The Parshall flume is the classic primary device for measuring flow in an open channel, so common in wastewater that many plants simply call their influent meter the flume. It is a specially shaped section of channel that constricts the flow through a narrow throat, and the depth of water it produces at a defined point converts directly to a flow rate. This page details how the flume is sized by its throat width, the standard head-discharge relationship that turns depth into flow, the crucial distinction between free and submerged flow and the submergence correction, and how the level sensor and flow computation are set up in a SCADA system.
Parshall Flume in one line: A Parshall flume is a primary flow-measuring device consisting of a channel that narrows to a throat, drops through it, and expands again, forcing a repeatable relationship between the water depth upstream and the flow rate. Its size is designated by the throat width, and a standard head-discharge equation for that size converts a measured upstream head into a flow. A single upstream level measurement is enough to compute flow as long as the flume is running with free discharge.
A Parshall flume has a characteristic three-part shape: a converging inlet section that funnels the flow inward, a narrow throat section with a downward-sloping floor where the flow accelerates, and a diverging outlet section that lets the flow expand and recover downstream. This geometry forces the water through a controlled constriction so that the depth of water upstream rises and falls in a defined way with the flow rate, which is what makes the flume a measuring device rather than just a channel. The shape is standardized, so a flume built to the specification behaves like every other flume of that size, and its head-discharge relationship is known without having to calibrate each installation individually.
The single number that designates a Parshall flume's capacity is its throat width, the width of the narrow section, and flumes are made in a series of throat widths from a few inches up to several feet and larger. The throat width sets the range of flow the flume can measure: a narrow throat suits small flows and would drown out at high flow, while a wide throat handles large flows but loses resolution at low flow, so choosing the right throat width for the expected flow range is the essence of sizing a flume. A flume sized too large reads a very shallow, hard-to-measure head at normal flow, while one sized too small backs water up and runs submerged, so matching throat width to the flow range is a real design decision rather than a formality.
Because the shape and dimensions are standardized for each throat width, the flume must be built and installed to those dimensions and set dead level to perform as specified. A flume that is out of level, undersized or oversized for its flow, distorted, or damaged no longer matches the standard geometry its equation assumes, so the flow it computes will be wrong even if the level is measured perfectly. This is why flume installation quality matters so much, and why a suspect flow reading is often traced back to the physical flume, its levelness, its condition, and whether it was the right size, before the level sensor is even questioned.
The Parshall flume works because of its head-discharge relationship, an equation that gives the flow rate as a function of the upstream head for each throat-width size. The head is measured at a defined point in the converging section upstream of the throat, at the location the flume specification designates, and the equation for that flume size converts that single depth into a flow. This is why a Parshall flume needs only one level measurement in normal operation: the standardized shape guarantees that the upstream head alone determines the flow, so measure the head at the right place and apply the right equation and you have the flow. Published tables and equations exist for each standard throat width, so the flow computer simply needs to know which size flume it is reading.
That simplicity holds only when the flume is running with free flow, meaning the water discharges freely through the throat and downstream without the downstream water level backing up into it. Under free flow the throat controls the flow independently of downstream conditions, so the upstream head is a clean, single-valued indicator of flow. Free flow is the normal, intended operating condition, and a well-designed installation keeps the flume free-flowing across its expected range by ensuring the downstream channel can carry the water away without ponding back into the flume.
When the downstream water level rises high enough relative to the upstream level, the flume becomes submerged, and the free-flow assumption breaks. Under submergence the downstream backwater slows the discharge, so the same upstream head now corresponds to a lower flow than the free-flow equation would give, and reading the upstream head alone would overstate the flow. The correction is to measure a second head downstream in the throat, compute the submergence ratio, and apply a submergence correction that reduces the free-flow value appropriately. There is a limit beyond which the flume is too submerged to measure reliably at all, so recognizing submergence, applying the correction when it is within range, and fixing the downstream restriction when it is not, are essential to a trustworthy Parshall flume reading.
Turning a Parshall flume into a live flow signal requires configuring the level measurement and the flow computation correctly, and the configuration must match the specific flume. The level sensor, commonly an ultrasonic or radar device mounted above the measuring point or a submerged pressure sensor at it, is set up so its zero corresponds to the flume floor at the head-measurement location and it reads true water depth there. The flow computation, whether in a dedicated flow meter, an integrator, a PLC, or the SCADA system, is told which Parshall throat-width size it is reading so it applies the correct head-discharge equation, and it integrates the instantaneous flow into a running total for reporting.
Several configuration details separate a flume flow point that reads correctly from one that is subtly wrong. The head measurement must be taken at the designated point, not wherever the sensor happens to be mounted, because the equation is anchored to that location. The zero and the flume size must be entered correctly, since a wrong size selects a wrong equation and produces a plausible but incorrect flow. Where submergence is possible, the system needs the second downstream level and the submergence correction configured, or it will silently overread whenever the flume drowns. These are exactly the settings that, if entered carelessly at commissioning, produce a flow that looks reasonable but does not match reality.
For SCADA and cloud monitoring, the value of exposing both the head and the computed flow becomes clear at a flume, because the flow is derived and the derivation can go wrong in ways the flow number alone hides. A cloud SCADA platform such as Merobix records the measured head, the instantaneous flow, and the running total, so an operator at a distant plant sees the reported flow and total while retaining the ability to look at the underlying level. When a flume flow reads oddly, seeing the head lets staff tell apart a genuine flow change from a level sensor that has lost the surface, a flume that has silted or is running submerged, or a configuration that no longer matches the physical flume, so the reported influent or effluent total stays defensible rather than quietly drifting.
The flume narrows the channel to a throat that constricts and accelerates the flow, forcing a defined relationship between the upstream water depth and the flow rate. A level sensor measures the depth, or head, at a designated point upstream of the throat, and a head-discharge equation for that flume's throat width converts the depth into a flow rate. Because the shape is standardized, a single upstream head is enough to compute the flow whenever the flume is running with free discharge.
Under free flow, water discharges through the throat without the downstream level backing up into it, so the throat controls the flow and the upstream head alone determines it, which is the normal operating condition. Under submerged flow, downstream backwater slows the discharge, so the same upstream head corresponds to a lower flow and reading it alone overstates the flow. Submergence is handled by measuring a second downstream head and applying a submergence correction, up to a limit beyond which the flume cannot measure reliably.
A Parshall flume is designated and sized by its throat width, the width of its narrow section, chosen to match the expected flow range. A throat too narrow backs water up and drowns out at high flow, while one too wide produces a very shallow head at normal flow and loses resolution, so the throat width is selected so normal flows produce a well-measured head without submerging the flume. The flume must also be built to the standard dimensions and installed dead level to perform as specified.
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