Using the "Correct for Partial Area Effects" Calculation Option in StormCAD (GVF-Rational Solver)


Product(s):StormCAD, CivilStorm, SewerGEMS
Version(s):CONNECT Edition, V8i
Area:Calculations

How does "Correct for Partial Are Effects" work?

The "Correct for Partial Area Effects" Calculation Option is available with the GVF-Rational solver in StormCAD, CivilStorm and SewerGEMS:

This option can be useful in situations where a large impervious area with a small time of concentration is connected to a network downstream of a small pervious area with a large time of concentration. Without using the "correct for partial area effects", the traditional logic applies and the StormCAD solver adopts the largest of the system times at a junction point, which can result in a reduction in flow (larger time = lower intensity). This is because the rational method traditionally assumes that peak flow occurs at the time when the whole upstream catchment area contributes to the flow.

If this calculation option is enabled and there are multiple rational flow sources entering a node (see list below), the model will check all incoming flows to find out the largest rational flow from the sources. This identified maximum flow is called the "partial area flow". The combined rational flow is calculated by the normal procedure and then is compared with the partial area flow. If the partial area flow is smaller than the combined rational flow, the combined rational flow will be used. If the combined rational flow is smaller than the partial area flow, the partial area flow is used as the new system rational flow at this point.

A larger 'partial area' flow will be carried downstream until the 'total area' flows exceeds it.

For any downstream node, the final combined system rational flow should not be smaller than the single maximum rational flow from contributing sources; these contributing sources include:

  • Incoming and tributary pipes
  • Captured surface inflow from catchments and gutters
  • The inflows (rational portion) from diversions
  • External rational flows entered directly in the catchbasin

Note: currently the other properties, such as the system CA, System Flow Time, System Intensity, etc. are not updated when the partial area flow is chosen. 

When a partial area flow is used it also serves as a minimum rational flow for the downstream elements. The downstream elements still follow the normal calculation procedure and the calculated rational flow will be compared with any upstream partial area flows. If the normal calculation procedure results in a flow that is smaller than the partial area flow, the system flow is replaced by the larger partial area flow.

Please see the Help topic "Partial Area Effects" for examples illustrating how this calculation option works in various situations.

Troubleshooting

Note: currently properties such as the system CA, System Flow Time, System Intensity, etc. are not updated when the partial area flow is chosen. This can potentially lead to confusion when comparing the system time and CA with the calculated flow.

If you are experiencing unexpected flow results downstream of a diversion or pond when using this option, please upgrade to a version greater than 10.04.00.158, or contact Technical Support for a patch for version 10.04.00.158 which fixes two issues related to these specific situations (reference #1107735 and 1133878)

See Also

How is flow balanced at junctions with the GVF Rational Solver (StormCAD)?

Flow Attenuation - Why does flow or velocity sometimes decrease when moving downstream?