Calculation of overbank losses

[Ted in NJ]

A simple question I can't find the answer to:

You input Manning's n-values for overbank areas (and channel) at each cross-section. Does HEC-RAS use those values to calculate losses in the segment upstream of the section, the segment downstream of the cross-section, or does it make a composite between sections? And is this dependent on flow regime, subcritical vs. supercritical?

Thanks for your help.

Ted

[dhales]

RAS in a steady state mode is a backwater program. That means that it does the computations in an upstream direction. When you provide an "n" value, it will be applied to all the following upstream sections until you provide a new value. Than the new value will be used till the end or until you change it again. That is the reason that you must provide an "n" value at the first x-section.
I hope this answers to your question.

[Ted in NJ]

That's what I figured, but perhaps I should have been more specific. Particularly, I'm interested in the multiple n-value option. If an overbank is, say, 1000 m. wide, with n=0.04 for 600 m. and n=0.05 for 400 m., does it use these values all the way up to the next X-section, even if the next overbank is 2000 m. wide, all n=0.08?

[TimW]

Because RAS is one-dimensional, it does not compute friction losses in between the cross sections. Therefore, the n-values in your example would be used to compute the WSEL at that cross section, then the n-values for the next section would be used at that location, without regard to the n-values at the previous section. The only element of the downstream cross section that is considered is the computed WSEL, which becomes the y1 in the standard-step calculation for the upstream section.

[Ted in NJ]

OK. As I thought. So I assume it use the n-values at a X-section and the overbank distance to the next upstream X-section to compute losses. Therefore, input n-values only apply to the segment upstream of a given X-section. Is this accurate? That's the real question: do n-values at a cross-section apply to the area upstream or downstream of the section?

I'm trying to figure out how to adjust n-values at cross-sections when there is a large area between cross-sections with a different ground cover.

[jarvus lunt]

The n values are used at a cross section to compute an "instantaneous" friction slope. While doing backwater (or forewater) computations for subcritical (or supercritical) flow, the instantaneous friction slope is averaged (using one of the friction slope methods) with the friction slope of the upstream (or downstream) cross section to compute a friction loss.

So one way of thinking of it is that the n-values at a cross section extend halfway to the next downstream cross section and halfway to the next one upstream.

If you haven't already, browse through the hydraulic user's manual available under Help (or download from HEC's web site if the manual wasn't installed).

[Ted in NJ]

Thank you very much. That's just what I needed to know.