Weirs – effect of nappe condition on discharge Compared to a freely falling (aerated) nappe, the discharge over a sharp-crested weir with a depressed nappe is typically 6–7 percent __________. Choose the correct relation.

Introduction / Context:Nappe behavior over a sharp-crested weir influences the effective head and discharge. A “free” (aerated) nappe separates cleanly from the crest, while a “depressed” nappe clings due to sub-atmospheric pressure beneath it if aeration is inadequate.

Given Data / Assumptions:

• Sharp-crested weir with insufficient nappe aeration leading to depression.
• Similar upstream head and geometry for comparison.
• Standard laboratory observations and rating corrections.

Concept / Approach:When the nappe is depressed, the pressure under the nappe drops below atmospheric. This effectively increases the driving head over the crest, yielding a higher discharge than the free nappe at the same upstream head—typically by several percent (commonly quoted as about 6–7%).

Step-by-Step Reasoning:

Verification / Alternative check:Many design manuals specify aeration vents to prevent nappe depression and maintain calibrated ratings, implying that lack of aeration artificially increases discharge readings.

Why Other Options Are Wrong:

• “Less than” contradicts physical behavior.
• “Equal to” ignores the pressure change effect.
• “Indeterminate” is not accurate for the stated typical condition; the increase is well documented.
• “Zero” is physically impossible.

Common Pitfalls:Using free-nappe coefficients when the nappe is not properly aerated, leading to systematic flow overestimation.

Final Answer:more than