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For an accurate flow measurement in open channel, an arrangement for aeration under the nappe is necessary when the weir used is:

For an accurate flow measurement in open channel, an arrangement for aeration under the nappe is necessary when the weir used is: Correct Answer Suppressed rectangular

Suppressed Rectangular Weir Problem:

  • The thin jet of water passing through the weir opening is called the nappe
  • There should always be air under the nappe of a sharp-crested weir
  • An arrangement for aeration under the nappe is necessary when the weir used is suppressed rectangular weir
  • This may require the installation of an air vent pipe through the sidewall of the channel to provide air under the nappe
  • If the nappe is not aerated, the weir will deliver more flow than that actual flow

[ alt="weir 2" src="//storage.googleapis.com/tb-img/production/19/05/weir%202.PNG" style="width: 264px; height: 171px;">

[ alt="nappe1" src="//storage.googleapis.com/tb-img/production/19/05/nappe1.PNG" style="width: 203px; height: 403px;">

Note:

There are different shapes of sharp crested weir but the most commonly used are

  1. Rectangular Weir
    1. Contracted Rectangular
    2. Suppressed Rectangular
  2. Triangular weir
  3. Trapezoidal weir

[ alt="weir" src="//storage.googleapis.com/tb-img/production/19/05/weir.PNG" style="width: 356px; height: 360px;">

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