Since its development at the beginning of the last century, the Parshall flume has proven to be a versatile and long-lived invention for the measurement of water in open channels. More Parshall flumes are sold for water measurement than all other flume types combined.
In developing the Improved Venturi (what would later become the Parshall) flume, Dr. Ralph L. Parshall sought to overcome limitations of the devices of his time:
- Weirs (proper weir pool formation, sedimentation, inability to handle larger flows)
- Orifices (development of correct discharge constants, contraction distances)
- Rating flume (deposits, downstream flow conditions, impact of vegetative growth)
Modifications made by Parshall to the Venturi flume include: reducing the convergence angle of the inlet walls, lengthening the throat, reducing the divergence angle of the discharge walls, and introducing a drop (and subsequent partial recovery) through the throat. The fundamental idea behind the design of the Parshall flume is a desire to produce supercritical flow through the throat. The result was a flume that: experiences head-loss 25% that of a weir, passes sediment and debris easily, is able to operate under free and submerged flow condition accurately, and reduces the impact of variances in approach velocity.
While Parshall patented instrumentation that he developed to use in conjunction with this Improved Venturing flume, he never patented the flume itself. The flume, its dimensions, and discharge tables have thus moved into the public domain – available and accessible to all.
Parshall flumes are used to measure a wide range of applications, including:
- Wastewater treatment plant flows
- Surface water measurement
- Irrigation runoff
- Storm water runoof
- Dam seepage monitoring
- Industrial pre-treatment discharges
- Spring discharge
- Leachate monitoring
- Mine dewatering
In applying a Parshall flume flume to sanitary flows remember that only throat widths 3-inches [7.62 cm] and larger should be used. 1-inch [2.54 cm] and 2-inch [5.08 cm] Parshall flumes will clog when used on sanitary flows.
Parshall flumes were originally built of either wood or concrete, or, for smaller flumes, of sheet-metal. Today fiberglass is the material of choice; offering high strength, easy customization, repeatable dimensional tolerances, and corrosion resistance. Wood is rarely used and concrete only on large (15-feet [4.572 m] and up] flumes. Occasionally galvanized sheet is used, but issues regarding adherence to dimensional tolerances, fabricator application expertise, and the rapidity with which galvanized installation rust are all concerns. For applications where abrasion or resistance to aggressive chemical constituents is required, stainless steel is the material of choice.
Montana (Short Parshall) Flumes
A less-known modification of the Parshall flume is the Montana flume. The flume is primarily used to measure irrigation flows, although it adapts well to certain industrial discharge and municipal sewerage applications where free-fall conditions occur.
The Montana flume modifies the Parshall by deleting the throat and discharge sections – leaving only the flat-floored converging inlet section. With no throat or discharge sections, the Montana flume has no resistance to submergence and requires free-fall conditions off the end of the flume at all times. Like the flume from which it is derived, the Montana flume is sized by the throat width alone. Montana flumes share the same discharge equations and tables as the same sized Parshall flume.
Advantages of Parshall Flumes
- Widely used / recognized
- Good submergence resistance (in larger sizes)
- Good solids passage
- Flat-bottomed inlet (easier calibration)
- End adapters available to connect to pipe / condition flow
Disadvantages of Parshall Flumes
- 1-Inch and 2-Inch Flumes will clog when used on sanitary flows
- Drop required (limits retrofit applications)
- Length (particularly with end adapters)
- Cost (particularly with end adapters)
When to Consider A Different Flume
- Low flows (HS / H / HL, Trapezoidal)
- Flat channels (Cutthroat)
- Limited space (HS / H / HL, Trapezoidal, Palmer-Bowlus)
- Free-spilling discharge (Montana, HS / H / HL)
Parshall flumes, although similar in shape, are not scale models of each other. The discharge equation for each flume had to be obtained by direct calibration. Thus, it is important that Parshall flumes are constructed to the specified dimensions and that intermediate sizes (30-Inch and 42-Inch) flumes are either not used or have specific, empirically derived flow tables / equations developed for them.
With 22 sizes available, coving flows from 0.005 cfs [0.1416 l/s] to 3,280 cfs [92,890 l/s], there is a Parshall flume for just about any flow rate you need to measure.
Find out more about Parshall flumes, including: