HEC-RAS & Manning's n

[RHOeng]

When coding cross sections, does Manning's n value to be used between 2 sections go in the upstream or downstream section data? The HEC-RAS Hydraulic Manual says the expansion contraction coefficients go in the upstream section, but I can't find anything for the n value placement.

 

[Bayou]

HEC-RAS as well as HEC-2 are backwater model programs. They use the standard step method for calculation of the
enery grade to calculate the water surface elevation. Although the model runs up the channel, everything within the model is looking downstream. Therefore any Mannings, coefficients, flows, etc. is taken from the upstream
cross-section downstream to the next.

 

[n3rei]

Hi!
I just wanted to irreterate that HEC2 is does down or upstream depending if you select SUBCRITICAL or SUPERCRITICAL modeling.

 

[Bayou]

Actually, in the HEC-2 & HEC-RAS models, it doesn't matter if the flow is sub- or supercritical. The cross-sections still looks downstream.

The only difference is the calculation of the energy grade elevation runs downstream to upstream in subcritical & upstream to downstream in supercritical.