Open-channel flow measuring methods include velocity-area calculation, weirs, and Parshall flumes. Which device is used to measure flow by constraining water to a fixed cross-section?

• A. Parshall flume
• B. Pitot tube
• C. Venturi meter
• D. Orifice plate

Answer: (A)

Parshall flume

In open-channel flow, a Parshall flume forces the water to pass through a fixed throat cross-section. Because the throat area is fixed, the discharge depends primarily on the head (water level) upstream of the throat. By measuring that head and using a standard calibration, you can determine the flow rate reliably. This fixed cross-section feature is what distinguishes the Parshall flume from other devices. Pitot tubes measure velocity rather than discharge, and Venturi meters or orifice plates are designed for closed conduits (pipes), not open-channel flow.