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Excessive Approach Velocity in Parshall Flume Flow Measurement Applications

Since the 1930s, wastewater professionals have relied on the Parshall flume as the primary device for open channel flow measurement applications. Parshall flumes provide a very repeatable relationship between water level (Ha) and flow rate; but, only so long as the approaching flow conditions are appropriate.

For any flume design, the hydraulics must cause a condition of subcritical flow with as much energy, velocity, and speed removed before entering the flume.

  • Subcritical Flow – a hydraulic condition in which inertia forces are less than gravitational forces. In this state gravitational forces are predominant and the flow has a low velocity which can be described as tranquil and streaming.

The geometry of a Parshall flume is such that it induces supercritical flow by means of a restriction in the flume’s sidewalls and a drop in the flume floor. This allows for a single point head measurement to be made upstream of the throat at Ha in order to calculate the flow rate. However, excessive approach velocities diminishes the designed purpose of this restriction point in the flume pushing through the flume faster than it should, and as a result registering a lower depth at the Ha metering location.

Recently, we encountered a flow measurement application in a wastewater collection system with excessive approach velocity at a Parshall flume. We were originally called to demonstrate the accuracy of the Teledyne Isco LaserFlow Non-Contact Flow Meter.

The LaserFlow Flow Meter installation was about 200 feet upstream of a metering vault with an 18 inch Parshall Flume which would serve as a comparison flow measurement. Real time flow data was monitored using 2105CI modems installed in the field which pushed data to a remote server.

Almost immediately, we noticed that the Parshall flume reported flow rates consistently lower than the LaserFlow; and during periods of high flow, the delta was often in excess of +/- 10%.

 During the LaserFlow installation, it was noted that the approach velocity seemed very high; however, we did not inspect the downstream vault to see if velocity remained high entering the flume. Once the LaserFlow vs. Flume delta pattern was established, we measured the approach velocity within the vault with an electromagnetic point velocity sensor; both ahead of and in multiple locations within the flume. Measurements were made at 3 depths to confirm that the velocity was increasing with depth with the following results; explaining how the error was greater during higher flows.

Depth Velocity
0.5 Feet 3.8 ft/sec
0.5833 Feet 3.9 ft/sec
0.6667 Feet 4.4 ft/sec


To determine if the approach velocity was too high we first had to calculate the critical velocity which equal to the Square root of the depth of water in throat of flume x 32.2 (gravity acceleration). In this application, the critical velocity of a Parshall Flume at 0.6667 feet of head is calculated as follows:

Critical Velocity = √0.6667 x 32.2 = 4.633

Plasti-Fab, Inc. recommends that flumes should have an approach velocity that is half (1/2) the critical velocity. In this instance, 2.3 feet per second is the recommended speed of water entering the flume with 0.6667 feet to achieve the highest accuracy of measurement.

For existing sites with excessive approach velocity, there are available remedies.

  1. Installing a Plasti-Fab Energy Absorbing Manhole upstream of the flume.
  2. Installing a Plasti-Fab Magic Bbottom inserted into the Parshall flume. A Magic Bottom raises the floor of the flume which in some cases may cause enough change in the critical depth upstream of the flume to allow the velocity to become subcritical. Magic Bottoms are more commonly applied in submerged flow conditions but can be a solution for excessive velocities. To read more on magic bottoms follow this link

For more information, please call us at 1-800-333-2252 or visit our website, https://cclynch.com./


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