| Product(s): | StormCAD, SewerCAD, CivilStorm, SewerGEMS |
| Version(s): | 10.XX.XX.XX and higher |
What does the following User Notification warning mean, when trying to compute a model with ponds and outlets using the GVF-Rational (StormCAD) or GVF-Convex (SewerCAD) solver?
"Tailwater elevation is higher than pond outlet's initial elevation. Calculations assume a free outfall, therefore pond outflow may be overestimated"
This message appears when using the StormCAD (GVF-Rational) or GVF-Convex (SewerCAD) solver (see "Active numerical solver" in calculation options. With these solvers, the pond calculations do not account for tailwater elevation in the pond outflow calculations.
With the StormCAD solver (peak flow/steady state) or with a steady state analysis in SewerCAD, it simply takes the initial elevation entered in the pond properties and looks that up on the outlet structure rating curve under the assumption of free outfall (not tailwater effects). But, the solver checks the HGL at the downstream element and compares it to the pond water surface elevation, in case the tailwater is higher. If that happens (HGL in downstream element > pond WSE), it generates that message to inform you that the flow out of the pond is probably going to be higher than in the real system.
Meaning, if the tailwater elevation was accounted for (which would require iterations that the StormCAD and SewerCAD solvers currently do not do for ponds), that restriction would reduce the outflow on the outlet rating curve at the given pond water surface elevation.
So, you may want to consider a different solver that can account for tailwater and other pond effects (Implicit or SWMM solver in SewerGEMS or CivilStorm) or increase the initial water surface elevation of the pond (for peak flow /conservative design purposes users often set it near the pond top) or increase the downstream pipe capacity to reduce the tailwater.
Pond and pond outlet assumptions in StormCAD, SewerGEMS, CivilStorm and SewerCAD