Adding a bridge with no rise requirement

[Anonymous]

I am adding a proposed bridge to a USCOE Hec-Ras model of an existing river reach.
The project requires a no-rise solution.
I have started by adding the bridge between two conveniently located existing cross-sections.
My plan was to single span the existing 100 year flood limits, but I cannot get a no-rise even if my low beam is significantly higher than the existing 100 yr WSE, and my bridge abutments are outside the existing flow limits.
I have tried lowering my roughness coefficients for the internal bridge cross-sections.
The increased backwater will not go away.
Any suggestions?

[Anonymous]

higher mannings n will decrease this. But what are you expansion and contraction coefficients for this?

[Anonymous]

I have not changed the expansion and contraction coefficients from the default .1 and .3 since the bridge I added does not restrict the existing flow. I also did not add ineffective flow areas because I am not looking at anything past the 100 year flow. Could this make a difference?

[Author]

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