| Product(s): | StormCAD, SewerGEMS, CivilStorm | |
| Version(s): | CONNECT Edition, V8i | |
| 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)?
Solution
A sudden increase in the system flow time may be due to a low velocity (and high pipe flow time), and/or due to how the GVF-Rational solver adopts the highest system time at junction points.
Pipe Flow Time
For example starting at the first pipe after a catchment/catchbasin, the time of concentration is added to the pipe travel time "Time (Pipe Flow)" under the Results (Hydraulic Summary) for pipes, which determines the system time. The pipe flow time is based on the velocity, so if the pipe flow time is high, check the flow, pipe size and diameter. Also consider the average velocity method being used.
Controlling Time at Junctions
As stated in the Help documentation, when combining rational loads at junction points (multiple Rational flows meeting together), the controlling time (also known as the system time) is adopted as the greatest of the individual incoming Rational flows' system times. Therefore the system time at a given node 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 node 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.
Note: The "Correct for Partial Area Effects" Calculation 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. See more here: Using the "Correct for Partial Area Effects" Calculation Option in StormCAD (GVF-Rational Solver)
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: