Monday, August 4, 2014

Oscillations in SWMM 5 and InfoSWMM

I made this example to show how oscillations occur for legitimate reasons and the best ways to test the oscillations in InfoSWMM and SWMM 5:

Oscillations due to downstream obstructions, downstream mixed solutions and downstream boundary conditions:
In this case the oscillation in some links is due to the culvert comparison which limits the flow based on the inlet equation.  You can see the flow changes if you turn on show control actions in the run manager.  The program will then show the changes in the HTML report file.  The flow oscillation is due to the water being stored at the upstream node of the culvert followed by the excess flow going into the culvert.  

How to check  the validity of the solution:  Use a Map Display of the Link Result Variable Link Volume.  If you do a map display of the link total volume you will see that the total volume passing through each link is in balance.  You were using a 30 minute report time step, you can see and understand the oscillations better if you use a 1 minute report step.   In the culverts even though the flow in the culvert oscillates the mean flow is being passed downstream – that is why the downstream flows are constant.  I am trying a different way of modeling the steady state flow = this model does not have a continuity error but since the inflow to the culvert is oscillating between two solutions you are getting an uncertain flow.
Figure 1 Control Actions
Figure 2 Mass Balance

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