Manning's n values, why more than three?

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Manning's n values, why more than three?

Pallo
Hallo.

I don't see very well why there can be more than 3 values (i.e. page 203 in Users Manual). I understand the values for ROB, Channel and LOB, why are there more?

Thanks,
Pallo
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Re: Manning's n values, why more than three?

Chris G.
Administrator
You may find that you have very different land conditions in one or both of your overbanks.  For example, for one of the overbanks, you could have riparian forest next to the main channel and farmland further out in the overbank.  Two very different n-values.  You can also have different n-values within the main channel, although RAS has autmated methods for compositing main channel n values into one single n value, under certain circumstances.  To put in more than the standard "3" n-vlaues, go to options in your cross section editor, and select "Horizontal Variation in n Values".  Then fill in the new column called n Value, that will show up in your Cross Section Coordinates table.  
Chris G.
@RASModel
www.therassolution.com
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Re: Manning's n values, why more than three?

Pallo
Thanks Chris.

The model can have several n-values (>3), if I use the "flow roughness factor" in Geometry Data or (how you say) "horizontal variation n Values" in Cross Section Data, can't it?
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Re: Manning's n values, why more than three?

Chris G.
Administrator
Yes
Chris G.
@RASModel
www.therassolution.com