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One of the more misunderstood flumes used to measure open channel flow is the Montana (Short Parshall) flume…but it doesn’t have to be! 

The Montana flume is a modification to the Parshall flume where both the throat and discharge sections have been removed – leaving only the flat-bottomed, tapering width approach section.  Like the Parshall flume from which it is derived, the Montana flume is sized by the throat width alone.

stainless steel 3-inch Montana flumes

The Montana flume is not, however, a Parshall flume where only the discharge section has been removed (the approach and throat remaining).  That configuration is correctly termed a Short-Section Parshall, on which little research has been conducted.
 
Several sources layout  the configuration of the Montana flume:

  • MT 9127 (AG) Montana (Short Parshall) Flume (Part 1)
  • MT 9128 (AG) Montana (Short Parshall) Flume (Part 2)
  • Water Measurement Manual, 3rd Edition, United States Bureau of Reclamation, Department of the Interior

Montana flumes are successfully used in a wide range of applications:

  • Sanitary flows 
  • Industrial discharges
  • Stream gauging
  • Irrigation studies
  • Drainage studies
  • Seepage monitoring
  • Spring discharge
  • Mine discharge
  • And more…

In applying a Montana flume to sanitary flows remember that only throat widths 3-inches [7.62 cm] and large should be used.  1-inch [2.54 cm] and 2-inch [5.08 cm] Montana flumes will clog when used on sanitary flows.

Advantages of Montana Flumes

  • Uses the same discharge equations / flow tables as the Parshall flume
  • Shorter lay length
  • Flat-bottom makes installation easier 
  • Reduced cost
  • Ease of fabrication
  • Available with standard Parshall inlet end adapters

Disadvantages of Montana Flumes

  • Not as well know as the Parshall flume
  • Requires free-spilling discharge under all flow conditions
  • Cannot correct for submergence
  • Lack of discharge end adapter to collect flow

Historically the limiting factor against the wider use of Montana flumes is the requirement to have free-spilling discharge.  The absence of the throat and energy recovering discharge section mean that the flume cannot withstand submergence;  but where space or budgetary constraints preclude the use of a Parshall flume (and free-spilling discharge can occur), the Montana flume may be a good choice.

learn more about Montana flumes

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