Varying n value along the channel

[hshahad]

Hello,
At first, thanks to all the contributors to this forum. This is an excellent source of knowledge.

I am modeling a channel, which transitions from grass lined to concrete lined and back to grass lined.
Lets say:
XS1 (most US cross section)–> the channel begins as grass lined,
XS2 (next cross section DS)–> grass channel ends and concrete lining begins,
XS3 (next cross section DS)–> concrete lining ends and grass channel begins,

As n values, I will be using 0.04 for grass channel and 0.015 for concrete lined portion. The way I have the n values is as follows: (To simplify my question, I am ignoring the overbanks)

XS1 –> 0.04
XS2 –> 0.015
XS3 –> 0.04

Can anyone please confirm if I am correct in my approach? To the best of my knowledge, every cross section represents the reach starting from that point until it reaches the next cross section downstream. I need your help to confirm if my understanding is correct.

Thanks a lot…

[cameron]

HEC-RAS interpolates between cross-sections and does not use a single xs until the next xs. You would need additional xs to account for the roughness changes. I would just add additional xs 1 ft away for the areas where roughness is changing.

[hshahad]

Thank you so much…

I ran a quick test. First, with only one cross-section near the change in roughness with n = 0.04, where the channel changed from concrete to grass. In the other one, I had two cross-sections. One XSec with 0.015 where the concrete ends, another one 1 ft downstream of it for the beginning of grass channel with n = 0.04.

The resultant water surface profile for both scenarios are identical. Any idea?

Thanks again…

[cameron]

could you post the model somewhere?

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