Open-channel discharge measurement: the volumetric flow rate of a river (free-surface flow) is most appropriately measured using which hydraulic structure or device?

Introduction / Context:
Rivers and canals convey water as free-surface (open-channel) flow. Measuring discharge in such systems uses hydraulic structures that relate upstream water level to flow rate through empirically or theoretically derived equations. Weirs and flumes are the classic choices for field hydrometry.

Given Data / Assumptions:

• The flow is open-channel with a free surface.
• We seek a practical field method for measuring total river discharge.
• Steady conditions and calibrated geometry are assumed.

Concept / Approach:
An open weir (e.g., sharp-crested, broad-crested, or V-notch) creates a controlled overflow. The upstream head over the crest is measured and used in a discharge equation of the form Q = C * b * H^(3/2) (for sharp-crested weirs; variations exist). By surveying water level and applying the correct formula and coefficients, operators determine river flow with good accuracy. In large rivers, current-meter or acoustic Doppler profiling is also used, but among the listed options, only the open weir is an appropriate open-channel device.

Step-by-Step Solution:
Install or identify a calibrated weir section.Measure upstream head (H) at the proper location to avoid velocity head errors.Apply the correct discharge equation and coefficient C.Compute river discharge Q and record stage–discharge relationship for repeat use.

Verification / Alternative check:
Portable flumes (Parshall, Palmer–Bowlus) serve similar purposes. Rating curves (stage vs. discharge) validated by current-meter traverses are standard hydrometric practice.

Why Other Options Are Wrong:
Pitot tube / vane meter / nozzle / Venturi tube: Designed for internal, pressurized pipe flow, not open-channel conditions.

Common Pitfalls:
Incorrect head measurement location, neglecting approach velocity corrections, or using coefficients for the wrong weir geometry can cause significant error.

Final Answer:
Open weir