| Product(s): | StormCAD, SewerGEMS, CivilStorm |
| Version(s): | 08.11.XX.XX and 10.00.XX.XX |
| Area: | Layout and Data Input |
Problem
How is the carryover Tc in a catch basin determined?
Why is the flow in the downstream gutter at-odds with the upstream carryover flow and inlet capture efficiency?
Solution
In StormCAD (or the GVF-Rational solver in SewerGEMS and CivilStorm), carryover loads are assumed to have the same time of concentration (carryover Tc) as the surface catchment load as is indicated in the help topic "Surface Carryover Loads". So, the carryover and bypass calculations use the attached catchment's Tc as the controlling System Time to determine the Intensity from the rational method Q=CIA. This means that if there is no catchment attached to the catch basin, the carryover Tc property will be 0.00. In older versions of the software, before version 08.11.03.83, the solver used to display the 'minimum Tc' value entered in the calculation options if there was no upstream catchment. If there was an upstream catchment the 'carryover Tc' used this value. This was changed in the later versions to help avoid confusion.
The times of concentration from their original catchments are not considered, so the time used for the carryover calculations is consistent with the time used for the other rational flow calculations at the inlet. Otherwise, you might have carryover calculations performed for, say, a 15 minute duration and catchment runoff calculations performed for, say, a 5 min duration at the same inlet, so there is an inconsistency there. The GVF-Rational solver doesn't use the 15 minutes for everything in this case because, since catchment runoff is usually the dominant factor (i.e. catchment runoff is usually higher than carryover flow), The GVF-Rational solver doesn't want to underestimate the catchment runoff by using the higher time, since that results in a lower average storm intensity.
It is unlikely that there is not some sort of catchment area connected to a catch basin, however small, draining to the bypass target. For example, the rainfall that lands directly in the gutter between the two inlets could be made into a catchment area.
If you'd like to see the 'carryover Tc' value in the catchment that is not displaying it, a workaround would be to attach a small catchment (for example zero or 0.01 acres) with a Tc equal to the value you want to use to compute carryover.
For example, here is a model that does not apply a Carryover Tc to the catch basin downstream of a gutter:
Here is the same model with the workaround (explained above) applied.
Carryover Modeling Method - V8i SELECTseries 4 and greater
Starting in V8i SELECTseries 4, a new option was included to specify how the bypassed carryover flow is conserved as it moves downstream: Carryover Modeling Method. This option provides two choices, "As CA (Traditional)" and "As Flow (HEC-22)". The default selection is "As CA (Traditional)".
This option allows you to specify how to add the carryover rational flow (upstream gutter flow) to a receiving inlet. If "As CA (Traditional)" is selected the calculation uses the bypassed CA and the receiving inlet local surface intensity to calculate the carryover flow which is added as part of the surface rational inflow. This sometimes can result in significant difference between the carryover flow and the flow in the bypassing gutter. The use of the second choice ("As Flow (HEC-22)") then can improve such use case; this will make the calculation directly use the bypassing gutter's flow as the carryover flow into the inlet, and accordingly the carryover CA is derived by the flow and local inlet intensity.