Mannings values

[Anonymous]

Hi,
Why does the Mannings n value for the total main cross section (Mann Wtd Total) is always lower than the Mannings n values I enter myself as input data. For example, I enter 0.07 for LOB, Channel and ROB Mannings value in a cross section and the Mann Wtd Total n gives 0.063 in the Output Table. How is that possible and why?
Thanks

[Chris G.]

That’s because hydraulic radius for the cross section as a whole is not equal to the sum of the hydraulic radii for the LOB, CH, and ROB. When RAS runs it’s computations, it sums the conveyances calculated for each individual sub section (LOB, CH, ROB) to arrive at the total conveyance. Then to get the weighted n value for the whole cross section, RAS takes the entire conveyance and backs out the n value with Manning’s equation. Because the total hydraulic radii is different in these two approaches, the weighted n value will be different.

[Ali Ercan MORKAN]

Can i ask you a question?
How can i calculate manning wtd total by manuel ?
when i use formula of hec-ras hydraulic reference manual page 2-7, But even so hecras results not equal to formula of page 2-7.

[Author]

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