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I am currently attempting to model a stormwater system that has a high likelihood of backflow because the proposed basins are interconnected and there are no intervening structures which would impede flow from moving backwards in larger storm events.

To model this, I've connected my ponds to each other with primary discharges going towards downstream basins, however, I've added identical discharges in the downstream basins (as a secondary and tertiary discharge) which are identical to the upstream flow controls (usually some kind of broad-crested weir).

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For smaller storms (i.e. 2-year), the output data has been pretty clean and not indicated that there are any issues worth noting.

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However, when I crank this up to large storm events (i.e. 100-year), the oscillations are absurd to the point that I'm questioning whether these results are valid. The overall hydrograph appears correct, however, I have been unable to completely eliminate the oscillations.

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At this point, I've attempted the following:

  • Reduced the time step all the way down to the minimum of 0.01 hours.
  • Ensured that none of the basins are undersized and have sufficient defined volume.
  • Applied a phase-in depth of 0.01' to all infiltration basins.
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Having tailwater dependencies does not necessarily mean that you have reversing flows. So I suggest that you start with DSI routing, and normal outlets going in the "downstream" direction. Then check the results. HydroCAD will issue a warning if there is any potential for reverse flows. If no such warning occurs, then DSI routing is sufficient, and you have no need for reverse outlets or SimRoute.

If there are significant reverse-head warnings, then you can try to model the reversing flows. But make sure the your DSI results are clean before making these changes. Any problems or instability will only get worse with SimRoute, so get it fixed first. And remember that significant oscillations generally mean that the results are not meaningful - that's the reason for the warning message.

As an alternative to SimRoute, you should also consider combining any adjacent ponds that experiencing reversing flows.

For further details please see www.hydrocad.net/tailwater.htm

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  • $\begingroup$ Apologies, I should've mentioned that I did attempt a DSI routing initially but was seeing downstream errors indicating Basin H having an elevation in vast exceedance of upstream basins (2-3 feet). This was causing me to lose a lot of effective volume in the upstream basins. $\endgroup$ Jul 21, 2022 at 12:59
  • $\begingroup$ Remember than SimRoute will be less stable, especially with small pond volumes, and may require a smaller time step. It's hard to comment further without seeing the HydroCAD file. $\endgroup$
    – psmart
    Jul 22, 2022 at 14:35

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