Similar to a time of concentration, a system time (or controlling time) is the amount of time it takes for all contributing parts of the storm sewer to reach a given location. This includes a catchment's time of concentration, and pipe travel times. When combining rational loads, the controlling time is the greatest of the individual loads' system times. This system time is used as the duration of the storm when determining peak intensity, and therefore peak flow.

To avoid unreasonably low storm durations and unreasonably high rainfall intensities, many regulatory agencies impose minimum storm durations, typically 5 or 10 minutes. Bentley StormCAD allows you to specify a minimum storm duration and uses this as the controlling time when the computed time is too low. In these cases, Bentley StormCAD carries the computed system time throughout the system, but continues to calculate intensity based on the minimum allowed time (until the system time rises above the minimum).

For example, consider a catchment at I-1 with a time of concentration of 4 minutes, and a minimum allowable duration of 5 minutes:

I-1 Catchment time of concentration: 4.0 minutes

Bentley StormCAD computes flow based on: 5.0 minutes

P-1 Pipe travel time: 0.5 minutes

J-1 System time (4.0 + 0.5): 4.5 minutes

Bentley StormCAD computes flow based on: 5.0 minutes

P-2 Pipe travel time: 1.0 minutes

O-1 System time (4.5 + 1.0): 5.5 minutes

Bentley StormCAD computes flow based on: 5.5 minutes

This 5.5 minutes is used as the duration in the intensity vs. duration equation used to calculate i in determining the flow using:

See Flow Balance at Junctions for more information.