Parshall Flume Flow Rate Calculator

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Parshall Flume Flow Rate Calculator

1 Inch (2.54 cm) 2 Inches (5.08 cm) 3 Inches (7.62 cm) 6 Inches (15.24 cm) 9 Inches (22.86 cm) 1.0 Foot (30.48 cm) 1.5 Feet (45.72 cm) 2.0 Feet (60.96 cm) 3.0 Feet (91.44 cm) 4.0 Feet (121.92 cm) 5.0 Feet (152.40 cm) 8.0 Feet (243.84 cm) 10.0 Feet (304.80 cm)
Feet (ft) Inches (in) Meters (m)

Flow Rate Results:

Cubic Feet / Sec (cfs): –
Gallons / Minute (gpm): –
Million Gallons / Day (MGD): –
Liters / Sec (L/s): –
Cubic Meters / Hour (m³/h): –

Understanding Parshall Flume Flow Measurement

A Parshall flume is a fixed hydraulic structure used to measure the flow rate of surface water, wastewater, and industrial discharge. Developed by Ralph L. Parshall in the 1920s, it is one of the most widely used devices for measuring open-channel flow because of its ability to operate with minimal head loss and its resistance to sediment buildup.

How the Calculation Works

The flow rate (Q) through a Parshall flume is calculated using the empirical formula for free-flow conditions:

Q = C × Han

  • Q: The discharge or flow rate.
  • C: A coefficient based on the throat width of the flume.
  • Ha: The primary head (depth) measured at the specific convergence point.
  • n: An exponent based on the flume size.

Installation and Measurement Tips

To ensure accurate readings, follow these critical guidelines:

  1. The Measurement Point: The head (Ha) must be measured at a specific point in the converging section, typically 2/3 of the way back from the throat.
  2. Level Installation: The floor of the flume's entrance must be perfectly level.
  3. Free Flow vs. Submerged Flow: This calculator assumes "Free Flow" conditions. If the downstream water level rises enough to back up into the flume (submergence), the calculation requires a correction factor.
  4. Approach Conditions: Flow entering the flume should be smooth and tranquil, not turbulent or surging.

Example Calculation

If you have a 6-inch Parshall flume and measure a head (Ha) of 0.5 feet:

  • For a 6″ flume: C = 2.06, n = 1.58
  • Q = 2.06 × (0.5)1.58
  • Q ≈ 2.06 × 0.334 = 0.688 cfs
  • Converting to GPM: 0.688 × 448.83 = 308.8 gpm
function calculateParshallFlow() { var widthSelect = document.getElementById("flumeWidth").value; var headVal = parseFloat(document.getElementById("headValue").value); var headUnit = document.getElementById("headUnit").value; if (isNaN(headVal) || headVal <= 0) { alert("Please enter a valid positive head measurement."); return; } // Convert all inputs to Feet for the standard formula var headInFeet = 0; if (headUnit === "ft") { headInFeet = headVal; } else if (headUnit === "in") { headInFeet = headVal / 12; } else if (headUnit === "m") { headInFeet = headVal * 3.28084; } // Parshall Constants (C and n) for standard formula Q (cfs) = C * Ha^n var C = 0; var n = 0; switch (widthSelect) { case "1in": C = 0.338; n = 1.55; break; case "2in": C = 0.676; n = 1.55; break; case "3in": C = 0.992; n = 1.547; break; case "6in": C = 2.06; n = 1.58; break; case "9in": C = 3.07; n = 1.53; break; case "1ft": C = 4.0; n = 1.522; break; case "1.5ft": C = 6.0; n = 1.53; break; case "2ft": C = 8.0; n = 1.55; break; case "3ft": C = 12.0; n = 1.566; break; case "4ft": C = 16.0; n = 1.578; break; case "5ft": C = 20.0; n = 1.587; break; case "8ft": C = 32.0; n = 1.607; break; case "10ft": C = 39.38; n = 1.6; break; default: C = 0; n = 0; } if (C === 0) return; // Calculation (Free Flow) var flowCFS = C * Math.pow(headInFeet, n); // Unit Conversions var flowGPM = flowCFS * 448.831; var flowMGD = flowCFS * 0.646317; var flowLS = flowCFS * 28.3168; var flowCMH = flowCFS * 101.941; // Display results document.getElementById("resCFS").innerText = flowCFS.toFixed(4); document.getElementById("resGPM").innerText = flowGPM.toFixed(2).replace(/\B(?=(\d{3})+(?!\d))/g, ","); document.getElementById("resMGD").innerText = flowMGD.toFixed(4); document.getElementById("resLS").innerText = flowLS.toFixed(2); document.getElementById("resCMH").innerText = flowCMH.toFixed(2); document.getElementById("flumeResult").style.display = "block"; }

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