A Parshall flume is a fixed hydraulic structure used to measure the volumetric flow rate of water and wastewater in open channels. A Parshall flume in wastewater treatment provides continuous, accurate flow measurement at the plant inlet — a critical requirement for process control, regulatory compliance, and operational management. With no moving parts, no power supply required for measurement, and minimal maintenance needs, it is one of the most reliable flow metering solutions available for open channel applications.
Parshall flumes are widely used at the entrance of wastewater treatment plants, where influent flow must be measured and monitored constantly. They are also used at discharge points, within treatment processes, and in irrigation and water management applications globally.
What Is a Parshall Flume?
A Parshall flume is a modified version of the Venturi flume, developed by Dr. Ralph L. Parshall of the U.S. Soil Conservation Service in the early twentieth century. The design accelerates flow by contracting both the parallel sidewalls and dropping the floor at the flume throat — the narrowest section of the structure. This controlled acceleration creates a predictable relationship between the water depth upstream of the throat and the volumetric flow rate passing through the flume.
As a Parshall flume manufacturer and worldwide supplier, Vortex Engineering offers standard dimensions and stainless steel grades to meet different channel sizes, flow ranges, and installation requirements.
The key advantage of the Parshall flume over a simple weir or orifice is its ability to pass sediment, solids, and floating debris without accumulation — making it particularly suitable for raw wastewater applications where upstream solids accumulation would compromise measurement accuracy or block the structure.
How Does a Parshall Flume Work?
The Parshall flume measures flow by exploiting the relationship between water depth and flow rate through a calibrated hydraulic structure. Under free-flow conditions — where downstream water levels do not influence the flow through the throat — this relationship is fixed and well-defined for each flume size.
Converging section. Wastewater enters the flume through a converging inlet section, where the channel walls narrow progressively toward the throat. This narrowing accelerates the flow and raises the water surface slightly upstream.
Throat section. The throat is the critical measurement zone. The floor drops at this point, and the walls reach their minimum width. The combination of wall contraction and floor drop creates supercritical flow conditions through the throat — a flow regime that is independent of downstream conditions under free-flow operation.
Diverging section. Downstream of the throat, the channel widens again and the floor rises, allowing the flow to decelerate and recover partial pressure before continuing downstream.
Depth measurement. A staff gauge or ultrasonic level sensor measures the water depth at a specific point in the converging section — upstream of the throat. This depth reading is converted to a flow rate using the calibrated flow equation for that flume size. The conversion can be performed manually using published tables or automatically by a flow controller or datalogger connected to the level sensor.
Parshall Flume Applications in Wastewater Treatment
Plant inlet flow measurement. The most common application — measuring the total influent flow entering the treatment plant. Accurate inlet flow data is essential for process control, chemical dosing, and regulatory reporting. Most environmental permits require continuous flow measurement and recording at the plant inlet.
Effluent flow measurement. At the plant outlet, Parshall flumes measure the treated effluent discharge. Effluent flow data is typically required for environmental compliance reporting and may be subject to real-time monitoring requirements.
Inter-process flow measurement. Within the plant, Parshall flumes measure flow between treatment stages — for example, between primary and secondary treatment, or between biological treatment and sludge handling. This data supports process optimisation and load balancing between parallel treatment trains.
Return activated sludge flow. In activated sludge processes, Parshall flumes can measure the return activated sludge flow from secondary clarifiers back to the aeration tanks — a key process control parameter.
Industrial discharge monitoring. Industrial facilities discharging to municipal sewers or water bodies use Parshall flumes to measure and record their effluent discharge in compliance with trade effluent consent conditions.
Irrigation and water management. Beyond wastewater, Parshall flumes are widely used in irrigation networks, river gauging stations, and water supply systems where open channel flow measurement is required.
Parshall Flume Sizing and Standard Dimensions
Parshall flumes are manufactured in a range of standard sizes, defined by the throat width. Standard throat widths range from 25 mm (1 inch) for small laboratory or low-flow applications up to 15 metres or more for large channels. Each size has a defined flow range and a specific calibrated flow equation.
Selecting the correct flume size for a wastewater application requires knowing:
Design flow range. The flume must measure accurately across the full range of expected flows — from minimum dry-weather flow to peak wet-weather flow. A single flume size must cover this entire range without exceeding its calibrated limits or producing readings below its minimum detectable flow.
Channel dimensions. The flume must fit within the existing channel geometry. Standard flume dimensions are published and must be matched to the available channel width and depth.
