Ineffective flow limits at bridge crossing with channel bend

[Anonymous]

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 runs perperdicular to the roadway upstream of the bridges, but 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. However, if I extend the downstream ineffective flow limits from the bridge, the ineffective flow limit on one side 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 ineffective flow limits to account for this bend in the downstream channel?

[Lonnie A]

It may be the way the sections are oriented. From what I visualize in your description the cross-section overbank should be running up the roadway embankment on one side of the channel where the embankment parallels channel. (excuse the crude pictures)

Like this

Not this

[jg2011]

Lonnie-

You’re pretty close with your sketch – the channel doesn’t run parallel to the roadway, but at an angle from the roadway. I have created a .jpg of the model to help clarify the issue.

The floodplain is wide, and is constricted by the existing bridge. The calculated length of expansion is just under 600′, which takes me down to the 2nd cross section (2nd from the most downstream cross section). I’m trying to determine how to model the downstream left ineffective flow area – based on your sketch, are you suggesing that I stop the ineffective flow area just downstream of the bridge (i.e., HEC-RAS cross section 2)?

[Lonnie A]

I would still include ineffective in the left overbank. But would apply it along something like the redline is showing. I don’t have a equation for how I came up with it. I’m just visualizing how flow would exit the crossing and then need to navigate around the hill in the left overbank with consideration of how the channel alignment is.

[jg2011]

Lonnie – thanks for your help on this…I came up with a similar ineffective flow area on the downstream left side, lining it up with the location of the floodplain constriction downstream. I appreciate you offering your thoughts on this!

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