Parshall Flume: What It Is and How to Buy the Right One

A Parshall flume is a fixed-geometry, open-channel flow-measurement device — a specially shaped constriction built into a channel, ditch, or pipe outfall that converts the water level upstream of the constriction into a known, repeatable discharge rate. It is one of the most widely specified primary measuring devices in irrigation, wastewater treatment, and environmental water-quality monitoring, precisely because it needs no moving parts and, in free-flow conditions, requires only a single head reading to derive flow. For a procurement officer or lab/facility manager, the buying decision is less about “which flume is best” and more about correctly matching throat size, material, and installation geometry to the actual flow range and site conditions — get any one of those wrong and the flume will either under-range, over-range, or read inaccurately under submerged conditions regardless of how well it was manufactured.

What Is a Parshall Flume?

A Parshall flume (also called a Parshall measuring flume) is a specific, standardized channel shape — a converging section, a constricted “throat,” and a diverging section — that forces flow through a critical-depth transition. Because the relationship between upstream water depth and discharge through that transition is fixed by the flume’s geometry, a single depth measurement at a defined point upstream of the throat (denoted Ha) is sufficient to calculate flow rate under free-flow conditions, without needing a separate velocity measurement.

The device was developed by Ralph L. Parshall of the U.S. Department of Agriculture’s Soil Conservation Service (predecessor to today’s Natural Resources Conservation Service), with development beginning around 1915 and refinement through the 1920s for irrigation-water measurement in the western United States; the American Society of Civil Engineers formally adopted the name “Parshall Measuring Flume” in 1930. It remains, roughly a century later, one of the standard reference devices cited by environmental regulators and wastewater permit writers for open-channel flow monitoring.

How Parshall Flume Flow Measurement Works

Flow through a Parshall flume is governed by a head-discharge relationship of the general form Q = C·Ha^n, where Q is discharge, Ha is the upstream head (gauge height) measured at a fixed stilling-well or staff-gauge location, and C and n are empirically derived coefficients specific to that flume’s throat width — they are not the same across sizes, because a Parshall flume is not simply a scaled-up or scaled-down version of another size. This is a critical procurement point: you cannot interpolate a custom throat width between two catalog sizes and expect the standard head-discharge table to remain valid. Buy (or have engineered) a flume in one of the standardized throat widths, or commission and independently verify a site-specific rating if a non-standard size is genuinely unavoidable.

Two flow regimes matter for both selection and ongoing accuracy:

• Free flow — downstream water level is low enough that it does not back up into the throat and affect the upstream reading. Only Ha is needed, and the standard published tables/charts apply directly.
• Submerged flow — downstream water level rises enough (generally above roughly 60-70% submergence, varying somewhat by throat size) to affect the upstream head reading. In submerged conditions, a second head measurement downstream of the throat (Hb) is required, and a submergence correction must be applied to the free-flow discharge value. A site prone to variable tailwater — tidal influence, downstream weir control, surcharged sewers — should be specified and purchased with a downstream (Hb) stilling well from the start; retrofitting one after installation is avoidable cost.

Because submerged-flow correction is more error-prone than a straightforward free-flow reading, site selection and flume sizing that keep the installation in free-flow conditions across the expected flow range is usually the better engineering and procurement decision than buying a smaller, cheaper flume and living with submergence corrections.

Reading a Parshall Flume: Charts, Tables, and Instrumentation

Historically, and still commonly on manually-read irrigation installations, flow is read directly off a staff gauge at the Ha measuring point and converted using the published free-flow discharge table (a “Parshall flume chart”) for that specific throat width — these tables are typically supplied by the flume manufacturer or drawn from the governing standard and give discharge directly for a range of head readings, avoiding the need to compute the C·Ha^n relationship by hand. For continuous or telemetered monitoring — the norm in wastewater permit compliance and most modern environmental monitoring — the staff gauge is supplemented or replaced with a non-contact ultrasonic or radar level sensor, a submerged pressure transducer, or a bubbler system feeding a flow totalizer/data logger, with the head-to-flow conversion programmed into the instrument rather than read manually. When procuring instrumentation to pair with a flume, confirm the vendor’s conversion firmware or configuration actually uses the correct published coefficients for your specific throat width and flow regime (free vs. submerged) — a generic linear or default curve fit will not match the true Parshall relationship and will produce a systematic measurement bias.

