Why is the system time at one of my nodes much larger than the upstream nodes Tc or system time?

Product(s): StormCAD, SewerGEMS, CivilStorm
Version(s): 10.00.xx.xx, 08.11.XX.XX
Area: Calculations

Problem

Why is the system time at one of my nodes (catch basin, transition) much larger than the upstream nodes time of concentrations or system time?

or

How is flow calculated for the GVF-Rational Solver (StormCAD)

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

Problem ID#: 64653

Solution

Most likely the reason why this is happening is the following:

The time of concentration is added to the pipe travel time a.k.a (Time (Pipe Flow)) under the Results (Hydraulic Summary) for pipes, which determines the system time. As stated in the Help documentation, when combining rational loads, the controlling time (aka system time) is the greatest of the individual loads' system times. Therefore the system time at a given transition X with one upstream node and a pipe connecting to it would be computed as follows: Catch Basin Y
has a time of concentration of 5.00 minutes + the Time (Pipe Flow) for conduit Z which would be 71.193 min = a system travel time of at transition X of 76.193 min.

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. 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, 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

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

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

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 in determining the flow using:

Q = i (CA)

See Also

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