Ras 6.0 Bridges through embankments

[Ian Sharp]

I have successfully developed several 2D models with bridges through areas where the terrain represented a channel through the bridge opening. I am now trying to add a bridge at a location where the terrain shows the embankment of the road without having the channel cut through the embankment. In other words, the road is a dam in the channel. In this case, HEC-RAS will not route flow through the bridge as it seems to be seeing the cell face elevations at the top of the road.

I am curious if anyone else has run into this issue, and if there is a work around that you have come up with.

[Luis Partida]

I thought, the work around was to open your internal cross sections and copy and paste the US and DS cross sections into the US and DS internal bridge cross sections. RAS was not computing available flow area based on your internal Bridge cross section because it is most likely falling on the elevated roadway itself. BUT this did not work. Im going to investigate this further and get back to you

[Luis Partida]

It seems you are right. I cannot find a work around for this. This is disappointing, and the work around would literally have to be terrain modifications. 🙁

[Ian Sharp]

I’m glad someone else is seeing this as I do, thanks for the responses Luis. Terrain modifications seem to be the solution, and thankfully they are not that hard in Mapper, but it seems this is a place for HEC to improve.

[Luis Partida]

I just found the work around with no Terrain mods…

So our limitation here and the problem is the cells, it is reading elevations that are blocking flow. So what I did was add a large refinement region around the bridge and its XS’s. From here i solved any associated cell errors and ran. But as you can imagine, if you need to span cells 300-ft so it ignores the high points, your transition back to your normal cell size would cause Courant variations depending on the original size of your model. Should i make a post on this? I have not seen anyone else solve this or bring up this issue yet

[Ian Sharp]

Your solution is probably a good one, I’ll have to give it a try. If I understand it correctly, you are essentially moving the cell faces to avoid the high ground. I think for most situations this will work fine, but as you observed you may find some with Courant issues, etc. You will still get some strange results when Mapper plots the depths, but I think that is acceptable.

I think your solution deserves a new post. Good work.

[Luis Partida]

I did get acceptable errors and now i would like to test it with 1D bridge results. But ok thanks for working back n forth with me.

I am going to refine the test so it isn’t as sloppy. Have a good one!

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