Flow Measurement · Weirs · Flumes · Wastewater · Irrigation · Open Channels
Open Channel Flow Measurement: 5 Essential Devices, Weir Formulas and Proven Selection Guide
Open channel flow measurement covers rivers, canals, sewers, drainage channels and irrigation ditches : anywhere liquid flows with a free surface exposed to atmosphere. Unlike closed pipe flow, open channel flow uses the liquid head above a restriction to calculate flow rate. This guide covers all 5 essential measurement devices: V-notch weirs, rectangular weirs, trapezoidal weirs, Parshall flumes and Palmer-Bowlus flumes : with formulas, CSS infographics and a live flow calculator.
Open Channel Flow Measurement vs Closed Pipe Flow: Key Differences
Pipe completely full. Pressure drives the flow. Fluid completely fills the cross-section. Instruments measure pressure differential, velocity or mass directly.
Instruments: Orifice plate, Coriolis, magnetic flow meter, vortex, turbine.
Driving force: Pressure difference between upstream and downstream.
Liquid flows with a free surface. Gravity drives the flow. The cross-section is only partially filled. Only the liquid level (head) above the restriction is measurable.
Instruments: Weirs, flumes, ultrasonic level sensors measuring head H.
Driving force: Gravity acting on the hydraulic gradient.
Open channel flow measurement is used in wastewater treatment, irrigation networks, stormwater drainage, river gauging, effluent monitoring and cooling water channels. The fundamental principle in all cases is the same: create a controlled restriction in the channel, measure the upstream liquid head H above the restriction, and apply the device-specific formula to calculate flow rate.
5 Essential Open Channel Flow Measurement Devices at a Glance
How Open Channel Flow Measurement Works: The Head-Flow Principle
Every weir and flume works on the same hydraulic principle. When flow approaches the restriction, the liquid backs up behind it and rises to a head H above the weir crest or flume throat. This head H is the only measurement needed. All open channel flow formulas take the form:
Where:
Q = volumetric flow rate (m³/s or L/s)
C = discharge coefficient (dimensionless, device-specific)
K = device geometry constant (depends on notch width, flume size etc.)
H = measured head above the weir crest or flume datum (metres)
n = exponent (1.5 for rectangular/trapezoidal weirs, 2.5 for V-notch, varies for flumes)
The head H is always measured UPSTREAM of the weir or flume at a specified distance (typically 4-6 times the maximum expected head). Measuring too close to the restriction gives a false low reading because the water surface accelerates and drops as it approaches the weir crest.
from this side
(4-6x H_max upstream)
over weir crest
Weirs in Open Channel Flow Measurement: Types, Formulas and Applications
A weir is a barrier built across an open channel with a precisely shaped opening (notch) through which all the flow passes. The three standard weir notch shapes each have different characteristics and flow formulas. Weirs are the most common open channel flow measurement device for clean liquids with no solids content.
The most accurate weir for low and medium flow rates. The triangular notch gives a high sensitivity at low flows because a small head increase produces a significant notch width increase.
Notch angles: 22.5°, 45°, 60°, 90° (most common), 120°
Q = (8/15) x Cd x tan(theta/2) x sqrt(2g) x H^2.5For 90° V-notch (standard, Cd = 0.611):
Q = 1.38 x H^2.5 (m³/s)Simple, robust, high-flow capacity. The rectangular notch passes much more water than a V-notch at the same head. Less sensitive at low flows but handles high-flow conditions reliably.
Types: Full-width (no end contractions) or contracted (with end contractions)
Q = (2/3) x Cd x L x sqrt(2g) x H^1.5Francis formula (Cd = 0.623, contracted):
Q = 1.84 x (L - 0.1nH) x H^1.5The Cipolletti weir uses side slopes of 1:4 (H:V) that compensate for the end contractions, giving a simpler flow formula than contracted rectangular weirs. Widely used in irrigation.
Side slope: 1 horizontal to 4 vertical on each side
Q = 1.859 x L x H^1.5 (m³/s)Where L = bottom width of the notch
Flumes in Open Channel Flow Measurement: Parshall and Palmer-Bowlus
Flumes are hydraulic structures built into the channel floor rather than across it. Unlike weirs, flumes do not create a full dam. The channel narrows through the flume throat, accelerating the flow to critical depth. This makes flumes self-cleaning and suitable for liquids carrying suspended solids, making them the essential open channel flow measurement device for wastewater and sewage applications where weirs would clog.
