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Palmer-Bowlus Flume Accuracy

The accuracy of the Palmer-Bowlus flume is +/-3-5% under normal operating conditions – where the head in the flume is large in comparison to the length of the throat.  For lower flows, where the head is low in comparison to the length of the throat, the accuracy decreased to +/-5-6%.

The decrease in accuracy reflects the increased importance of influence of the boundary layer at low flows.

Where the upstream / downstream / installation conditions are less than optimal or where the flume is out of dimension will exhibit accuracies less than above and may require field calibration.

Unlike the Parshall or Cutthroat flumes, the are no corrections for Palmer-Bowlus flumes that have settled (or been installed at a slope).

The above holds true for Palmer-Bowlus flumes experiencing free-flow.  For installations where downstream conditions restrict the flow out of the flume, submergence may become a factor.  There are no submergence corrections for Palmer-Bowlus flumes and as such submerged flow conditions should be avoided.

Palmer-Bowlus Flume Flow Equations

The general free-flow discharge equation for Palmer-Bowlus flumes may be expressed as:

Free flow discharge equation for Palmer Bowus Flume

Throat Width

K (CFS)

K (L/S)

n

4”

1.68

468.34

1.9

6”

2.18

607.7

1.9

8”

2.59

722.03

1.9

10”

2.96

749.9

1.9

12”

3.31

922.75

1.9

15"

3.79

1056.5

1.9

18"

4.23

1179.22

1.9

21"

4.61

1285.16

1.9

24"

5.03

1402.25

1.9

 

Palmer-Bowlus Flume Flow Tables 

Free-flow discharge tables for Palmer-Bowlus flumes can be found in the Discharge Tables section.

Palmer-Bowlus Flume Flow Resolution

While the accuracy of the Palmer-Bowlus flume is comparable to that of a Parshall flume, the Palmer-Bowlus flume does produce a smaller change in level for a given change in flow than would otherwise result in a Parshall flume.  

Changes in the flow rate, then, are less distinguishable with a Palmer-Bowlus than with a Parshall.  The Palmer-Bowlus is said to have less resolution than a Parshall.

Palmer-Bowlus Flume Operating Range

The Palmer-Bowlus is best when used within a defined band of levels. 

  • The minimum head at the point of measurement should be no less than the larger of 0.1 L (L = throat ramp top length) or 0.2-feet (0.06 m).
  • The maximum head should not exceed 0.6 L.

For Openchannefllow Palmer-Bowlus flumes, the throat ramp top length (L) is equal to the flume width (D – also the flume size).

Substituting and consolidating, the recommended head should be between:

0.1 D < H < 0.6 D

Remember that H is measured at the same level (although upstream) as the top of the throat ramp (which is itself D/6 above the floor of channel in the upstream portion of the flume).

Therefore, if an operator is measuring from the water's surface to the bottom of the flume, the total reading should be:

0.27 D < H < 0.77 D

Sediment / Debris Passage

At low flows when the solids content is high, sedimentation may occur upstream of the throat ramp.  This sedimentation may not clear on its own.  Flumes experiencing sedimentation may require periodic maintenance.

All sizes of Palmer-Bowlus flumes are suitable for use on unscreened sanitary flows.