HEC-RAS Bridge Modeling question ....

[Knarf1Man]

I'm working on a project at the junction of two large rivers (30,800 CFS). There are multiple existing bridges over each of the two rivers (all bridges are skewed, 2 are at 40-degrees). My question is how should I model the bridges? The Section lengths are much longer/larger than the bridges.
1. Do I revise my HEC-RAS Section so that it is perpendicular to flow at the bridges and adjust the bridge dimensions accordingly? If so, how should I show my analysis Section on the drawing?
.... or .....
2. Should I simply use the "Skew Bridge" command from the Bridge Editor? Would this option affect the entire Section length (i.e., beyond the bridge extents)?

All responses/advice/opinions appreciated. Thanks.

 

[gbam]

I typically cut my xsecs in accordance with the flow direction and model the bridge as if it were perp. I have tried the parallel to bridge approach w/skew but typically revert back to the perp condition.

I am not sure there is a correct answer both are accepted in my area.

 

[francesca]

The skew function in HEC-RAS reduces the horizontal distance by the cosine of the skew angle. It is calibrated for up to 30 degrees. I have used it for up to 40 degrees on narrow flood plains with flows under 10,000 cfs.

The problem with excessive skews is that you have one side of the flood plain outside the contraction reach while the other side is already going through the bridge. The local hydraulics would be best modeled in a 2D model.

On a river of the size you're dealing with, with several existing bridges, is there not an existing FEMA model? It would at least let you calibrate your existing conditions model.

I have one rather delightful project where I'm modeling a bridge at 45 degree skew where there is road overflow at the 10-year flow condition and the entire road is under water in the 100-year condition. My "skew" through the "bridge" section varies from 45 degrees to 80 degrees and the point at which overflow occurs is really three cross-sections downstream.

The modeling approach was discussed with the reviewing agency before I cut my cross-sections. I didn't use the HEC-RAS skew function, I used a spreadsheet to adjust the horizontal distance by the skew at that part of the cross-section. In other words, I manually projected a perpendicular cross-section at the bridge. The rest of the cross-sections were cut perpendicular. Strictly speaking, my internal "bridge" cross-sections crossed these, but there was no other way to model the road overflow.

Without road overflow, most of the cross-section is inside the ineffective areas and the only part that matters is right near the opening.