For a Palmer Bowlus Flume to operate as designed, the flow out of the flume must be free and not restricted by downstream conditions.  When downstream resistance to flow increases above a certain point and now reduces the upstream velocity through the flume, a backwater effect is created.   This transition from free, unrestricted flow to one of backwater / slowed velocity is known as the submergence transition (S_t).

It is important to remember that although a Palmer Bowlus Flume may have initially designed for free-flow, changes in the downstream hydraulics can submerge the flume.  For Palmer Bowlus Flume, these changes are typically the result of sediment deposit, added hydraulic structures, or reconfigured channels.  Don't assume that a once free-flowing flume will always be a free-flowing flume.