Yes, certain flume types allow for the nesting of smaller flumes inside of larger flumes.  

The ability to nest flumes inside each other can be useful in situations where design or process changes have resulted in an existing flume being oversized for the current flow rates, or alternately, it can allow for systems to be optimized for current low flow rates with an eye towards expanding to larger flow rates in the future.  Additionally, operations that experience regular, extended seasonal fluctuations (i.e. resorts) may also benefit from the use of nested flumes.

Nested flume inserts are intended to fit level in the larger flume.  The nesting flume is typically provided with inlet and outlet wing walls so that it fits snuggly into the larger flume.  The connection between the two flumes should be caulked or sealed so that the join is watertight.  

Stainless steel fasteners may be used to permanently secure the nesting flume to the larger flume.  Finally, the spaces between the exterior of the nesting flume and the interior of the larger flume may be filled with sand capped by grout (or similar materials) to reinforce the smaller flume and ensure that water does not collect between the two flumes.  

Flumes available for nesting within each other include:  Parshall, Palmer-Bowlus, Montana, and Cutthroat flumes.  Combinations of dissimilar flumes are also possible (i.e. Trapezoidal in Parshall).