HECRAS Help WSEL around bridge seems unrealistic 1

[fg71]

I'm modeling the 100-year storm in a river using HEC RAS but the water surface elevation immediately downstream of a bridge seems unrealistic. The model shows the bridge being overtopped by about 8 feet (WSEL = 28.21 ft). Just downstream of the bridge, the WSEL dips down almost 10 feet to 18.24 ft (presumably going into supercritical flow even though the Froude # is < 1). The next cross section downstream, the WSEL goes up to 20.21. This dip and subsequent hydraulic jump seems unrealistic in this situation. I would expect the WSEL to go down a bit after it goes over the bridge but not to the extent the model is showing (and certainly not into supercritical flow). What could be causing this? Is there a way to change the model settings/parameters so that I would get more realistic results?

Thanks

 

[francesca]

Have you done your contraction/ineffective flow areas upstream and downstream? Also are you using the pressure flow option?

 

[fg71]

The ineffective flow areas were set and are OK. I did change it to pressure flow (didn't realize the energy only option had been chosen) which helped with the upstream WSEL but I'm still getting that dip and hydraulic jump downstream of the bridge. The WSEL is now:
22.91 at the bridge
18.24 just downstream of the bridge
20.23 at the at the next cross section downstream

The bridge deck is approximately 20.20. I think the entire profile is more accurate now other than the one cross section immediately downstream of that bridge. It seems unrealistic that this stream would experience a hydraulic jump in that location during high flows.

Thanks again.

 

[francesca]

Instead of ineffective flow area I should have said expansion reach, so if you managed to decipher that, thanks.

I would expect your water surface elevation to drop through/after the bridge and then rise again due to the contraction, increased velocity (continuity equation), consequent increase in velocity head and therefore decrease in potential energy (energy equation).

I'm guessing that 18.24 ft is not supercritical, so it's probably an okay answer, but to be sure you can/should add more cross-sections. If it is a product of channel geometry adding cross-sections will not change it.