Basic question

[Shaul]

Hi all. I am new to HEC RAS so forgive me if my question seems a little basic.
I model an existent 3 km concrete channel with constant xs except a few places where they created narrowings of the channel in order to provide upstream water level for irrigation users. I know from the field that subcritical flow occurs along all channel length except in some of these narrowings, which i model as culverts, where supercritical flow occurs. When I run the steady flow input I choose mixed flow mode and enter also my known water surface elevatioms at both ends, as well as an estimated (yet not known) flow rate value which I understand is a must to put in.
My question is: since the resistance of the channel is known (the physical structute of the cannel which I entered), the boundary conditions are known, and the flow rate is also entered – there is no free parameter in the equation. So what does the program do? It does give me solutions(water profile and water level in x aections and all other run results) but I suspect them because who assures me that the flow I chose is the right one for the two boundary conditions?.
Can anyone clarify this issue for me, please?
Thanks in advance
Shaul

[Chad Ballard]

The boundary conditions force the model at the locations specified. The model will compute the water surface elevations and velocities in the channel for the given boundary conditions. The boundary conditions are the “knowns” of the model they can supported by a hydrology study or even better measured data.

[Anonymous]

Thanks Chad but this still does not answer the question that if I have the boundary conditions and the program requires me to enter the flow between them as an input, then what is left to compute?

[Anonymous]

Wetted perimeter, velocity, WSEL’s between the boundary conditions just to name a few. Remember there is still the cross sectional geometry and bed friction and you said yourself it narrows at a point.

Also as a side note you should not be modeling the narrowing of an open channel as a culvert unless it is a culvert. Better to just adjust the geometry of the cross section where the channel narrows. Culvert hydraulics are different from open channel hydraulics and if that culvert goes into orfice flow or weir and orfice flow you are not comuting the correct parameters within.

[Anonymous]

For steady flow, if you specify an upstream water surface and the upstream water surface is subcritical, I don’t think RAS will use it.

For mixed flow, if you specify an upstream supercritical water surface, a downstream subcritical water surface, and a flow, RAS will compute the locations(s) of the hydraulic jump. The location of the hydraulic jump is a free parameter.

For steady, subcritical, flow:

If you want to specify the upstream water surface and have RAS compute the flow,

Then: Add a storage area to the upstream end. Connect the upstream reach directly to the storage area and enter the desired water surface elevation for the storage area. Go to flow optimizations and turn on the Reach-Storage area optimization. RAS should compute the flow that is needed to get the correct water surface at the upstream end.

[Anonymous]

Thank you all. Helped a lot.

[Author]

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