Which two devices are used to measure open-channel flow?

• A. Weir and Parshall Flume
• B. Venturi meter and orifice plate
• C. Float gauge and ultrasonic sensor
• D. Magnetic flowmeter and turbine flowmeter

Answer: (A)

Open-channel flow is measured by devices that relate the water surface height to discharge. A weir creates a known head over its crest, so the discharge can be calculated from that head using standard weir formulas. A Parshall flume is a specially shaped converging chamber that creates a measurable head difference as water flows through, allowing accurate discharge calculations for larger open channels. Venturi meters and orifice plates are intended for fully enclosed pipes and rely on pressure changes in a closed conduit, not open channels. A float gauge or ultrasonic sensor only measures water surface level and must be paired with a stage–discharge relationship to infer flow, not by itself providing the direct discharge measurement open-channel devices offer. Magnetic and turbine flowmeters require a contained pipe and aren’t practical for open-channel flow. Therefore, the two devices used to measure open-channel flow are a weir and a Parshall flume.