Standards Governing Parshall Flume Design and Use

Three standards are the ones most often cited on spec sheets, calibration certificates, and regulatory permit language for Parshall flumes:

• ASTM D1941, Standard Test Method for Open Channel Flow Measurement by Parshall Flume — the standard most commonly referenced in U.S. environmental and wastewater permitting.
• ISO 9826, Measurement of liquid flow in open channels — Parshall and SANIIRI flumes — the international reference standard, relevant when equipment or data must be defensible under an ISO-based quality system.
• JIS B7553 — the Japanese Industrial Standard covering Parshall flumes, relevant primarily for installations and instrumentation sourced under Japanese specifications.

For a laboratory or utility operating under ISO/IEC 17025 accreditation, the flume itself is a measurement device feeding data into an accredited test or monitoring method — procurement documentation should specify which standard the flume’s dimensions and rating table conform to, and the accredited lab’s quality manual should trace that conformance the same way it traces any other piece of measurement equipment.

Standard Throat Widths and Selecting a Size

Parshall flumes are manufactured in a range of standardized throat widths — commonly cited as spanning roughly 1 inch up to 50 feet across the full family of published sizes — each with its own discharge range and its own C and n coefficients. Selecting a throat width is a matter of matching the site’s expected minimum and maximum flow to a size where that range sits comfortably within the flume’s rated free-flow operating range, not simply picking the size that fits the physical channel width. An undersized throat will surcharge (back up, and potentially go submerged or overtop) at high flow; an oversized throat will produce a very shallow, hard-to-resolve head reading at low flow, degrading measurement accuracy exactly when regulatory or process-control attention to low-flow accuracy often matters most (e.g., minimum-flow permit conditions, leak/infiltration monitoring). Site data — actual historical or projected flow range, not just pipe/channel capacity — should drive sizing, and an engineer familiar with the specific application (irrigation delivery, wastewater influent/effluent, stormwater) should confirm the selection before purchase, since undersizing or oversizing is a design error that installation cannot fix afterward.

Fiberglass Parshall Flumes vs. Other Materials

Material selection affects both procurement cost and long-term measurement accuracy, since dimensional tolerance directly affects the validity of the standard rating tables:

• Fiberglass (fiberglass-reinforced plastic, FRP) — the most common material for prefabricated, smaller-to-mid-size flumes in wastewater and environmental monitoring applications. Lightweight (easier and cheaper to ship and install than concrete or steel), corrosion- and chemical-resistant, and manufactured to close dimensional tolerance in a mold, which matters because the published head-discharge tables assume the flume’s dimensions match the standard geometry closely. Verify the manufacturer’s dimensional tolerance and, ideally, a certificate of conformance to ASTM D1941/ISO 9826 dimensions as part of the purchase, not just a nominal “Parshall flume” product description.
• Concrete (cast-in-place or precast) — typically used for large throat widths (utility-scale wastewater plants, large irrigation canals) where fabricating in fiberglass or metal is impractical. Durable and low-maintenance once installed, but far less forgiving of construction/forming error — dimensional accuracy depends on formwork quality, and a poured flume that deviates from standard dimensions will not match the published rating tables without a site-specific calibration.
• Stainless steel or galvanized steel — used for portable, temporary, or high-temperature/aggressive-flow installations, and where mechanical durability under debris loading or physical impact is a bigger risk than in a typical wastewater channel.
• Aluminum or plastic — used for lightweight, portable, or laboratory/pilot-scale flumes where permanence and load rating matter less than ease of handling.

For most environmental and wastewater-lab procurement decisions, the practical choice is between fiberglass (small-to-mid sizes, most common) and concrete (large sizes, permanent installations) — request dimensional tolerance documentation either way, since a flume that is nominally the right throat width but out of tolerance will produce a flow reading that is wrong by a fixed, silent bias no amount of downstream instrumentation calibration will catch.

