Intermediate Time Steps

  Applies To 
  Product(s): WaterGEMS, WaterCAD, SewerCAD, SewerGEMS
  Version(s): V8 XM, V8i, CONNECT Edition
  Area:  Modeling
  Original Author: Dan Iannicelli, Bentley Technical Support Group

Problem

Why do I see extra time steps inserted between main timesteps, per the hydraulic timestep?

For example the hydraulic timestep is set to 1.0 hours, but results are reported at 1.10 hours.

Solution

In an Extended Period Simulation (EPS), WaterCAD, WaterGEMS and the GVF-Convex solver in SewerCAD and SewerGEMS can insert intermediate timesteps, up to one tenth of a hydraulic timestep. Meaning, it can add additional timesteps between the step defined in your calculations.

This happens when the program determines that a tank will fill or drain between the calculation timesteps, or if a logical control (not Simple) will trigger before the next scheduled timestep (per the calculation options). This is done for better accuracy. Click on the timestep in question in the Calculation Summary, then click the Information and Status Messages tabs to see what tanks or controls are causing it.

If you'd prefer to not see these additional time steps, you can choose "Constant" for the "Reporting Timestep" calculation option and set the reporting timestep equal to the hydraulic time step. Note that this option is currently only available for WaterCAD and WaterGEMS, not GVF-Convex (SewerCAD).The intermediate timestep results will still be calculated (and subsequent timestep results can be impacted accordingly), but graphs and other places will essentially skip over them.

Note that excessive intermediate timesteps are often an indicator of problems with tanks and/or controls. For example a tank may be hovering near 100% or 0% full (a graph of all tanks can determine this), which initiates a built in altitude valve that may be closing an opening. Check to make sure your controls are reasonable and that they are configured such that tanks do not become full or empty.

A note on EPANET

EPANET will also insert intermediate timesteps in the same fashion, though it may be difficult to visualize within the EPANET reporting results. The EPANET reporting does not show intermediate timesteps directly in the report Table. However you can visualize the Status Report to see that the intermediate events being triggered. To visualize the Status Report in EPANET, go to Hydraulics Options > Edit and change the Status Report option to FULL.

 

See Also

Rapid flow oscillation between hydraulically close tanks

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