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Cutthroat Flumes

The Cuthroat flume is a class of flumes developed in the 1960's at the Utah Water Research Laboratory, Utah State University, to overcome the limitation of the Parshall flume in flat gradient applications.

Development of the Cutthroat Flume

In the 1960's, Gaylord Skogerboe, Leon Hyatt, Ross Anderson, and Keith Eggleston, backed by funding from the United States Department of the Interior, Office of Water Resources Research, sought to develop a flume that was suitable to installation on flat gradients that operated satisfactorly under both free and submerged flow.  

Up until their research into what would become the Cutthroat flume, the common approach to measuring flows in flat gradient applications was to use a 

Cutthroat Flume for Open Channel Flow Measurement

Parshall flume with the throat / discharge sections removed (which eliminated the need to raise the flume to accomodate the drop through the flume).  

While removing these sections from the Parshall flume produced good flow results, the resulting jet of energetic water from the throat (now the outlet) of shortened flume frequently resulted in downstream errosion problems.

The developers of the Cutthroat flume relied heavily on previous studies into a range of topics to develop the flume.  

These included:

 

Building on flat-bottomed design, which was well suited to the flat gradient channels the flume was to be designed for, the developers incorporated Ackers' and Harrison's 3:1 inlet convergence ratio with Hyatt's 6:1 divergence ratio to set the basic shape of the flume.  

Since the Hb depth, the downstream depth used for calculatiing and correcting for submergence,  was to be measured in the discharge of the flume (and not in the throat as is the case with the Parshall flume, there appeared to be no benefit to having a throat section.   

Design of the Cutthroat Flume

The Cutthroat flume is a rectangular open-channel constriction with a flat floor and no throat length (the flume simply converges at a 3:1 ratio to the throat and then immediately diverges at a 6:1 ratio). As the flume has no throat length, it was given the name “Cutthroat” by its developers.


 

2D Generalized Layout of the Cutthroat Flume

 

Cutthroat flumes are available in two different styles:  the Rectangular Cutthroat Flume and the Trapezoidal Cutthroat Flume.  The difference between the two styles is the shape of the cross-section of the flume.

For the Rectangular Cutthroat Flume, the sidewalls are vertical.  Combine this with the flat floor of all Cutthroat flumes and the result is a rectangular cross-section.  For Trapezoidal Cutthroat Flumes, the sidewalls are angled out at 45º.  The resultant cross-section is trapezoidal in shape.

Both flumes have their own advantages:  the Rectangular Cutthroat Flume is available in (4) standard lengths and (4) standard widths per length - for a total of (16) standard sizes - allowing the flume to be applied to a wide range of applications.  The Trapezoidal Cutthroat Flume is only available in one standard length and three standard widths, but provides much greater low flow sensitivity than similarly sized Rectangular Cutthroat Flumes. Trapezoidal Cutthroat Flumes also conform more readily to the shape of natural channels and irrigation ditches.

The simplicity of the Cutthroat design means that it is less costly to construct than other types of flumes.  In addition, the flat floor allows it to be easily retrofitted into existing channels, reducing installation costs and eliminated the need to either set the flume above the channel floor (increasing the upstream pool and head loss) or modifying the downstream channel.

A further advantage of the Cutthroat flume is that the angles of convergence / divergence are the same for all sizes of Cutthroat flumes, thereby allowing intermediate flume sizes of Rectangular Cutthroat Flumes to be developed by simply moving the sidewalls in and out as necessary and interpolating from the existing ratings.

Cutthroat Flume Applications

Cutthroat flumes have proven to be suitable to a wide range of applications, including:
  • Water Rights
  • Irrigation Flows
  • Stream Gauging
  • Dam Seepage Monitoring
  • Industrial Pre-Treatment Discharges
  • Sewage Treatment Plants
  • Spring Discharge
  • Proportional Flow Splitting
  • Well Pumping Tests
  • Mine Drainage
  • Tailwater Measurement

Many of these applications are possible because the flumes are specifically designed for use in flat gradient channels under both free-flow and submerged flow conditions.  With transition submergence, st, ranging from 60% to 80%, the Cutthroat flume can operate under free-flow conditions much longer than other similarly sized flumes. 

When used in submerged flow applications, two head measurements are made:  one in the converging section of the flume, the other in the diverging section of the flume. Due to the added instrumentation costs and operational complexity of operating under submerged flow conditions, Openchannelflow recommends operating flumes under free-flow conditions whenever possible.

Cutthroat Flume Accessories

Openchannelflow offers full and custom (half or extended) height Cutthroat flumes (in select sizes), nested (smaller flumes nested in larger flumes), and “magic bottoms” (replacement floors for existing Cutthroat flumes to raise the zero flow elevation). 

Half height flumes provide a convenient solution to applications where the anticipated flows do not warrant a larger flume, but where the flow channel is wider than one in which a smaller flume can easily be installed. 

Extended height flumes offer higher flow capacities than standard height flumes, but in a smaller footprint than a larger flume would occupy.  For accurate flow measurement, the recommended ratio of flow depth to flume length is recommended to be equal to or less than 0.4 for Cutthroat flumes.  Should extended height flumes be used, therefore, it should be noted that there will be a decrease in system accuracy above the 0.4 ratio level.

Available Materials of Construction

At its core, Openchannelflow believes in choice.  

As such, we are material neutral, believing that the material that a flume is made from should match our Customer's needs - not just what we make.  We are not just a fiberglass shop, nor are we just a a steel fabricator, what we are is a solutions provider - brining you flow products that best suite your particular needs.  

Openchannelflow Cutthroat flumes are available in a vareity of materials, including:

  • Aluminum
  • Fiberglass (FRP / GRP) 
  • Galvanized steel
  • Lexan (small sizes)
  • PVC (small sizes)
  • Stainless steel

Cutthroat flumes for Water Rights and Irrigation Flow measurment are typically fabricated from galvanized steel, while those for Sewage and Industrial Pre-Treatment flows are commonly fabricated from fiberglass or stainless steel.

In fiberglass, Cutthorat flumes range in price from $480 for an 18-inch L x 1-inch W flume to $13,410 for the large 108-inch L x 72-inch W flume.  Naturally prices will vary based upon the material of construction, accessories, and other factors.

Cutthroat Flume Evaluation Table

Sources:

Skogerboe, G., Bennett, R., Walker, W., Installation and Field Use of Cutthroat Flumes for Water Management, Water Management Technical Report No. 19, U.S. Department of State, Agency for International Development, PB 224 293, AER71-72GVS-RSB-WRW 4, March 1972.

Sewer Flow Measurement A State-of-the Art Assessment, U.S. Environmental Protection Agency, Municipal Environmental Research Laboratory, EPA/600/2-75-027, November 1975.