HEC-RAS critical depth problem

[bbengrs]

I am currently trying to model a channel in HEC-RAS but keep running into the same problem. At one of the cross sections the model calculates the flow to be at critical depth and then at the next section it goes to supercritical flow. At that point, it the flow runs out of the channel and to a lower elevation of the next cross section. It doesn't seem to account for the channel banks at all. From a hydraulic perspective, it should not turn to supercritical flow anywhere in the channel.

I have added more cross sections (every 50 feet at this point) which should be enough, but I can't seem to figure why it would model it this way.

Any help would be appreciated!

 

[francesca]

Check your geometry inputs. One extra zero in a Manning's coefficient or one too few on the bottom slope would be all it takes. Have you specified your bank stations correctly? Are your downstream reach lengths correct? Do you have ineffective flow areas and if so, are they correctly specified?

When you say "next cross-section" do you mean upstream or downstream? HEC-RAS calculates from the downstream end up, so check your starting depth (which should be Normal Depth) on the downstream end. HEC-RAS requires a few additional x-sections downstream of your area of interest to allow the depth calculation to settle, so make sure you have sufficient x-sections downstream of your study area.

Have you run a mixed-flow calculation and achieved the same result? Why do you believe that it shouldn't turn to supercritical flow?

 

[bbengrs]

Thanks for the reply. I checked the manning's n's, and they are all ok, as are the bank stations and reach lengths. I don't have any ineffective flow areas.

I will double check my downstream sections. i was focusing on the stations that change from critical to supercritical.

I ran mixed, super, and sub and reached the same result each time. In addition, it goes to supercritical even at a very low flow (i.e. 1 cfs). With the size of this channel and the relatively high mannings number, it just shouldn't go super.

The most confusing thing is that it is flowing outside the channel banks. It flows in the channel, and then gets to supercritical and all of a sudden turns out, which is impossible unless you are considering groundwater.

 

[francesca]

What version of HEC-RAS are you using? Could it be a bug? What happens if you delete the offending cross-section?

 

[bbengrs]

I am using version 3.1.3 (the latest except for the beta).

I took out the cross section and it still does the same thing. Its really puzzling me. I am now going over each bit of data that I entered to look for any inaccuracies.