Open-channel measurement – Why prefer a control meter over a weir?\r Identify the most comprehensive reason a control meter is preferred to a weir in field channels, especially when streams are silt-laden and may carry debris.

Introduction / Context:
In irrigation and drainage engineering, flow-measurement devices must work accurately under real field conditions. Control meters (critical-depth flumes and similar controls) are often preferred over sharp-crested weirs for silt-laden channels that also carry floating debris. This question checks understanding of practical selection criteria.

Given Data / Assumptions:

• Silt-laden flow is common in earthen canals and field distributaries.
• Debris can accumulate on sharp crests and appurtenances of weirs.
• A control meter creates and maintains critical flow with a shaped control, not a thin sharp crest.

Concept / Approach:
Control meters are designed with gradually varied transitions that produce a stable head–discharge relationship under a range of approach conditions. They are less sensitive to floating debris and sediment accumulation than thin-plate weirs, which require clean crests and free nappe aeration for accurate calibration.

Step-by-Step Reasoning:
Silt issue: A control’s geometry promotes higher approach velocity at the throat, discouraging silt deposition, so measurements remain reliable.Debris issue: Control meters lack a delicate sharp crest; debris is less likely to affect calibration or damage the structure.Field robustness: The head–discharge relation of control meters remains dependable across varying field conditions.

Verification / Alternative check:
Design manuals specify that flumes (e.g., Parshall, cutthroat) are preferred where sediment and debris are present. They maintain accuracy without the strict crest and aeration requirements of sharp-crested weirs.

Why Other Options Are Wrong:

• Each of (a), (b), and (c) is correct but partial; the comprehensive answer that encapsulates all benefits is (d) All of the above.
• (e) contradicts established field practice.

Common Pitfalls:

• Selecting weirs for channels that cannot guarantee crest cleanliness or free-flow (submergence) limits.
• Ignoring sediment transport and maintenance realities in device selection.

Final Answer:
All of the above