An Introduction to Open Channel Flow Measurement

An Introduction to Open Channel Flow Measurement

  • A variety of techniques are available for measuring open channel flow with Primary and Secondary devices
  • Primary devices (Flumes, Weirs, Nozzles etc.) in conjunction with Secondary devices such as level measurement systems, this introduction only covers the most common weirs and Flume types.
  • The Flow Meter is programmed with the type of Primary device and it’s size
  • The Flow meter (secondary device) then converts the Level in the Hydraulic structure (primary device) to flow based on a known relationship of liquid level and the flow rate of the stream
  • Flumes and weirs are used extensively in wastewater treatment plants
  • Care must be taken when the system is installed to ensure accuracy


  • A measuring weir is simply an overflow structure built perpendicular to an open channel axis to measure the rate of flow of water.
  • Weirs are also classed as dams
  • A properly built and operated weir of a given shape has a unique depth of water at the measuring station in the upstream pool for each discharge.

Broad Crested Weirs

  • Broad crested weirs are robust structures that are generally constructed from reinforced concrete and which usually span the full width of the channel.  They are used to measure the discharge of rivers, and are much more suited for this purpose than the relatively flimsy sharp crested weirs.  

Humber River Weir

Humber River Weir

V-Notch weir

V-Notch Weir


  • Must take care in manufacture and installation
  • Must know flow range to size correctly
  • The measurement of head on the weir is the difference in elevation between the crest and the water surface at a point located upstream from the weir a distance of at least four times the maximum head on the crest.
Weir Point of Measure


  • Flumes are shaped, open-channel flow sections that force flow to accelerate. Acceleration is produced by converging the sidewalls, raising the bottom, or a combination of both
  • Flumes range in size from very small-1 inch (in) wide-to large structures over 50 feet (ft) wide
  • Flume head loss is less than about one-fourth of that needed to operate a sharp-crested weir having the same control width, and in some long-throated flumes, may be as low as one-tenth. Another advantage compared to most standard weirs is that for a properly designed and installed flume, the velocity of approach is a part of the calibration equations

Flume Types

  • Short Throated Flumes, Parshall, Palmer Bowlus, Leopold Lagco etc
  • Long-throated flumes control discharge rate in a throat that is long enough to cause nearly parallel flow lines in the region of flow control.
  • Long-throated flumes can have nearly any desired cross-sectional shape and can be custom fitted into most canal type site geometries.

Flat bottom from entrance to exit for better head conversion.
Do not require a free-fall discharge to operate correctly.

Natural shape of flume mimics many earthen and concrete-lined ditches.

Flat floor and sloping wall allows solids to pass through throat without being trapped.

Palmer-Bowlus Flume

  • Wide range of flow in each throat size
  • Insert type can fit in the benched half pipe in a manhole or in a section of open pipe
  • Three types to suit a variety of applications
  • Insert, Exit or Cutback and Permanent
  • Must know flow range to size correctly
  • Constructed from Fibreglass Reinforced polyester (FRP)
Palmer Bowlus Sizes

Palmer Bowlus Flume installed

Palmer Bowlus Installed

Insert type Palmer Bowlus Flume

Insert Type Palmer Bowlus Flume

Palmer-Bowlus Flume Exit type

Exit Type Palmer Bowlus Flume
H Flumes

                        Originally developed to measure agricultural water runoff, H flumes have a “V” shaped design that allows for measurement of a wide flow range. Additionally, H flumes are used to measure irrigation water, snow melt, holding pond overflow, industrial process discharge, as well as sanitary and storm sewer discharge.

Hinged wing walls direct flow through the flume

H Flumes

Since 1922, the Parshall Flume has been the most widely used type of flume for fixed flow monitoring installations. This special shaped open channel flow section can be installed in a ditch or canal to measure the flow rate.

  • Can be constructed from Concrete, Steel or Fiberglass Reinforced Polyester (FRP)
  • Since an FRP flume is produced from a mold the characteristics of each flume remain constant and true to design criteria
  • The most widely known flume and still the most widely used for permanent installations. Used in monitoring sewage, plant effluent and irrigation water. Available in sizes 1″ through 12 feet.

Large Parshall Flume

Level Measurement Point

  • With all flumes there is a drawdown curve, that is the surface level falls as the water accelerates to the discharge point, and it is therefore essential that the measurement of depth of flow must be made at precisely the specified distance upstream from the control section

For further information please contact the author Peter Smyth

Peter Smyth

Technical Sales

2851 Brighton Rd.

Oakville, ON. Canada L6H 6C9

T.F. 1-800-215-4469 Ext.225

Tel: 1-905-829-0030 Ext.225



Skype: peter.smyth27 Information contained herein has been assembled from various open sources but in particular from the USBR Water Measurement Manual