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I've set up a model with numerous sub basins, links, junctions, etc.
When I run an SCS-TR-20 analysis i get some strange results. For example, one of my sub basins is only 0.4 acres with a computed peak flow of 1.02 cfs for a 2 yr-24hr storm. The inlet it drains to also shows a 1.02 cfs peak flow. However, the single 12 inch pipe it drains to shows a 14.17 cfs peak flow during analysis. Pretty weird i'd say or am i missing something? Thanks.
Hydrodynamic analysis? You will need to post your file for the best analysis, but I am going to venture a guess that your time steps are too small or lengthening is too short and the model didn't balance properly.
Matthew Anderson, PE CFM Product Manager Autodesk (Innovyze)
Does the program alert you if it doesn't balance properly? After analysis the dialog box shows Continuity Error: Hydrology 0%, Hydraulic Routing -0.05%
I tried to attach my data file to this message but the file extension (.spf) is not allowed.
Attached is my file with the extension changed to .dwg. I have deleted all subbasins, nodes and links except one where there appears to be a problem although there were others in my model that had similar discrepancies. After i run the 2 year 24 hour analysis the peak flow from the basin in about 1 cfs but the flow in the next link downstream is about 14 cfs.
Do you intend to have that last link at such a critical slope? 3.1602 ft/ft? This violates the maximum pipe drop pushing flow in the pipes into supercritical and messing with the balancing of the hydraulic routing.
The biggest problem is really the sag inlet and the lack of ponded area area over it. By providing additional area over the inlet, the peak flow for the downstream pipes reflect something more appropriate to the runoff.
Matthew Anderson, PE CFM Product Manager Autodesk (Innovyze)
So edited some of the data to give the inlet some ponding area (10 sf) and dropped the pipe slopes to less than 5% and when i run the analysis i still get 1.02 cfs from the subbasin to the inlet, 1.02 cfs at the inlet and 14.17 cfs in the pipe. Same numbers i getting before the edits. How is that possible?
10 sq ft of flooding is what the program assumes when you don't enter anything.
100 sq ft allows the program to smooth the transition between weir/orifice flows for your inlet.
The red line in the attached image is 10 sq. See how jerky the depth on the inlet is - its bouncing around trying to solve the flow at the inlet. The green line is 100 sq ft of flooding about the inlet (that's only 10 feet by 10 feet) of flooding, which I would assume you have for this inlet at 0.3 feet deep.
Matthew Anderson, PE CFM Product Manager Autodesk (Innovyze)
I changed the ponded area to 100 sf and now the analysis looks ok. Thanks. I'm trying to analyze overall site post develoment outflow for comparisons to pre developed conditions and wasn't trying to be very precise with my data entry for now. Thanks again.
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