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Flume Selection Guidance

 Parshall Flume DesignOne of the most frequent requests that we get from civil engineers during WWTP design is for flume selection guidance. While there are many complex aspects that need to be considered in flume selection and outfall channel configuration, there are a few basic considerations that can be evaluated to streamline the process.  While not all inclusive, the following considerations should help guide you to selecting the most appropriate flume for the application.  Additional guidance can be found on our resource center at www.cclynch.com/resource-center.php.


  1. Define the minimum and maximum flow rates expected at the facility. It is important to consider the minimum and maximum instantaneous flow rates, not just minimum and maximum daily total flow rates.
  2. Identify the flume type and size that accommodates the anticipated range in flows.  In many cases, more than one flume will accommodate the range in flows.
  3. Consider the shape of the channel or pipe.  In a collection system, particularly in an existing manhole, it will be easier to install a Palmer-Bowlus Insert flume than a Parshall flume.  The transition from a trapezoidal channel to a trapezoidal channel will be smoother and most likely easier to install than a Parshall flume in a trapezoidal channel.
  4. Evaluate the head loss through the flume.  Can the channel upstream of the flume handle the max water surface elevation at peak flow?
  5. Consider the amount of solids in the flow stream.  Some flume types pass solids through them better than others.
  6. At this point in the evaluation the list of potential flumes is typically narrowed down to perhaps 2 or 3 flume sizes and types.  Compare the flow charts for each flume type and determine which flume has the best resolution.  Many times you’ll find that one flume type may have better resolution at the low end of the flow range but not as good at the high end (for example a Trapezoidal or H-flume compared to a Parshall flume).  We recommend selecting the flume that has the best resolution near the average flow rate. Sometimes it may be critical to capture flow rates near 0 GPM, in which case, a sacrifice on flume resolution at the higher end might need to be made.


Consideration should also be given to the channel immediately upstream and downstream of the flume.   Accurate flow rate measurement is highly dependant upon the approaching flow conditions.  The approaching flow should be well distributed, laminar, and free of surface waves.  To aid in the transition of flow from what is typically a wider approach channel into the flume, the use of curved wingwalls or flared transitions should be used to guide the flow into the converging section of the flume without creating eddies or surface waves.  Right angles should always be avoided.

In addition, the approaching velocity should be subcritical.  In some cases the use of tranquilizer racks or energy absorbing plates can be used to reduce the velocity immediately upstream of a flume.  These criteria are more significant than simply stating a number of straight run requirements upstream of the flume.  Some installations can handle less than 10 times the channel width straight run rule of thumb so long as the flow is laminar and the velocity is not excessive.  To determine if your approach velocity is too high there are a few basic calculations you can perform.

            -Critical velocity through the throat of the flume = (Gravity X Head )1/2

              Where Gravity = 32.3 fps

            -Recommended approach velocity should be less than half the critical velocity.  Calculated by:

              fps = 0.5√(32.2) (Head)

Make sure that the channel downstream of the flume will carry water freely away preventing a submerged flume condition.  Submerged flow occurs when resistance in the flow downstream of the flume is significant enough to reduce velocity and increase water depth causing a backwater effect at the flume and an over reporting of flow.  In some cases the flume is installed at an elevation higher than the channel in order to create a drop in the channel immediately downstream to help carry the flow away from the flume.  In the images below you can see the same trapezoidal flume but on the left the flume is in a free flow “normal” operating state while on the right the flume is submerged due to an overgrowth of vegetation within the channel downstream.

Trapezoidal Flume

Trapezoidal Flume With Free Flow

Submerged Flume

Same Trapezoidal Flume Under Submerged Condition Due To Downstream Vegetation In Channel















C.C. Lynch & Associates, Inc. has represented Plasti-Fab, Inc. for 40 years and has experience with all standard flume types in most all applications.  Additional information on various flume types can be found at www.cclynch.com/products/flumes.php.  Contact us today to discuss your flume application.


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