HEC-2 skewed bridge

Here's a question for all the old HEC-2 users out there-

I am converting an old FEMA HEC-2 model to HEC-RAS. The HEC-2 run includes a bridge without piers modelled using the special bridge method.

The GR data from the cross section at the downstream face of the bridge is "repeated" at the upstream face (meaning that it is not explicitly input again). No skew factor was applied to the GR data, either at the downstream or upstream X1 card.

The BT data (low chord and top of road) have stations that match the GR data. However, the BT data are then skewed by a factor of 0.89 by the X2 card at the upstream face of the bridge.

This looks to me like an old error in the HEC-2 model. Wouldn't you have to also skew the GR data using the X1 card by the same factor so that the stations of the BT and GR points would match? Or does HEC-2 know to also skew the GR data?

I couldn't find this issue discussed explicitly in the HEC-2 manual. Anyone know the answer? What are the impacts (if any) of not also skewing the GR data?

Thanks,
Chris Hauser, P.E.
chris.hauser@fpsengineering.com

Dear Chris Hauser :

Since you own RMS for AutoCAD, RMS will automatically process the BT data and apply the skew angle to the data and correct it. If you are working with Special Bridge Data, RMS will correct the problem automatically.

If you have trouble with this, please let me know.

Sincerely,

Chris E. Maeder, M.S.
Senior Technical Engineer
chris.maeder@bossintl.com

It's been a while since I used HEC-2, but here is my understanding of HEC-2 and skewed bridges...

First, I don't think the BT stations have to match the GR stations for Special Bridge. This only applies to Normal Bridge.

If you would like to apply the skew to the roadway only, then use X2.8. For a high flow condition (i.e. weir & pressure flow), the X2.8 value will not change the pressure calcs (BAREA), but it will skew the BT stations to give you a shorter (X2.8 value < 1) weir length. HEC-2 will compute an Energy Elevation based on the pressure and weir flow calculations and then apply this energy to the unskewed cross section to determine the CWSEL at the upstream face of the bridge.

To skew both the roadway and cross section, you need to use the same factor for both X1.8 and X2.9. HEC-2 will not automatically skew the GR stations if you only use a skew factor in X2.9. The X2.9 value is used to skew the BT stations only and the X1.8 value is used to skew the GR stations. Most likely these values will be the same. So, to reduce the cross section flow area and the weir length, you need to apply a skew factor on both the X1 and X2 records.

Chris Krebs, PE

Can anyone answer the question for those of us not using RMS? I deal with a lot of old HEC-2 data, that I convert to HEC-RAS for FEMA flood studies, and would also like to know how HEC-2 interprets the data set that Chris Hauser described.

Dave Briglio, P.E.