Flow Measurement · Glossary

What is a Weir?

open channel flow elementFlow Measurement

A weir is a barrier with a notch in an open channel; the height of water above the crest gives the flow rate.

Weir Explained

Water rises behind the weir and flows over the crest or through a notch. The upstream head, measured by an ultrasonic or radar level sensor, sets the flow through a known equation.

V notch weirs suit small flows and rectangular weirs larger ones. Weirs need a free fall (ventilated nappe) and the head must be measured far enough upstream.

Weir Formula

Q = (8 ÷ 15) × C_d × √(2g) × tan(θ ÷ 2) × h^2.5

Q	flow (m³/s), V notch weir
C_d	discharge coefficient, about 0.58 for 90°
θ	notch angle
h	head above the notch (m)

Example: 90° notch, h = 0.2 m, C_d = 0.58: Q ≈ 0.0245 m³/s, about 24.5 L/s.

Calculators for Weir

CalculatorWeir Flow CalculatorFree online calculator with formula and worked example.CalculatorParshall Flume CalculatorFree online calculator with formula and worked example.CalculatorManning Equation CalculatorFree online calculator with formula and worked example.

Frequently Asked Questions

What is a Weir?

A weir is a barrier with a notch in an open channel; the height of water above the crest gives the flow rate.

Weir or flume?

Weirs are cheaper and accurate, but trap solids. Flumes such as the Parshall pass solids and have lower head loss.

Where is the head measured?

Upstream of the crest, at least about 4 times the maximum head, where the water surface is not drawn down.

Learn more on the blog

Types of flow meters

A practical guide on instrumentationblog.in with more detail on weir and how it is applied on real plants.

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More Flow Measurement Terms

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