Hec-Ras no rise question 1

[sam74]

It has been a while since I have used Hec-Ras and could use some suggestions concerning problems I am having. I have an existing FEMA model of the river in question. We are proposing to add top of bank walking trail, minor walls, and two pedestrian bridges over the river.

I have no problem running the duplicate effective model and have interpolated new sections and added back up topo from LIDAR and field survey information to come up with the new sections I will be needing. So I now have an existing conditions model.

However, when I try to begin the proposed conditions model I can try to add a new ped bridge and set the deck high cord and low cord elevations higher than any available WSEs so that there should be no change from the existing conditions model. However, there is an increase in the WSE shown on the profile report at the sections immediately above and below the bridge station even though there is no visible interference.

Any ideas on what I might be doing to cause this?

 

[RWF7437]

Without seeing your model it is difficult to guess what the problem(s) might be. Here are a few guesses anyway.

Is the bridge opening smaller on area than the "before" cross section ?

Have you added some ineffective flow areas upstream and downstream of the bridge opening ?

Have "n" values changed ?

Has the "wall" reduced the flow area available ?

good luck

 

[sam74]

Nope, that is the kicker. All I did was save the Existing Conditions run out at the Proposed Conditions run (and the geometry as proposed for this run) and set a bridge deck above all WSEs for the upstream or downstream sections. No piers to see that rise but it is only a few hundreths. Then if I add piers of course there is a larger rise. But if I try to mitigate by obtaining some cut volume from the existing sections it forces the WSEs up higher. I'm baffled.

 

[sam74]

If anyone would like to take a look at this I've got the model files posted here. It includes an excell spread sheet listing various runs and WSEs.

http://www.keepandshare.com/visit/visit_page.php?i=255185

My new bridge that I am trying to add is at river station 41815. I do not understand the problem. If you run the Proposed Conditions Model and compare it two the Existing Conditions Model you will see the difference. The bridge has been input at a ridiculously high level that in no way should impacts the WSE but it still does. If you delete the bridge and compare to the WSEs on the excell spread sheet for the Existing conditions model the numbers match again. What could be causing this?

Thanks in advance if you have the time to input. I hope I am overlooking something simple.

 

[gbam]

I did not look over your input; but I have a question. Are you adding/inserting the 4 cross sections at the bridge for the proposed condition? If so, try adding those to the existing condition model.

 

[sam74]

Did that already as the files show.