SSA - varying peak flow during analysis

[Anonymous]

The particular pipe system is for a piped irrigation ditch with a Q=105 cfs. There is no variability due to runoff events. The flow is set. I set the External inflow at the uppermost node to be Baseline inflow=105 cfs. 

When I perform the analysis, the program is calculating varying Peak Flow during Analysis varying from 107-120 cfs at each Link and Junction. What am I missing? It seems like a designer should be able to set the flow rate for exactly what they require for their design and the software should not be injecting biased results when not instructed to do so! 

[pat.lowe]

Hi @Anonymous

Are the Reporting and Routing time set to the same value?

[Anonymous]

No, I attached a screen shot of the Analysis Options. Thanks for responding.

[g_k50]

Do the pipe sizes vary? or the inlet conditions, or the Mannings? or the grade?

[Matt.Anderson]

 Began -

The problem is you are using a time-varying hydraulic simulation with an expectation that time does not change.

SSA is not gradually varied flow tool  - so your variability is the shock occurring in the system when you turn the hose on full blast at t=0. 

The "Rational" method you are using is a triangular hydrograph with a set time step of 30 seconds. 

[Anonymous]

Thank you. Based on the available options, what method do you recommend to analyze an irrigation pipe system with a constant Q?

[Matt.Anderson]

Hydraflow Storm Sewers or the "Analyze Pipe Networks" functions are the steady flow tools in the Autodesk portfolio for pipe systems.  Check & Question the Junction Losses as both tools can produce energy "gain" in a gravity network - which should not happen.  (Negative Energy Loss).

River & Flood for surface "channels" - but it's HEC-RAS within Civil 3D.

If you want to understand what SSA does - the EPA published the technical manual for Hydraulics which is what SSA is based:  https://www.epa.gov/water-research/storm-water-management-model-swmm#documents 

[fcernst · Accepted solution]

If you want to use SSA, use a Hot Start Interface file for this..to initialize the conditions.

If it's a pressurized distribution system you can use EPANET.