Montana flume


A Montana flume, is a popular modification of the standard Parshall flume. The Montana flume removes the throat and discharge sections of the Parshall flume, resulting a flume that is lighter in weight, shorter in length, and less costly to manufacture. Montana flumes are used to measure surface waters, irrigations flows, industrial discharges, and wastewater treatment plant flows.
As a short-throated flume, the Montana flume has a single, specified point of measurement in the contracting section at which the level is measured. The Montana flume is described in US Bureau of Reclamation's Water Measurement Manual and two technical standards MT199127AG and MT199128AG by Montana State University.
As a modification of the Parshall flume, the design of the Montana flume is standardized under ASTM D1941, ISO 9826:1992, and JIS B7553-1993. The flumes are not patented and the discharge tables are not copyright protected.
A total of 22 standard sizes of Montana flumes have been developed, covering flow ranges from 0.005 cfs to 3,280 cfs .
Lacking the extended throat and discharge sections of the Parshall flume, Montana flumes are not intended for use under submerged conditions. Where submergence is possible, a full length Parshall flume should be used. Should submergence occur, investigations have been made into correcting the flow.
Under laboratory conditions the Parshall flume - upon which the Montana is based - can be expected to exhibit accuracies to within +/-2%, although field conditions make accuracies better than 5% doubtful.

Free-Flow Characteristics

The Montana Flume is a restriction with free-spilling discharge that accelerates flow from a sub-critical state to a supercritical one.
The free-flow discharge can be summarized as
Where
Montana flume discharge table for free flow conditions:
Throat WidthCoefficient Exponent
1 in.3381.55
2 in.6761.55
3 in.9921.55
6 in2.061.58
9 in3.071.53
1 ft3.951.55
1.5 ft6.001.54
2 ft8.001.55
3 ft12.001.57
4 ft16.001.58
5 ft20.001.59
6 ft24.001.59
7 ft28.001.60
8 ft32.001.61
10 ft39.381.60
12 ft46.751.60
15 ft57.811.60
20 ft76.251.60
25 ft94.691.60
30 ft113.131.60
40 ft150.001.60
50 ft186.881.60

Free-Flow vs. Submerged Flow

Free-Flow – when there is no “back water” to restrict flow through a flume. Only the single depth needs to be measured to calculate the flow rate. A free flow also induces a hydraulic jump downstream of the flume.
Submerged Flow – when the water surface downstream of the flume is high enough to restrict flow through a flume, the flume is deemed to be submerged. Lacking the extended throat and discharge sections of the Parshall flume, the Montana flume has little resistance to the effects of submergence and as such it should be avoided. Where submerged flow is or may become present, there are several methods of correcting the situation: the flume may be raised above the channel floor, the downstream channel may be modified, or a different flume type may be used. Although commonly thought of as occurring at higher flow rates, submerged flow can exist at any flow level as it is a function of downstream conditions. In natural stream applications, submerged flow is frequently the result of vegetative growth on the downstream channel banks, sedimentation, or subsidence of the flume.

Construction

Montana flumes can be constructed from a variety of materials:
Smaller Montana flumes tend to be fabricated from fiberglass and galvanized steel, while larger Montana flumes can be fabricated from fiberglass or concrete.
In practice, is it usual to see Montana flumes larger than 48-inches as the need for free-spilling discharge can not usually be met, downstream scour would be excessive, or other flume types better handle the flow.

Drawbacks