Skip to main content

Why It's Important to Pour a Bench in a Metering Manhole

Let’s be honest:  metering manholes (from an upfront, material cost standpoint) aren’t cheap.  While no one would argue that metering manholes don't provide substantial benefit to their users, part of what you are paying for is a unit that is ready for immediate installation and use. 

Unfortunately, an overlooked, but vital, part of the installation of a metering manhole is the pouring of a concrete bench around the flume. 

plasti-fab packaged metering manhole without a bench

The bench serves two purposes. The first is as a place for maintenance personnel to stand when they are at the bottom of the manhole servicing the flume and / or the flow meter.  The second is that should flow back-up into the manhole due to a downstream blockage, the bench allows flow to be directed back into the flume and out the manhole once the blockage has been cleared.

Nested Cutthroat flume manhole without a manhole bench

Without a bench or fiberglass shoulder in the manhole, water will overtop the flume and fill the void spaces between the flume and the manhole barrel.  If this happens, there is no easy way to get the standing water back into the flume.  The standing water becomes a source of odors, makes accessing the flume / flow meter more difficult and dangerous, and can act as a breeding pond for a variety of pests.

Trapezoidal flume metering manhole without a bench

Customer Feedback

In talking with our customers, we realized that about half of the time the manhole installer decided to skip this step and did not pour the suggested bench around the flume.  The result is that, for users without the bench, maintenance personnel must straddle the flume – standing on the top flanges.  This in inconvenient at the least and dangerous at the worst.

Openchannelflow's Stand

Openchannelflow feels so strongly about the need for a bench around the flume that our industry-leading (5) year FiberLock warranty now requires verification that either a concrete bench has been poured or our optional fiberglass bench has been used.

Plasti-fab fiberglass packaged metering manhole with concrete bench poured around a Parshall flume

Now we realize that from a contractor's standpoint, pouring a bench costs money.  You have to get a concrete truck and also a pump truck to do the pour properly and that takes time and money - particularly if the bench is installed after the manhole has already inplace.  

Fiberglass manhole bench / shoulder / surround in a packaged metering manhole

To assist our customers, Openchannelflow offers integral fiberglass benches.  These benches are factory molded and form a non-slip surface for operators in the manhole.  The integral benches also speed installation time and elminate the risk of deforming the flume duirng the installation of a traditional concrete shoulder.


Ultimately, our goal is to ensure that, as a customer, you have the best possible user-experience.  

We continually refine our products, processes, and materials and want to ensure that you receive maximum value for your money.  We believe that having a bench in our metering manholes helps to bring you that value.

learn more about metering manholes

Related Articles