Floodway Natural Profile Error

[Mark Roberts]

We are using HEC-RAS 3.1.3 to do a Floodway Analysis with Method 1 encroachments in a sub-critical steady flow model. The Natural (unencroached ) profile is giving different water surface elevations when compared to the 1% profile we ran in the multiple profile model. Same geometry, same Q's, same starting condition etc. If we put a third 1% unencroached profile in the floodway model, its results match the multiple profile results. It appears to occur at sections where critical depth is being calculated. Any ideas?

[jarvus lunt]

Are the critical depths themselves getting different values?

If so, from the steady flow run time editor, go to options/critical depth computation method and click on Multiple critical depth search and then run both data sets again.

[Mark Roberts]

That did the trick. Thank you. I guess my only question (rhetorical) is why does it report two different answers for the very same situation? Perhaps we should by default use the multiple critical depth calculation rather than the parabolic method.

[jarvus lunt]

If you want to know more about what it is doing, you can read the hydraulics reference manual (should be available under help).

It is possilbe to get more than one spot on the energy curve that acts like critical depth (see the reference manual).

I think HEC should change the default to the multiple critical depth method. When I think about it, I usually change it for my models.