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In this article: flume, how it works

While it is important to understand the various sections of a flume and their associated functions in proper flow measurement, from a practical standpoint it is also important to be able to identify the functional elements of a flume and the purposes they serve.

Parshall Flume Components

1)  Dimensional bracing

2)  Interior flow surface

3)  Anchor clip

4)  Top / end flanges

5)  Stiffening rib

6)  Flume exterior

Dimensional Bracing

Dimensional bracing is provided at the top of the flume to help to ensure that a flume maintains the proper dimensions at the converging / throat / diverging sections.   At the base of a flume the floor / sidewall connection holds the flume rigid and dimensional stable. At the top, there is no floor to hold the flume rigid and dimensionally stable.  Bracing serves this purpose will stilling allowing access to the interior flow surfaces of the flume.  Without bracing the walls of the flume would want to come together – reducing the cross-sectional area of the flume as you go up the sidewall.

Dimensional bracing typically takes the form of L or U-shaped angles when mounted above the top of the flume, but can also consist of round or square rods (hollow or solid) when mounted just below the top of the flume.  The bracing may be bolted, chemically bonded, or welded to the flume depending upon the construction of the flume and bracing.  Even if removable, dimensional bracing should always be left on the flume during installation.

If the bracing is removed after installation, it is important to verify the dimensional accuracy of the flume.

For flumes with integral bolt-down covers, dimensional bracing may be omitted as the covers themselves serve the purpose of keeping the top of the flume in alignment. 

Remember that the inclusion of dimensional bracing does not alleviate the need to brace or crib the flume internally during installation.  Bracing and cribbing is intended to keep the sidewalls and /or floor from distorting during installation and must be removed after a flume is installed.

Interior Flow Surface

The interior flow surface of a flume is just that – the surface that is exposed to the process flow.  On fiberglass flume, the flow surface is the “gloss” surface of the part.  The interior of any flume should be smooth and free of scum, build-up, and vegetative growth.  Flow surfaces should be inspected periodically and cleaned as necessary. 

Anchor Clips

Anchor clips are L-shaped angles attached (chemically bonded or welded) to the exterior of a flume.  Each anchor clip has a hole drilled through it (typically 5/8” [1.5875 cm] in diameter) to aid in the installation of the flume. 

When installing a flume in concrete, the ancho clips are used to key the flume into the concrete.  Wire or rebar may be run through the holes in the clips to secure the flume to the channel structure prior to concrete encasement. 

It is important to note that anchoring clips are not intended to prevent the flume from floating or shifting during installation.  Sandbags or other materials are necessary to accomplish this task. 

On small flumes anchoring clips may be used to secure and level the flume.  Threaded rod can be run through the holes in the anchoring clips and secured in place through the use of a nut/washer combination.  This method of securing / leveling a flume is not recommended for medium and large size flumes.

h flume suspended by threaded rod through the anchor clips and top flange

Top / End Flanges

Flanges on a flume provide rigidity, particularly along long, straight edges.  They also aid in securing (or keying) a flume into the channel.  In earthen installations, end flanges can be extended to help stop scouring at the ends of the flume as well as flow bypassing the flume.

A secondary use of flanges is to provide a convenient mounting location for dimensional bracing, flume accessories, and end transitions (end adapters and wing walls).  Flanges may be drilled as necessary to accommodate these items.

The top flanges can also be used to run threaded rod through for a suspended flume installation.  This approach works only for smaller flumes.

suspended h flume

Flanges are not critical flow elements of a flume and as such limited trimming may be allowed so long as the rigidity of the flume is not affected and any cut (for fiberglass and galvanized steel construction) is sealed.

Stiffening Ribs

Stiffening ribs are provided along the sides and floor of a flume to provide stiffness along flat, unbroken surfaces. 

In welded construction flumes (PVC or steel), stiffening ribs are typically L-shaped angles welded to the outside of the flume. A welded rib is a stiffening rib and is integral to the structure. 

For fiberglass constructed flumes, stiffening ribs are made by securing a form to the outside of the flume and then laminating over the form.  Here the fiberglass laminate over the form that provides the strength – not the form itself. Rib shapes may come from cardboard, PVC, or engineered foam.  Of these materials, engineered foam is the preferred method as the shape of the foam may be tailored to the particular needs of the laminating process. 

Cardboard and PVC rib forms come from standard shapes (U-shaped, round, square) and do not take into account the needs of the laminating process.  Poor laminate bonding and interstitial voids around rib forms made of cardboard and PVC are common.

Flume Exterior

The flume exterior is simply the outside of a flume where anchor clips and some accessories (i.e. attached stilling wells) may be attached.  As the surface is not exposed to flow, it may be patterned (fiberglass) or smooth (PVC, galvanized, or stainless).  If the exterior of the flume is exposed to direct sunlight / atmosphere and is constructed from fiberglass, it is important that the exterior surface be protected with U.V. inhibited gel coat.

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