What to Evaluate When Procuring a Parshall Flume

• Throat width matched to actual flow range — sized from real site flow data (historical, modeled, or permit-specified minimum/maximum), not channel dimensions alone.
• Governing standard and documented conformance — ASTM D1941, ISO 9826, or JIS B7553 as applicable to your regulatory or quality-system context, with a manufacturer’s certificate of dimensional conformance, not just a catalog listing.
• Material fit for the installation — fiberglass for most small/mid prefabricated installations, concrete for large permanent structures, corrosion-resistant metal for aggressive or high-temperature flows.
• Free-flow vs. submerged-flow site conditions — confirm downstream tailwater conditions before finalizing size and instrumentation; specify a downstream (Hb) stilling well and submergence-correction capability if there is any realistic chance of submerged operation.
• Instrumentation compatibility — level sensor/transducer and totalizer configured with the correct head-discharge coefficients for the specific throat width and flow regime purchased, not a generic or default flow curve.
• Installation and leveling tolerances — the flume must be installed level and free of upstream/downstream disturbances (bends, drops, obstructions) close enough to the structure to affect approach flow; manufacturers and the governing standards specify minimum straight-run distances.
• Calibration and verification program — a plan for periodic verification that the installed flume and its instrumentation continue to read accurately, particularly if the flume is part of a regulatory-reportable measurement (NPDES/wastewater discharge monitoring, water-rights accounting). Many of the same calibration-traceability principles that apply to laboratory instruments apply here — see CASRAI’s guide to calibration certificates and metrological traceability for what “traceable” documentation should actually contain.

Distributors and manufacturers of environmental monitoring equipment — general laboratory/environmental supply distributors such as LAC Health among others in that market — typically carry or can source Parshall flumes alongside the level sensors, flow totalizers, and calibration services needed to bring an installation into service; when evaluating any supplier, the criteria above (documented dimensional conformance, correct instrumentation coefficients, and a real calibration/verification pathway) matter more than price alone, since a flume that reads inaccurately produces silently wrong data rather than an obvious failure.

Frequently Asked Questions

What is a Parshall flume used for?

Measuring open-channel water flow rate — most commonly in irrigation water delivery/accounting, wastewater treatment plant influent and effluent monitoring, and environmental/stormwater flow monitoring — by converting a single upstream water-depth reading into a known discharge rate under free-flow conditions.

How does Parshall flume flow measurement differ from a weir?

Both are primary open-channel flow-measurement devices, but a weir measures flow as water overtops a fixed obstruction (a dam-like structure), while a Parshall flume measures flow through a converging-throat-diverging channel constriction without requiring water to pond or overtop anything. Parshall flumes are generally preferred where sediment or debris carryover is a concern, since they are largely self-cleaning (the accelerated flow through the throat tends to scour rather than deposit solids), whereas weirs can accumulate sediment behind the obstruction.

What is a fiberglass Parshall flume, and when is it the right choice?

A fiberglass (FRP) Parshall flume is a prefabricated, molded unit — the standard choice for small-to-mid throat widths in wastewater and environmental monitoring applications where a lightweight, corrosion-resistant, close-tolerance prefabricated structure is preferable to pouring concrete on site. It is generally not used for the largest throat widths, where concrete construction is more practical.

What does a Parshall flume chart show?

A Parshall flume chart (or discharge table) is the published free-flow head-to-discharge conversion for a specific throat width, derived from that size’s C and n coefficients in the Q = C·Ha^n relationship. It lets an operator read discharge directly from a measured upstream head (Ha) without calculating the equation manually, and is specific to one throat width — charts are not interchangeable between sizes.

Do Parshall flumes need calibration?

The flume’s geometry is fixed and doesn’t drift the way an electronic sensor does, but any instrumentation attached to it (level transducers, ultrasonic sensors, totalizers) requires periodic calibration and verification, and the installation itself should be periodically checked for physical wear, sediment buildup, or structural damage that would invalidate the standard rating table. For regulatory-reportable flow data, document both the flume’s original dimensional conformance and the ongoing instrumentation calibration record.