The most widely used flume in wastewater treatment. Has a converging inlet section, a throat section and a diverging outlet. The throat is where critical flow occurs. Head is measured in the inlet section at a specific location upstream of the throat.
Sizes: 1 inch to 50 feet throat width. Each size has a unique flow formula.
Parshall flume formula (6-inch throat, typical):
Q = 2.06 x Ha^1.58 (m³/s, 6-inch flume)Where Ha is the upstream head in metres at the standard measurement location.
Designed specifically for circular sewer pipes running partially full. The flume fits inside the pipe and creates a constriction that generates a measurable head upstream. Available in standard sizes matching common sewer pipe diameters.
Key advantage: Installs inside existing circular sewer pipes without major civil works. The circular inlet matches the pipe geometry for minimal disturbance.
Flow formula: Depends on pipe diameter : normalised tables provided by ISO 9826.
Level Sensors Used in Open Channel Flow Measurement
The weir or flume is just the primary element. To get a flow reading, the liquid head H must be converted to an electrical signal. Three types of level sensors are used, each connecting via 4-20 mA output to a flow computer or DCS.
Most common. Non-contact. Measures time-of-flight of ultrasonic pulse from sensor to water surface and back. Mounted above the channel pointing downward. Automatically compensates for temperature. Output 4-20 mA to flow computer.
A constant air flow is bubbled through a tube submerged to the channel floor. The back-pressure in the tube equals the hydrostatic pressure of the water column. Excellent for sewers and channels with debris, foam or vapour that confuse ultrasonic sensors.
A sealed pressure sensor submerged at the channel floor or weir stilling well. Measures hydrostatic pressure P = rho x g x H directly. Simple, reliable, low cost. Used when the channel is covered or ultrasonic mounting is impractical. Must correct for barometric pressure changes.
Open Channel Flow Measurement Device Selection Matrix
Open Channel Flow Measurement Calculator: Weir Flow Rates
Enter the measured head H above the weir crest to calculate flow rate for each weir type. The calculator uses standard ISO 1438 formulas. The flow computer connected to your level sensor performs this calculation continuously and outputs the result as a 4-20 mA signal.
Quick FAQs: Open Channel Flow Measurement
- Turbidity Measurement: Essential Water Quality Parameter in Open Channel Systems
- 4-20 mA Current Loop: How Open Channel Flow Transmitters Connect to SCADA
- Reynolds Number: Why It Matters Even in Open Channel Flow Calculations
- pH Measurement: Key Quality Variable Alongside Open Channel Flow in Wastewater
- Venturi Tube: Closed Pipe Flow Alternative When Channels Are Converted to Pipes
External References
- ISO 1438: Hydrometry : Open Channel Flow Measurement Using Thin-Plate Weirs
- ISO 9826: Measurement of Liquid Flow in Open Channels : Parshall and SANIIRI Flumes
- USGS: Open Channel Flow Measurement Methods
- AutomationForum: Open Channel Flow Measurement Devices Reference
What we learn today
- Open channel flow measurement uses a weir or flume to create a measurable liquid head H above a restriction. Flow rate Q = C x K x H^n : the exponent n is 2.5 for V-notch weirs, 1.5 for rectangular and trapezoidal weirs. Head H is always measured upstream at 4-6 times maximum H to avoid drawdown error. A level sensor (ultrasonic, bubbler or submersible pressure transducer) converts H to a 4-20 mA signal for the flow computer.
- Five devices cover all applications: 90-degree V-notch weir (best for low flow, highest accuracy), rectangular weir (best for high flow, simple), Cipolletti trapezoidal weir (wide range, irrigation), Parshall flume (wastewater and sewage with solids), Palmer-Bowlus flume (in-pipe monitoring in circular sewers). Never use a weir in wastewater : solids accumulate behind the crest and invalidate the measurement.
- Device selection by application: clean water low flow = V-notch weir. Clean water high flow = rectangular weir. Irrigation channels = Cipolletti. Wastewater treatment plant = Parshall flume. Existing circular sewer pipes = Palmer-Bowlus flume. The level sensor of choice for open channels is ultrasonic (non-contact, simple, temperature-compensated), with bubblers as backup for channels with foam or heavy vapour.