Open channel flow metering, explained
A practical guide to how open channel flow meters work, which technology fits which site, and what keeps them accurate.
What is open channel flow?
Open channel flow is any flow with a free surface exposed to the atmosphere — sewers flowing partially full, channels, flumes, ditches and stormwater outfalls. Unlike a full, pressurized pipe, both the depth and the velocity of the water change constantly, so a flow meter has to measure more than one thing to calculate flow accurately.
How open channel flow meters measure flow
Area-velocity. The most common method in sewers. The meter measures water level to calculate the wetted area, measures average velocity, and multiplies them: Q = A × V. Hach Flo-Dar and Flo-Tote 3 sensors both use this approach.
Primary devices (flumes & weirs). A Parshall flume or V-notch weir creates a known relationship between level and flow, so only level needs to be measured. Common at treatment plants.
Manning equation. Estimates flow from level, pipe slope and roughness. Useful as a check, but less accurate than measuring velocity directly.
Contact vs. non-contact sensors
- Non-contact (Hach Flo-Dar): radar velocity and ultrasonic level from above the water. Nothing in the flow to foul, so it excels in raw sewage, high velocities and debris-heavy lines.
- Submerged (Hach Flo-Tote 3): sensor mounted on the pipe invert. Strong performance in shallow and low-flow conditions and can detect reverse flow.
Portable vs. permanent flow metering
Battery-powered loggers like the Hach FL900 are built for temporary flow studies and unpowered sites. Line-powered loggers like the FL1500 suit permanent installations that feed SCADA.
What makes flow data go bad
- Sensor fouling from grease, ragging or sediment
- Poor site hydraulics — turbulence, bends or drops upstream of the sensor
- Level offsets drifting from the original calibration
- Surcharge conditions the meter wasn't configured for
- Dead batteries, wet desiccant and damaged cables
Most of these are caught with routine maintenance and periodic verification against an independent measurement. Learn about our calibration service →
Have a site you're unsure about?
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