A Trapezoidal Flume installed in a concrete vault experiencing a downstream restriction.  

Flow in the picture is from left to right.  

Looking at the right end adapter you can see that while the flow is still under free flow conditions - as evident by the hydraulic jump in the discharge section of the flume, the boiling water present in the outlet end adapter (and wetness around on the vault floor because of it) indicates that beyond the current flow rate, the downstream pipe will not have enough carrying capacity to avoid submerging / overflowing the flume. 

Remember, when sizing a flume it is important to consider the carrying capacity of the downstream channel or pipe.