Critical-depth control meters: recommended throat length for reliable control For efficient operation of a critical-depth control meter (e.g., a long-throated flume), the throat length should be approximately how many times the critical depth to ensure stable critical-flow control?

Introduction / Context:
Critical-depth flow meters (long-throated or control flumes) establish a control section where flow passes at Froude number approximately 1, enabling discharge to be related to stage. The length of the throat is vital: it provides sufficient distance for flow to adjust and maintain near-critical conditions with minimal disturbance from approach and exit effects.

Given Data / Assumptions:

• Open-channel, subcritical approach flow transitioning to control at the throat.
• Well-formed approach and smooth, prismatic control section.
• Critical depth dc is the characteristic depth at the control section.

Concept / Approach:

A sufficiently long throat relative to dc helps ensure: (1) a uniform velocity distribution at control; (2) diminished influence of local eddies; and (3) robust rating stability. Practical design guidance recommends a multiple of dc rather than a minimal equal length.

Step-by-Step Solution:

Verification / Alternative check:

Empirical and field experience in long-throated flume design supports the 3 * dc guideline as a balance between hydraulic stability and construction economy.

Why Other Options Are Wrong:

• Equal to / 2 * dc: Often too short for stable control in field conditions.
• 4 * dc or 10 * dc: More conservative than necessary; adds cost/head loss without typical need.

Common Pitfalls:

• Ignoring approach conditions and surface roughness, which can necessitate slightly longer throats.
• Assuming lab-calibrated minimums apply unchanged to field installations.

Final Answer:

Three times the critical depth.