step interpolation between 2 xs

[mat A.]

Hello,

i have two xs in 100 meters. The flow regime is supercritical.
In first time, i interpolated between this 2 xs with a distance of 10m, then ws elevation was stable, but i had a warning : i must add xs.
So , in second time i interpolated between the 2 xs with a distance of 1m, and then results in ws elevation were very different and become instable (multiple oscilations!) Same thing with lower step.

My question is : how can i define the good interpolation’s step between xs? Is it possible that a very low interpolation’s step make oscilations in the water surface elevation?

Thank you for your help!

[Anonymous]

Hello,

May you share some print screens, such profile plot, boundary conditions?

[mat A.]

– step 1m in the first image
– step 2m in the second.

The 2 xs are located after a bridge that make flow be supercritical.

I dont understand why xs interpolation step can be so significant

[Robert]

Hi Mat,

Why do you choose a interpolation 1 or 2 m? You should estimate a distance between cross section using Samuel equation and Fread’s euatione (see manual hec).

Can you send me your model? (using File- Debug report) I think, that you have more bugs in your model (ineffective area, abrupt change of bed of channel, …)

regards
Robert

[mat A.]

Thank you Robert!

I thought Samuel’s equation must be used in case of unsteady flow data (relation time step and xs interpolation step)?

I choose a low xs interpolation step because of warnings in cross section Output “…this may indicate the need for additional cross sections” I thought results always where best with a low step?

best regards.

[mat A.]

MA_Tournefort_modele2.zip

[jadams]

Oddly I don’t seem to have the rapid oscillation when I run it on my machine. However I do still have the hydraulic jump after Section 130 for your higher flow profiles.

[Robert]

Hi Matt

Sorry, I am busy, therefore I write you today.

I was looking your model and your issue is a downstream condition (steady flow, reach boundary conditions).
Why did you choose a Know WS for the downstream condition? You cann´t use one WS for all flow (10,20,50, …)!!! You should use it, when you simulate one flow a you know a WS for this flow in the downstream cross section.
Do you understand me?

I changed:
upstream condition: from 0.01 to 0.006 Normal depth
downstream condition: from Know WS to Normal depth 0.01

Using URL I send you your edited model.
https://onedrive.live.com/redir?resid=76FE2B3DBFE48BD3!4657&authkey=!AHSbaEZxmdRgO2s&ithint=file%2czip

I hope, that I help you 🙂
regards Robert

[mat A.]

Hello,

OK for downstream condition, thanks! (My interest is near the bridge, downstream condition is far enough).

Sorry, oscillations appears with 0.5m step interpolation, under 1 m. (it’s also depending on manning value).

In this model, measuring station is unfortunately located near the bridge on supercritical flow, that’s why i must interpolate with very low step, and i founded that oscillation problem.

Best regards.

[Author]

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