The expansion takes place abruptly ( 0,10 m distance between upstream and downstream XS).
If I expand my cross section until around 2.2 m, the flow maintain its subcritical state in the whole reach and the water level rises after the expansion, as expected.
However, if i continue to increase the channel width a hydraulic jump takes place.
Initially i was modelling for mixed regime. Keeping the same geometry and changing to subcritical regime gives the following result:
I am not quite sure if i understand this situation.
My explanation follows:
Analyzing my first section, the flow profile should follow a M2 curve.
Expanding my channel width beyond 2,2 m cause the normal depth in the expanded section to be below the critical depth of the preceding one. Therefore the flow will be limited by the critical depth and a free overfall after the control section will happen.
That said, I am prone to believe that modelling the whole channel as subcritical gives me the most realistic result.
However I am not quite sure. Could someone shed a light on this?