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The beauty of fiberglass is its versatility.  This versatility allows Openchannelflow to incorporate a range of different accessories into our Fiberglass Manholes.


fiberglass bench in an Openchannelflow parshall flume flow metering manholeWhen a flume is installed as part of a Flume Manhole, a space is created between the outside of the flume and the inside of the manhole barrel.  If not filled or covered, this space can fill with wastewater should there ever be a blockage downstream (and there will be).  

Fiberglass benching eliminates the need to pour a shoulder around the flume – a step routinely forgotten.  With a molded-in diamond plate surface, Openchannelflow fiberglass benches provide a safe and secure environment for the operators to work from.


All Openchannelflow Fiberglass Manholes come with a 2-inch [5.08 cm] coupling, but if you need another (or larger) coupling to bring power, sample lines, etc. into the manhole, additional couplings can be provided.

Manhole Extension

extension for an Openchannelflow fiberglass flow metering manholeSometimes thing change, and on the job site it is not unheard of needing to move a manhole because of something found during excavation (gas line, granite outcrop, etc.).  When that happens, you may need to adjust the depth of your manhole.  

Manhole extensions include a slip coupling that goes around the existing manhole barrel, which is them bolted / sealed in place.  If necessary, a manhole extension can also include a different top style.

For those times, Openchannelflow offers manhole extension kits to help you field modify your Fiberglass Manhole.

Equipment Panels

For vertical mounting of flow meters, transmitters, analyzers, and communications modules.


fold-up equipment shelf in an Openchannelflow fiberglass flume manholeFiberglass shelves provide convenient storage for equipment that needs to stay inside the manhole.  From flow meters to samplers, Openchannelflow can design an equipment shelf to fit you needs.

Manhole equipment shelves can be fixed or can be designed to fold-up when not in use.

Normally made of fiberglass, Openchannelflow also offers equipment shelves in stainless steel.

Two-Piece Construction

One of the hallmarks of the Fiberglass Manhole is its single-piece nature…and for most sizes / configurations that's true.  However, sometimes when larger flumes or other structures are integrated into a Fiberglass Manhole, the manhole becomes difficult to ship or handle on site.  For these manholes Openchannelflow offers two-piece construction.  

Two piece construction Cutthroat flume flow metering manhole

Unlike competing systems which rely on hand made flanges to seal the two manhole sections together (and which can leak), Openchannelflow offers an adhesive channel that positively seals and chemically bonds the manhole sections together.

The system consists of a specially formed channel at the top of the base section of the manhole and a special adhesive that is poured into it.  The base section is set, the top section is moved into position, and then the channel is completely filled with an adhesive mixture – chemically bonding the two sections together.  The resultant bond is watertight and extremely strong.


To help circulate the air in a manhole and reduce the build-up of hazardous gases, Openchannelflow offers fixed ventilation systems.  These vents – usually 4-inches [10.16 cm] and larger in diameter – can be used alone or in pairs.

The vents can be flush to the manhole barrel wall or can be of a gooseneck design.