Ras unsteady simulation issue on low gradient channel

[jhpanster]

Hello there,

I ran into a problem where RAS unsteady simulation get stuck with high WSE when there is no energy slope. Here is a description of problem.

The storm channel is very flat (s = 0.00012 ft/ft). There is a reservoir at US end, and another storm channel at ds end (where the stage is typical lower than the study channel bottom. (see image).

The issue is due to the low gradient, when stage at ds end is about even with reservoir, flow stops, the unsteady momentum equation is computing near zero velocity. Thus leading to stagnant stage and unrealistic high flow over levee onto floodplain.

Has anyone seen this before? Any ideas to trick the model to start computing flow once it hits the high stage.

Thank you.
Jimmy P.

[Anonymous]

Perhaps the problem here is that the downstream boundary condition (stage) is lower than the channel invert. RAS needs to have a finite depth at all sections to solve the unsteady flow equations. If that is the case, perhaps a pilot channel should be used to insure that there is depth everywhere. The situation is described with having about the same upstream and downstream elevations, resulting in a nearly horizontal water surface that is above the levees. The computed results might be correct.

[Author]

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