Coupled pond mystery flow

[DuckEngineer]

I'm running HydroCAD 8 and am hitting a snag. I have two ponds, each about 1 square mile in size, that are connected by a 48" pipe. I am proposing a new structure in the downstream pond that will have an invert 10' above the connecting pipe. When I select the higher invert as a starting elevation, I start to have problems with the flow.

I need to assign the same starting water level to the upstream pond as the downstream, since they're connected. Stor-Ind routing didn't give me the error for tailwater, but I think the tailwater does have an impact. DSI and Sim Route show 130 cfs leaving the upper pond, but the summary shows no water going through the culvert (even if I model a reverse flow).

My concerns are that the stage discharge table shows heavy flows starting to occur 2' below my starting invert elevation. This combined with the culvert not showing any flow happening has me wondering if I'm asking HydroCAD to do a function that it can't handle. Or have I missed the obvious???

 

[psmart]

The stage-discharge report does not include the (variable) tailwater effects, but these are incorported in the routing calculations when you use DSI.

Make sure you set the starting elevation in both ponds. As long as there are no warning messages, the results should be valid. If you do have problems, you can send the file to HydroCAD support.

Also consider: If the interconnection between the ponds is large in comparison to the final (higher) outlet, then the two ponds will always be equalized, and are more easily modeled as a single pond, ignoring the interconnecting pipe.

 

[DuckEngineer]

I ended up running the upstream pond with a starting elevation using stor-ind and a fixed tailwater equal to the starting elevation in the downstream pond. Then I compared that to sim-route results with both ponds at the same starting elevation. The results were pretty close in terms of water surface elevations, so I just took the more conservative one. I am curious that the stor-ind did not show any warnings and the sim-soute indicated potential backflow. Thanks for the info.

 

[psmart]

The software will always generate a warning message if tailwater is present and you need to switch to another routing procedure. Start with Storage-Indication and check the messages. If there is tailwater, you should generally use DSI rather than SimRoute. SimRoute is inherently less stable, and has advantages only for reversing flows. Please see

http://www.hydrocad.net/simroute.htm

If you still have questions please email your file to HydroCAD support.