HEC RAS bank stations

[Crystalct1]

I have a meandering stream in superflood (the 100-year storm). My colleague and I can't seem to agree about the location of some of the bank stations. I want them at the obvious steep-to-flatter slope change on the cross-section, but my colleague likes to move banks stations to make the "picture" look nice. He likes all of the red dots on the Geometric Data window to be perfectly in line, even if that is not the actual bank.

How much does it matter where the bank stations are set in HEC RAS? Assuming I use horizontal variation for the n-values, changing n where the bank actually is, and not have the break in n-value at the red dot bank stations, what else does that bank station affect in computations? Could an incorrect bank station actually make the results inaccurate?

 

[francesca]

The bank stations affect where the channel is defined. Ultimately HEC-RAS compresses everything into a single "roughness" (or headloss) value for each cross-section (as it's a one-dimensional model), but it computes the channel and the overbanks separately.

You could do a sensitivity analysis to see how much impact it makes, but strictly speaking you are correct and your college's insistance that the geometric window look pretty is trivial. I never once even printed out my geometric window image. Also, anyone reviewing your model will notice straight away that your bank stations are in the wrong place as they show up on the cross-secion plots.

 

[Crystalct1]

Thanks. I agree. I don't think we've ever submitted that picture for anything either - but we do sometimes submit the entire HEC RAS run on a CD for approval by NJDEP. We only print out the cross-sections and that would look bad with a red dot at the wrong place. The next question a rewiewer would ask would be, "Is that where n-value changes?" Even though it wouldn't be, it would look strange to have the banks out of place.

I will do a sensitivity analysis and report the results here.

 

[blueoak]

Bank stations affect interpolation. It would seem having a model that can be used in the future without going back to correct things should justify placing bank stations with that in mind.

Someone may know if you get extra warning messages in HEC-RAS due to an expansion or contraction due to bank stations. I think improper bank stations placement affects floodway tests with Check-RAS.