The simplest device used to measure flow in an open channel is which of the following?

• A. Weir
• B. Ultrasonic meter
• C. Magnetic meter
• D. Orifice plate

Answer: (A)

In open-channel flow, discharge relates to a measurable hydraulic head over a crest. A weir does this in the simplest way: as water flows over the sharp edge, the head above the crest directly corresponds to the flow rate, and you can get Q by measuring that head with a staff gauge and applying a standard weir equation. It’s passive, has no moving parts, and is inexpensive and easy to install, making it the most straightforward choice for measuring flow in open channels. Devices like ultrasonic or magnetic meters and an orifice plate are more suited to closed conduits or require more complex instrumentation and setup, which isn’t as simple for open-channel measurement.