HEC-RAS output mystery

[vossmachine]

Hello,

I am trying to compared existing and proposed conditions and check for the difference in water surface elevations to try and obtain a "no rise" condition. However, I am seeing a rise of .02 feet and everything I think of to try and help my situation I get the same result. For instance, I have changed the N-values up (no dramatic change) N-value down (no dramatic change), I have added ineffective areas there may be no flow conveyance, I have used different combinations of n-values across my sections. The only difference between the two cross-sections is the is a rail embankment being added in the proposed conditions. It there any other ideas out there on how to tinker with HEC-RAS to get it to reduce the proposed water surface elevation?

Any ideas are appreciated,

 

[francesca]

You could take out the rail embankment.

 

[amb2002]

Does it matter? The flood studies that I have seen report elevations to the nearest tenth of a foot. Personally I would sign a no-rise certificate for a project if I only saw a change of 0.02 feet as you noted.

 

[vossmachine]

I tend to agree with you, amb2002, however I have been requested to obtain anything less than .01 feet.

 

[amb2002]

I understand being asked to show results that are more accurate that practically observable.

Considering that the surveyor's rod could easily be stuck into the ground 0.02 feet while the elevation is being recorded, or conversely maybe sitting on a small rock 0.02 feet above the ground, maybe you could slightly adjust your section elevations such that the 0.02 feet disappears from the calculations?

Similarily are your cross-sections input with elevations to two decimals? Maybe changing elevations to one decimal would still be valid and make the 0.02 fall out?

 

[vossmachine]

I understand there could be errors in the data provided. Shoot, even our survey was flown from an aerial survey, however, with all the data supplied the existing and proposed conditions should not vary that much. The only thing changing is the addition of a rail embankment throughout my site.

My cross-sections are at 0.02 decimal places. I guess I will just have to tinker around with the model in HEC-RAs until I get an acceptable "No Rise". There are flaws in HEC-RAS program itself at times. it's not perfect.

 

[cvg]

who is doing the requesting for 0.01' accuracy?

I would be prepared to argue that this is negligible. Stick to your guns.

 

[beej67]

I wouldn't consider fudging you model to produce the answer you're looking for very ethical.

If I were you, I'd look upstream, find the first section with no rise, and contact land owners to obtain an easement. That or convince your reviewer that .02 should qualify as no rise.

If you toy with anything, toy with providing some flood plain compensation on your project to mitigate your fill.

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