	Product(s):	SewerGEMS, SewerCAD
	Version(s):	08.11.XX.XX and higher
	Area:	Output and Reporting

Problem

When graphing the HGL or level of a wetwell along with the flow in downstream pumps, the results are not expected. For example the wetwell level appears to drop during times when the pump appears to be off.

Or, when graphing one of the "Maximum" statistics such as Overflow (Maximum) alongside the corresponding calculated result (such as Overflow), the calculated result never rises up to meet the maximum.

Or, the wetwell elevation goes beyond the pump on or off range.

Solution

This may be due to the granularity of the results output. If the output timestep is too large, you may be missing important details between the reporting steps. For example a pump may be cycling on between the reporting timesteps, so the graph appears as if the pump is off. To resolve this, ensure that your output increment (reporting time step) is small enough to capture these changes. A quick way to do this is to set it equal to the calculation timestep.

For example here is a case where the calculation timestep is 0.001 hours but the Output Increment is 0.25 hours:

Here are the results when both are set to 0.001 hours:

Wetwell level goes beyond pump on/off range

Another example might be when you see that the level of a wetwell goes beyond the elevation you set for the Pump On or Off setting. Generate a graph of wetwell elevation alongside pump flow (press the CTRL key to multi-select, then right click and choose to graph). Click the Data tab of the graph and you may notice that when the pump turns on or off, it overshoots the control elevation within about one timestep.

To address this situation, you must use a smaller timestep. This will enable the model to capture the exact time when the wetwell reaches the pump control set point. However, using a smaller timestep can impact performance and potentially model stability. If you notice a higher Continuity Error, you might need to adjust other Calculation Options as explained in this article: Troubleshooting unstable SewerGEMS and CivilStorm results using the implicit solver.

One must find a good "balance" between accuracy (from a smaller timestep) and model performance (run time, improved with a larger timestep).

Instability

Unexpected results with pumps could also be due to instability. Review the first two articles below for assistance with that.

Wetwell level drops when pumps are off, Result never reaches Maximum, or other unexpected results in

Problem

Solution

Wetwell level goes beyond pump on/off range

Instability

See Also