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

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

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

(OP)
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?

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

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. ponytails

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