HEC-RAS: Ineffective flow limits for bridges at channel bends

[jg2010]

I am working on a HEC-RAS analysis for a roadway widening project where we will be adding a second bridge downstream of the existing bridge. The existing bridge will remain in place. The HEC-RAS model for the existing bridge did not include ineffective flow areas, so I am updating the model to include them upstream and downstream of the bridge (among other things).

The creek bends approximately 45 degrees just downstream of the bridges. I have run some preliminary calcs for the length of contraction and expansion, as well as the contraction and expansion ratios. I have attached a .jpg of the cross section locations, along with approximate ineffective flow limits (assuming there was no bend in the downstream channel) - as you can see from the .jpg, the left ineffective flow limit would almost completely block the downstream channel by the time the flow has fully expanded.

My question - is there any guidance available on how to adjust the inefective flow limits to account for this bend in the downstream channel?

 

[francesca]

You need to identify the direction that water is flowing, which isn't perpendicular to your bridge. Your bridge is skew. Align your ineffective flow areas to the skew angle. I would consider the water flow to be oriented at something like your left 22.5-deg expansion limit, and would rotate the expansion and contraction reaches about that axis.

It's not a great alignment, and to paraphrase Jaws, I think you're going to need a bigger bridge.