Low Flow Dam Break in Series

[Anonymous]

Hello there! Anybody up for a puzzle?

I have an unsteady flow HEC-RAS model where we have 2 dams in series, where the upstream dam is modeled as a storage area, and the downstream dam is modeled using dynamic storage through cross sections.

I have run a PMF storm through the model, once without breaking the upstream dam and once with breaking the upstream dam. If the downstream dam is overtopped, I breach the structure and use Froehlich Method parameters. The model runs fine with the high flows.

When I do the same with an SDF storm (100-year intensity), and breach the downstream dam when it is overtopped, the model crashes on the cross sections directly downstream of the downstream dam.

I have tried everything I have read on blogs and forums for gaining stability on a low flow unsteady dam breach model, including:
-Check Boundary Conditions
-Check Ineffective Flow Areas
-Check Htab parameters
-Check Dam Breach Parameters
-Use Interpolated Cross Sections
-Modeled with and without downstream tributaries
-Adjusted Computation Intervals
-Make sure the bottom slope is steady
-Re-modeled the plans
-Adjusted Calculation Options and Tolerances
-Adjusted pilot flow of inline structures
-Use a Pilot Channel

What am I missing?

A link to the model is provided here:
https://www.dropbox.com/sh/0s7wt6z90wf34av/AABUWgruH5C14i3kr6mjqHvoa?dl=0

Any help if very much appreciated!!

Thanks,
-Jim M

[Anonymous]

Jim: I found that using a wider (5 ft.) and rougher (n=0.1) pilot channel helped. Also got rid of section 1078.42 based on it being too close to the bounding sections. RAS 5.0.1 completed the run at a time step of 5 seconds, but I didn’t look at the results. Might consider these suggestions.

[Anonymous]

Thank you Michael! I will absolutely look at that solution! Thank you for your suggestion!

[Author]

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