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How It Works
A Parshall flume uses a converging-diverging channel shape to create a predictable relationship between upstream water depth and discharge. The standardized geometry means no individual calibration is needed. This calculator applies the general equation for throat widths between 1 and 8 feet under free-flow conditions.
Example Problem
A Parshall flume with a 2 ft throat width measures an upstream depth of 1.5 ft. What is the flow rate?
- Q = 4 × B1.026 × h11.522
- Q = 4 × 21.026 × 1.51.522 = 4 × 2.036 × 1.855
- Q ≈ 15.1 ft³/s
Frequently Asked Questions
What is a Parshall flume used for?
It measures open-channel water flow in irrigation canals, wastewater plants, and industrial discharge points. Its self-cleaning design handles sediment better than weirs.
What is the difference between free-flow and submerged flow?
Free-flow means the downstream water level does not affect the upstream reading. Submerged flow occurs when tailwater backs up into the throat, requiring a correction factor. Most flumes are designed for free-flow conditions.
How accurate is a Parshall flume?
Under free-flow conditions with proper installation, Parshall flumes achieve accuracy of ±3–5%. Errors increase with submergence, improper approach conditions, or sediment buildup.
Related Calculators
- Venturi Meter Calculator — applies the same converging-flow principle in closed pipes.
- Manning Equation Calculator — calculate open-channel flow velocity upstream of the flume.
- Orifice Flow Calculator — an alternative flow measurement method using plate orifices.
- Hydraulic Radius Calculator — compute hydraulic radius for open-channel flow analysis.
- Chezy Equation Calculator — another approach to open-channel velocity estimation.
- Volume Unit Converter — convert flow rate units between gallons and liters per second.