More common on Parshall flumes, end adapters are also available on Cutthroat flumes from Openchannelflow. 

End adapters are specially formed transitions to allow water to flow into / out of a flume in a controlled manner.  On the inlet, end adapters are designed to spread, straight, and normalize the velocity pattern of the flow before it enters the converging section of the flume.  On the outlet, end adapters are designed to transition the flow back into the downstream piping (or channel).

As alluded to above, end adapters are most commonly used to transition PIPED flow into / out of a flume.  They can, however, be provided with OPEN ends to allow for flow transitioning / conditioning in channels, canals, and ditches. 

Openchannelflow’s Cutthroat flume end adapters can be readily modified to suit specific site needs. On the inlet, the most common modifications include the integration of energy absorbers (to dampen energetic pipe flows) and flow straighteners (to normalize the velocity profile before it enters the flume). In either case, the modifications are typically only used for clean flows (or those not carrying large particular matter) as the modifications can act as screens on all but the finest of solids. 

In the outlet, it is most common to see the end adapter extended vertically to allow the flow exiting the flume to rejoin a pipe or channel lower than the flume floor. While there is always some rise (inlet) / fall (outlet) to the Cutthroat flume end adapters (to help calm the flow on the inlet and to help avoid submergence on the outlet), occasionally this is insufficient and an extended outlet drop is required.

End adapters are available for standard length and widths of Cutthroat flumes offered by Openchannelflow and are also available for flumes of a custom width (within a standard given length).