Head loss. The Parshall flume introduces a fixed head loss — a drop in water surface elevation — across the structure. This head loss must be accommodated by the available hydraulic gradient in the channel. In flat sites with limited hydraulic gradient, head loss is a critical design constraint.
Upstream conditions. Flow measurement accuracy requires a settled, uniform approach flow upstream of the flume. Turbulence, bends, or obstructions close to the flume inlet reduce measurement accuracy. A straight approach length of at least 3–5 times the flume width is recommended upstream of the converging section.
Free Flow vs Submerged Flow Conditions
Parshall flumes are designed primarily for free-flow conditions — where the downstream water level is low enough that it does not influence the flow through the throat. Under free-flow conditions, the upstream depth measurement alone is sufficient to determine flow rate.
When downstream water levels rise sufficiently — due to backwater effects, tidal influence, or downstream obstructions — the flume enters submerged flow conditions. Under submergence, the simple upstream depth measurement underestimates the true flow rate. Accurate measurement under submerged conditions requires a second depth measurement downstream of the throat and a submergence correction factor.
The submergence ratio — the ratio of downstream to upstream depth — at which submergence corrections become necessary varies by flume size but is typically around 0.6–0.7 for standard Parshall flumes. Vortex Engineering can advise on submergence conditions for specific installation sites.
Parshall Flume vs Other Flow Measurement Devices
Several flow measurement alternatives exist for open channel wastewater applications. Each has advantages and limitations.
| Parshall Flume | V-Notch Weir | Electromagnetic Flow Meter | |
|---|---|---|---|
| Channel type | Open channel | Open channel | Closed pipe |
| Solids handling | Excellent — self-cleaning | Poor — solids accumulate | Good |
| Flow range | Wide | Narrow — best for low flows | Wide |
| Head loss | Moderate | High | Negligible |
| Maintenance | Very low | Low — but requires cleaning | Low |
| Installation | Permanent civil structure | Simple — weir plate in channel | In-pipe installation |
| Best for | Wastewater plant inlets, large channels | Clean water, low flow | Pressurised pipe flow |
The Parshall flume is the preferred choice for raw wastewater flow measurement in open channels because its self-cleaning geometry prevents the solids accumulation that compromises weir performance. For pressurised pipe flow measurement, electromagnetic flow meters are more appropriate.
Materials and Installation
Vortex Engineering Parshall flumes are manufactured from stainless steel — selected for durability and corrosion resistance in wastewater environments.
Flume body: Stainless steel AISI 304 standard; AISI 316 available for more aggressive environments or marine applications.
Construction: The flume is fabricated as a complete prefabricated unit, ready for direct installation in the channel. This eliminates the need for on-site formwork and ensures dimensional accuracy — critical for measurement precision.
Installation: The flume is set level in the channel and grouted or bolted into position. The approach channel must be straight and free of obstructions for the specified upstream length. A staff gauge or ultrasonic level sensor is installed at the designated measurement point in the converging section.
Flow indication: The depth reading from the level sensor is connected to a flow controller, datalogger, or plant SCADA system for continuous flow calculation, display, and recording.
Frequently Asked Questions
How accurate is a Parshall flume?
Under free-flow conditions with correct installation and calibration, Parshall flumes achieve measurement accuracy of ±2–5% of actual flow. Accuracy depends on the approach flow conditions, the quality of the level measurement, and whether submergence corrections are applied when needed.
Does a Parshall flume require calibration?
Standard Parshall flumes use published calibration tables that have been validated against physical models. No site-specific calibration is normally required provided the flume is manufactured to standard dimensions and installed correctly. For non-standard sizes or unusual conditions, independent calibration may be required.
Can a Parshall flume be installed in an existing channel?
Yes. Vortex Engineering Parshall flumes are supplied as prefabricated units that can be installed in new or existing channels. The channel must be sized to accept the flume and provide the required approach length upstream.
What level sensor should be used with a Parshall flume?
Ultrasonic level sensors are the most common choice — they measure the water surface level without contact and can be connected directly to flow controllers or dataloggers. Float-operated sensors and pressure transducers are also used in some applications. The sensor must be positioned at the designated measurement point specified for each flume size.
Can Parshall flumes handle raw wastewater with high solids content?
Yes. The Parshall flume’s self-cleaning geometry — particularly the throat drop — prevents solids accumulation that would compromise measurement accuracy. This makes it particularly suitable for raw wastewater applications where V-notch weirs or other structures would require frequent cleaning.
Vortex Engineering designs and manufactures Parshall Flumes as part of its complete Flow Control Equipment range and the full Wastewater Treatment Equipment lineup.
