Boundary condition not respected in steady flow analysis

[yann.viala]

Hello,

I have identified a strange behavior in steady flow analysis and I want to know what you think about it.

When the program computes for any reason a critical water depth at the downstream cross section, this water deph is used instead of the boundary condition.

The behavior is not the same in unsteady flow analysis, where the boundary condition is respected.

Have you ever seen such thing in your model ? Do you think it is normal even though it is not natural ?

Thank you for your answers.

[Hydraulics Tool]

Here's my understanding: In steady flow, prior to computations, RAS checks to make sure your downstream boundary is in the correct regime. If not, it defaults to critical depth so that it can continue with the computations. In other words, if you select subcritical flow regime, but your downstream boundary is a supercritical water surface elevation, than RAS will default that location to critical depth. To get around this, make sure your downstream boundary is correct, and if so, run the model in mixed flow.

[yann.viala]

Hello,

I post a new reply to this thread as I have always the same problem on another river model. I have seen that the same behavior of the model was posted by woodm on the 14th of november.

The answer I receive didn't satisfy myself as the model doesn't have to determined if the boundary condition is valid or not. The water level can be fixed by the water elevation in a reservoir, so it can be everywhere you want it to be, below or above the critical depth, whatever the channel geometry.

Thank your for your answers.

[jarvus lunt]

For steady flow, if the downstream boundary is below critical depth, the program will compute a subcritical profile based on the critical depth. If mixed flow is turned on, it will then try and compute a supercritical profile, if applicable. Look at section 4.6 ("Mixed Flow Regime") of the Hydraulic Reference manual. Available under Help, or off of HEC's web site.

Unsteady flow will use ("respect") the downstream water surface. However, this is not always hydraulically correct.

Imagine you have a storage area (a lake) at the bottom of a very high cliff. If the stream goes over this cliff (makes a waterfall), it would not be correct to force the water surface of the stream (at the top of the cliff) to the water surface in the storage area down below. But unsteady flow will do this. In reality, the downstream boundary (in this scenerio) should, somehow, be set to critical depth.

[yann.viala]

Thank you jarvus lunt for your answer. I have indeed found what I am looking for in the section 4.6 of the hydraulic reference manual.

But I keep on thinking that the model should not have to set a downstream condition other than the one seted by the user. The user theoricaly knows what happen in the flow downstream of the model, the software not.

In the example you are giving, if you end the model at the top of the cliff, the downstream condition should be set to critical depth, but if you decide to modeled the lake and the cliff (even if all hypothesis of equations validity are not respected !) the downstream condition should be set to the lake elevation.

[jarvus lunt]

If the stream water surface is the same as the lake and this water surface is below critical deth, then the flow is supercritical at that location.

For steady flow, the program can not do the subcritical [downstream to upstream] backwater starting from supercritical flow. So it starts at critical depth and goes upstream. However, if the flow really should be supercritical, and the user has the mixed flow regime flag checked, the program should, in theory, get the supercritical answer when it does the [upstream to downstream] supercritical pass. If it doesn't, then it probably means that the flow wasn't really supercritical. But in either case, it does not take any more work for the user.

Unsteady flow will allow you to enter (and use) a supercritical downstream water surface. However, if the flow really shouldn't be supercritical at that point, then you can get wild and bad answers.

Even if the downstream boundary really is supercritical, unsteady flow struggles to get good results with a downstream, supercritical boundary. In general, it would be better to find some location that was subcritical (or at least, at critical depth).

Actually, I have found that unsteady flow does a poor job in general with supercritical flow anywhere. However, checking the mixed flow flag does seem to help.