Manhole or catchbasin inflow is not equal to the outflow with the Implicit or Explicit solver

Product(s): SewerGEMS, CivilStorm
Version(s): CONNECT Edition, V8i
Area: Output and Reporting

Problem

Why doesn't the "Flow (Total In)" match the "Flow (Total Out)" result in the properties of a manhole or catchbasin when using the Implicit or Explicit (SWMM) solver in SewerGEMS or CIvilStorm?

Solution

First, a manhole is a storage element (similar to a pond), so the flow in may not be always equal the flow out because of storage effects, but the volume in should equal the volume out (accounting for change in storage between start and end and overflow volume).

If you are viewing these results in the properties of a manhole or catchbasin, you may be viewing the results at time zero when the model may be in a state of flux. Typically the results in the first hour or two of the simulation are not always accurate due to the dry pipes initially filling. It is generally best to graph the time series results and ignore the initial filling period. Using the same example above, we can see that the inflow and outflow results are equal after the initial timestep:

To ensure the volume mass balance is occurring, open the Hydraulic Reviewer tool (available for Implicit and Explicit solvers only, under Analysis > Analysis Tools > More), since this shows the mass balance based on volume for all storage element types (including manholes). Make sure that you account for any overflow, too.

If there is a large %deviation (continuity error) for that particular manhole in the Hydraulic Reviewer, it means that the model is unstable and you may need to tweak the calculation options to get better results.

See Also

 

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