Water Profile - Subcritical Channel with abrupt expansion

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Water Profile - Subcritical Channel with abrupt expansion

Hello everyone,

I was just playing with HEC-RAS, modelling some contractions/expansions reaches and stumbled upon a scenario, that i can't quite explain.

My model consists of a channel with two distinct cross sections, a normal and an expanded one.

I modeled the expanded cross section for different widths, varying from 2 to 3 m.

The boundary condition both for upstream and downstream was set as "Normal Depth = 0.005"

Cross Section 1 - Rectangular Channel
B=1.5 m
S=0.005 m/m

Cross Section 2 - Rectangular Channel

B=2,0 m / 3,0 m
S=0.005 m/m

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?

Best Regards,

Lucas Barbosa