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Storm and sanitary analysis: Instability of Weir Flow

Storm and sanitary analysis: Instability of Weir Flow

Storm and sanitary analysis: Instability of Weir Flow

I created an SSA model for my site. The site is next to a creek that is undersized and floods. I have been given a tailwater condition of this creek that puts the upper right corner of my site underwater. We are proposing a detention basin in this corner to provide extra storage capacity and protect a nearby development. I modeled the overland release of this detention basin as a weir. The system seems to work when the tailwater condition is low but with the high tailwater, the weir would start off submerged and stays submerged.

I ran the model a couple times with 1 sec for the time step of the analysis and adjusting the reporting time step from 5 min down to 30 sec and got a fair amount of variability.

5min reporting interval

30 sec reporting interval

When I run the model, I notice some instability in the system and it manifests itself near the final weir and outfall of the model. Any idea why this is happening? and how to fix this? Is there a setting that needs to be adjusted for a flooded weir condition? Or is it something else?

See attachments for the snapshot of the other intervals and the SSA model. Thanks in advance for any wisdom you can impart.

RE: Storm and sanitary analysis: Instability of Weir Flow

I do not believe that S&S can handle submerged weir flow. I suggest that you analyze outside of the S&S model and code a rating curve.

RE: Storm and sanitary analysis: Instability of Weir Flow

The help tutorial says that it can handle the submerged weir flow but that it has to flip over to the Villemonte Equation but it doesn't go into much detail.

So far the best I can come up with is reducing the width of the weir, seems to help the instability a bit but there are still some random blips.

RE: Storm and sanitary analysis: Instability of Weir Flow

Have you been able to reproduce the submerged weir results using external analysis? I have evaluated computations within S&S and found some issues. I typically augment my S&S analyses with external computations.

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