Modeling small piers in RAS

I have a question relating to modeling bridges using HEC-RAS. I am modeling a bridge which has four "fender piers" associated with it. These fender piers are circular piers about 9' in diameter. They are actually not attached to the bridge structure; they each lie about five feet from a corner of a bridge pier and are supported separately in the river bottom. The fender piers have a rubber covering and are intended to protect the bridge piers from collisions with ships. They are "inside" of the bridge piers, in the waterway opening. I'm wondering how to model these piers. I'm fairly new to RAS, but it looks like the pier modeling routine assumes that a pier extends the full length of the bridge deck. I don't see a way to represent a smaller pier that does not extend the full length of the deck (length referring to the direction parallel to flow). Should I use ineffective flow areas in the cross sections immediately upstream and downstream of the bridge deck to represent these piers? Or is there a better method? I'm also using RAS to perform a scour analysis of this structure, and I'd like to incorporate the effects of these fender piers, if any. Thanks in advance for any suggestions.

Aaron Volkening
aaron_volkening@earthtech.com

Since HEC-RAS allows you to change the width of the bridge piers at any elevation, you could simply make the lower portion of the bridge piers wider to represent the width of the fender piers. Then, at the top of the fender piers, reduce the pier width back to the structural piers only. This will work only if the centerline of the fender piers is lined up with the centerline of the structural piers. Otherwise, you can achieve the same thing by entering the fender piers as separate piers with their own individual centerline station.

You can specify the upstream and downstream pier widths and stations separately, so it is easy to represent piers that are only on the upstream or downstream side. I would avoid using the ineffective flow areas for anything like this. HEC-RAS gives you so many better options (such as the blocked obstructions, the cross-section lid, the sloping abutment, etc.)

Roy Dodson
rdodson@dodson-hydro.com