Reach roughness

[Anonymous]

We are trying to represent a design reach with a particular n-value in HEC-RAS. Wanted to confirm that when assigning a roughness value to an individual cross section in RAS, we’re actually representing the roughness between that section and the next downstream section? (since RAS uses the standard-step approach for steady state).

Does this vary with whether we’re running steady vs unsteady (or) subcritical vs mixed?

I would appreciate any feedback.

Thanks.

[Anonymous]

HEC-RAS like it’s predecessor HEC-2 averages the conveyance between cross sections. It does not use the n value entered at a particular cross section to represent the conveyance for the entire reach downstream of that cross section. There are several programs that use this method like SWMM, etc. if you need that type of approach for representing the conveyance between cross sections.

Check the HEC-RAS users manuals for details on how HEC-RAS averages the conveyance between cross sections. There is the default method and several alternative methods of averaging the conveyance that can be selected.

[Anonymous]

Thanks. Really appreciate your response and clarification.

One way we have tried to implement the reach based n-value is by having two cross sections bounding the reach with the desired n-value and providing short transitions on either side (i.e. both upstream and downstream).

[Author]

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