[Caroline]

Sorry, I thought you were modeling steady state. I have no clue then.
I guess you did already add the elevations in the boundary tab for unsteady flow?

[Caroline]

Hi,
I think I know what is missing. I had the same problem, of some reason you need to add the data not just on the boundary condition tab.

Try to look at the steady flow data. Go to Options and then “Storage Area Elevations”. There you can add the missing elevation data (again).

Hope that helps
Caroline

[Caroline]

Thank you! Your answer helped a lot for understanding.
I have started to look into those equations.

But first, I did some changes in the geometry file. When removing ineffective flow areas immediately upstream of the junction I got rid of the most increase. The rest could be removed by setting the exact same values of Manning’s number in the reaches on both side of the junction. The latter is unfortunately not consistent with the real geometry, which have walls after the junction and a river bed before.

[Caroline]

Hi,
I’m not that experienced with HEC-RAS myself, but you could start by looking into your Htab parameters on the geometry data. That will probably solve the error message:
”
***** Extrapolated above Cross Section Table at: *****

Arial Khan Arial Khan R.S. 01
Arial Khan Arial Khan Upper R.S. 6
”

You can find out how to do it in the blog:
http://hecrasmodel.blogspot.se/2011/03/more-on-htab-parameters.html

Best regards,
Caroline

[Caroline]

I have had some problems using the flow hydrograph, but the solution is actually more stable than the stage alternative.

Since my channel connects two water levels (upstream controlled lake, downstream sea) with larger deep upstream than downstream my slope gets a negative number which HEC-RAS refuses to use. Is there some way to overcome that problem?

By the way, what did you mean to write in the sentence ” And, as you noticed, sometimes the”? It ends pretty abrupt.

Have a nice weekend

[Caroline]

Hi,
Chris, thanks for your help and hec-ras model.

I’ve moved on now to analyzing the downstream side of the channel, i.e. the wave downstream the gate. The effect of the gate-closing is larger than on the upstream side. The water surface closest to the gate sinks approximately 5 times the rise on the upstream side, (~0.5 m compared to ~0.1 m).

As downstream boundary condition I use a flow hydrograph. A few sections upstream, I put an inline structure to prevent the channel from being dried out.
I do adjust the flow response in the flow hydrograph after looking at the xs flow in the unsteady flow spatial plot. I keep the flow constant until the decrease in flow translates to the channel-end. Then I start to reduce the outgoing flow.

Does it seem reasonable to use my boundary conditions?
Thanks!

[Caroline]

Hi,
Nice that you solved the problem! Exactly the same way I solved mine. The strange thing was that it worked well with special characters when running an steady flow simulation, the error appeared when I tried the unsteady simulation.

[Caroline]

I just realised, that when changing the upstream boundary condition to constant flow instead of constant water surface level, there is a shift, the water rises at some cross sections simultaneous at is it sinking in other.
(When using constant water surface, the result is as described above.)

But, the cross sections 200 m upstream still show a wave directly as the gate closes and not after a while, for instance after the time it would take a wave to travel that distance for the specific channel depth I’ve used.

[Caroline]

Thank you,
the hotstart now works very well as input to the simulation, eventhough the fluctuation is still there. Problem was that the boundary conditions of the real simulation.

[Author]

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