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In a submerged rectangular weir, an arrangement for aeration of nappe is essential for which of the following reasons?

In a submerged rectangular weir, an arrangement for aeration of nappe is essential for which of the following reasons? Correct Answer <span style="">For having a constant head-discharge relation independent of time</span>

Explanation:

Submerged weir:

  • When the water level on the downstream side of a weir is above the crest of the weir, then the weir is called to be a submerged or downed weir.
  • The total discharge, over the weir, is obtained by dividing the weir into two parts.
  • The portion between upstream and downstream water surface may be treated as a free weir and the portion between downstream water surface and crest of the weir as a drowned weir.

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  • 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 otherwise negative pressure will be created which may damage the weir structure on the downstream side.
  • 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.
  • Therefore, in order to maintain constant discharge, aeration is a must.

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