Which of the following is a common open-channel flow measurement device?

• A. Weir
• B. Ultrasonic meter
• C. Parshall flume
• D. Magnetic flow meter

Answer: (C)

Open-channel flow is measured by devices that translate the water level, or head, into a discharge reading without enclosing the flow. A Parshall flume fits this idea perfectly: it is a fixed, narrow throat device that constricts the flow and creates a known relationship between the upstream water depth and the discharge. By simply measuring the water depth at a specified location upstream of the throat, you can determine the flow rate using standard calibration tables. This makes Parshall flumes reliable, robust, and widely used in wastewater plants and other open-channel applications, across a wide range of flows and with minimal moving parts or maintenance.

A weir also measures open-channel flow, but it tends to be more sensitive to upstream conditions and debris, and its accuracy can vary more with approach conditions. Ultrasonic meters and magnetic flow meters are common for closed, full pipes and are not typical for open channels, where the Parshall flume’s design and calibration are specifically suited to measuring flow in an open conduit.