How do I model wet well overflow or backup? For example as a result of pump failure.
When a pump fails to operate, the upstream wetwell may overflow and cause flooding. SewerGEMS and CivilStorm can provide you the volume of overflow encountered within the wet well with the use of the Implicit or Explicit solver.
Basically, Implicit or Explicit solvers are best for analyzing problematic systems or flooding since they use dynamic numerical solvers that can calculate overflow rate and many other things.
Contrast with the GVF-Convex solver which uses the standard EPANET solver for pressure networks, allowing for solid stability with complex pumping situations like manifolds and pumps that cycle faster than the hydrologic time step. However, it is not intended for use with quantifying overflow, so you wouldn't be able to see the overflow with convex solver.
The backing up of water as a result of a wetwell overflow is accounted for automatically with the implicit and explicit solvers. If you are modeling a pump failure situation, a pump/outfall is not required downstream of the wetwell, although you can include it if you'd like, and set the pump on/off controls such that they never turn on. The backup effects from the overflow can be seen in profile view and other places.
The wet well overflow volume can be graphed and seen in the hydraulic reviewer with Implicit engine.
You can quantify overflow volume under Analysis > Analysis Tools > More > Hydraulic Reviewer (Or in V8i, Analysis > Hydraulic Reviewer)
Overflow could include flow at the surface area above the wet well, as it can influence the overflow rate and potentially pond up and recede.
To account for the surface above the wet well top:
A note on stability
Models with a large amount of overflow and/or surcharging can be difficult for the numerical solver to achieve stability. If you are experiencing high mass balance error (continuity error) or otherwise unexpected results, please see the tips in the below article written for a similar situation.
Modeling a flow restriction at an outfall
Storage volume of wetwell and pipe