Question

Which of the following statements regarding construction and installation of weirs are true?
(A). The velocity of approach of water to the weir should not be considered.
(B). Weir crest should be placed closer than 2H from the bottom of the channel.
(C). The head over the weir should be measured on the upstream side at a distance of at least 4 times the head of flow from the weir.
(D). The falling water surface from the weir should not have free flow.

Choose the correct answer from the options given below:

• (C) only.
• (A) and (B) only.
• (B) and (D) only.
• (B) and (C) only.

Hint:

Always measure head upstream at \(x \ge 4H\) from the crest to completely bypass the surface drawdown region near the weir crest.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Weirs are overflow structures used to measure water flow in open channels. Precise installation and hydraulic conditions are mandatory to secure accurate discharge measurements.

Step 3: Detailed Explanation:
Let us analyze each design standard:
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Statement (A): The velocity of approach increases the head and discharge over the weir. For precise measurements, the channel must be wide and deep enough to keep the velocity of approach negligible, or it must be mathematically corrected. Thus, (A) is false.
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Statement (B): To prevent the channel floor from restricting flow contraction, the height of the weir crest above the channel bottom must be at least \(2H\) (where \(H\) is the maximum expected head). Thus, (B) is false.
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Statement (C): As water approaches the weir crest, it accelerates and the surface draws down. To measure the true static head \(H\), the head measurement tap or gauge must be located upstream at a distance of at least \(4H\) from the weir plate. Thus, (C) is true.
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Statement (D): For standard weir formulas to be valid, the falling nappe must flow freely with atmospheric aeration underneath. A submerged or restricted flow alters the discharge coefficient. Thus, (D) is false.
Therefore, only statement (C) is correct.

Step 4: Final Answer:
The correct option is 1, which corresponds to (C) only.