Mannings values not following logic

[steven crisp]

Hello,

I have a normal stream channel with associated floodplain. I am running a 1D 100 year flood analysis. I am getting odd water surface elevations(WSE) during the flood when I change my n values.

For example: I decrease my channel n and my WSE skyrockets up. I increase my bank n and my WSE drops like a rock. Shouldn’t this be the other way around?

I am using a normal range of values for both channel and bank n values and they are having the opposite effect on my WSE. I’ve gone through about 25 iterations with channel from 0.013=>0.1 and bank from 0.03=>1 with small changes being made everywhere in between.

Am I missing something? Thank you.

[Eric R]

Hi steven crisp.
It’s hard to assess and say what could be going on with your model. There are many possibilities.
Have you ever tried to build a new model from scratch, following all the usual recommendations for one-dimensional models?

[steven crisp]

I have. I re-exported data from Civil 3D to Hec-Ras and updated all the necessary parameters. I still get odd changes in WSE that don’t seem right. When I lower mannings, water should move through the channel quicker, thus a lower WSE. I don’t know. I was just hoping somebody else might have seen this happen as well.

Thanks though

[Author]

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