Modeling skewed bridge piers in HEC-RAS/bridge replacement/parallel structures

[Bfrench]

Existing condition:
There are two existing parallel bridge structures with the south bridge carrying eastbound traffic and the north bridge carry westbound traffic. The channel bends just upstream of the existing bridge opening and flows through the crossing with the existing piers for both bridges skewed 10-20 degrees to the centerline of the channel. It appears that the channel was straightened when these bridges were constructed. The existing bridges do not overtop during the 100 year event.

Proposed condition:
The north bridge is being replaced with a wider bridge that will accommodate east and westbound traffic and the south bridge is to remain as a frontage road for adjacent properties. Because the proposed bridge is wider than the existing bridge, the bend in the channel will fall underneath the bridge. The proposed bridge is skewed at 20 degrees. The piers at the upstream face and the exit of the proposed bridge are aligned about 10-15 degrees to the centerline of the channel. The channel is a FEMA floodway, therefore, a no-rise certification is required.

Attached figures:
There are two figures that I’ve included with this post: existing and proposed condition. The layers in the figures include HEC-RAS, floodplain, topo survey, contours, bridge layouts, and proposed roadway overlaid onto an aerial. The link below is to this same post where the figures can be downloaded.

http://www.eng-tips.com/viewthread.cfm?qid=398329

Question/concerns:
Due to HEC-RAS assuming the piers as continuous, when the 10-15 degree skew angle for flow is applied to the proposed piers, it produces an excessively wide effective pier width. What should be the approach in accurately modeling skewed flow to the piers?

The cross-section 61095.00 is directly upstream of the proposed bridge and is cut parallel to the bridge face. Should I apply a skew angle to this section? Please see the proposed figure.

Are there any concerns with adversely affecting the existing bridge downstream with the configuration of the proposed bridge?

Thank you for taking the time to read this post. Please feel free to give any advice you may have as it relates to modeling the skew and the proposed bridge configuration.

[cameron]

What I have done sometimes is to actual skew the data outside of HEC-RAS and then add it in (XS and Bridge data). Some times it is easier and you have more control over what it does.

Another option since the bridge does not overtop is to not model it using a bridge, but just model the piers using blocked obstructions. You would need to add more cross-sections through the bridge so you can pick up where each pier/abutment begins and stops. Doing this would allow you to not have to worry about the bridge skew.

[Author]

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