Hec Ras - Modelling Bridge Parapet. 2

[macgarth]

Hello, I am trying to model a bridge replacement case, where the proposed bridge has a larger opening (3-sided culvert), and a parapet wall thats 3' tall. This is causing my WSEL upstream to rise in comparision to the existing. I have modelled the parapet as a deck with the high chord corressponding to the top of the parapet elevation.

Is there an alternate way of modelling the parapet, without having the model assume that the entire width (length when we speak hydraulically) of the bridge is at the same elevation as the parapet. Please advise.

 

[gbam]

macgarth - wouldnt the approaching flow have to reach the minimum elevation of the parapet before overtopping the bridge; hence creating a backwater condition? If not, is your bridge perched or can flow cross the road outside of the parapet/bridge; is this configuration represented in the highchord?

 

[macgarth]

Thanks Gbam . The road outside of the bridge is such that the flow can cross the road outside the parapet, and this is represented in the high chord.

 

[froude]

If I understand correctly, you've used high chord and low chord deck data to establish your roadway profile? Or in other words, looking at the bridge editor, there is a jump in the top of road profile at the beginning and end of the bridge (to represent the top of wall) and the profile approaching and leaving the bridge is at normal or proposed grade?

If that's the case, I think that is probably the best way to model it. If your existing roadway is overtopping and you are placing a solid wall/rail in the flow area, it could be likely that you are reducing weir flow and creating a rise in WSEL.

Unrelated...It would be nice if HEC would throw in an option to model slots or "windows" such as those used in decorative bridge railings.

Good luck!

 

[macgarth]

Thanks Froude. Your understanding of my model setup is correct. The proposed culvert opening is larger than the existing, and the bridge is replaced in kind. The parapet wall is reducing the weir flow. So what do you think are my options to get back the WSEL back to existing. I am already at the max culvert opening possible for the proposed bridge height.

Also, I am running the bridge comps under Energy Eqn. Any suggestions.

 

[francesca]

Tell the architect to nix the parapet?

Seriously, though, I had this issue on an in-kind replacement too - only they shifted the bridge 400 ft upstream and I had the same backwater elevation to meet. I guess your options are:
- longer sag curve to extend your weir flow area
- change to a box culvert
- add a secondary culvert on the overbank(s)

 

[macgarth]

thanks guys for your responses.

 

[gbam]

One thing you should evaluate is if the overflow area outside of the prapet is indeed weir flow. I have had models with perched bridges but low areas were outside of the bridge abutments. In these cases I used the multiple opening option to model these areas as conveyances. If the bridge does overtop, As Fransesca suggests, try to eliminate the parapet and replace with open railing.

 

[froude]

Pressure flow option is definitely worth a look if it's justifiable, which it sounds like it may be. Depending on how much conveyance you've blocked, the pressure flow scenario could still show a rise.

It really sounds like you need more bridge opening or potentially lower/change your railing. Your DOT may have approved bridge railings with enough openings that you may be able to increase conveyance.

What is your tailwater condition like? Will downstream grading help your case at all? Depending on your environmental regulations/impacts, lowering the flowline could be an option, if grades allow.

 

[macgarth]

Thanks Guys, I am trying to go with open railing and it seems to help. Thanks for all our help.