methods are the use of weirs and venturi-type flumes.

Weirs are the following: sharp-crested rectangular weirs, triangular or V-notch weirs and trapezoidal weirs. For weir to give accurate water discharge, the upstream face of the weir must be vertical and at right angles to the channel, and the crest of the weir must be horizontal. In addition atmospheric pressure should be maintained under the nappe, and the approach channel should be straight and without any obstruction. Corrosion of the weir crest or damage by floating debris may alter the weir coefficient.

When the water level downstream from a weir rises above the level of the weir crest, the weir crest is said to be submerged and under such conditions accurate flow measurement is not possible because surface disturbances of water downstream from the weir make it difficult to measure the depth of submergence.

The head on a weir is usually measured with a staff gage, though better results are obtainable with a hook or point gage in a stilling well. Inaccurate head determination will provide sizable error in measurements.

If water contains suspended particles, some of them will be deposited in the pool located above a weir, resulting in a gradual change in the weir coefficient. Moreover, the use of a weir results in a relatively large head loss. Both these difficulties are at least partially overcome by use of venturi-type flumes. For obtaining best results venture-flumes ought to be installed in a straight section of channel where flow conditions are relatively uniform. Usually venturi-flumes are made of galvanized iron, plain and reinforced concrete.