How Is Open-Channel Flow Measured?

Many systems determine flow by measuring water depth at a known shape. In a Parshall flume running in free flow, the depth is measured in the converging (narrowing) section at a point two-thirds of that section's length upstream of the throat. Debris, buildup, foam, or a poor approach can affect the result. The measuring point and channel must stay in good condition.

Rags collect before a flume and water backs up. The indicated flow may no longer represent normal conditions, so the operator follows the cleaning procedure.

The water approaching a flume or weir should be as smooth and even as the installation requires. Waves, downstream backup, deposits, and floating material can change depth at the measuring point. The level sensor must also look at the correct spot. During rounds, inspect both the structure and the sensor. A clean sensor cannot correct a damaged flume, and a perfect flume cannot correct a blocked sensor. Document conditions before and after cleaning.

Inspect the measuring structure and sensor as one complete flow system.

Key point: Open-channel flow is often found by measuring water depth at a known shape, such as a flume or weir. Debris or backup changes the depth and the reading.