For a flume to operate properly the flow entering it must be:

• Open channel (free surface)
• Well-conditioned
• and have a sub-critical velocity 

In below grade applications, one device can help a user do all three:  The Energy Absorbing Manhole.

The energy absorbing manhole is designed to condition high energy / potentially full pipe flows so that when they leave the manhole they are slower, no longer running full pipe, and are well conditioned.

Energy absorbing manholes are larger diameter manholes with an energy dissipating baffle plate in the front third of the manhole and two curved sidewalls in the last third.  As the flow enters the energy absorbing manhole, it hits the baffle plate, is slowed and then forced to flow around the plate. Once past the plate, the now slower flow is smoothly directed into the downstream pipe by the curved sidewalls.

Energy absorbing manholes can be used to eliminate the long upstream requirements usually recommended for hydraulic jumps (30-throat widths).  In some many cases, a flume or weir manhole can be placed immediately downstream of the energy absorbing manhole…greatly reducing installation costs and space requirements.