Can air flow in partially-full pipes be modeled in SewerGEMS or CivilStorm? For example:
The dynamic numerical solvers in SewerGEMS and CivilStorm can model partially-full pipes and the release/overflow of water using manholes, but they do not model the dynamics of air flow, such as the compression of air and complex air flow patterns.
Many factors can influence air flow such as drag at the fluid surface, temperature gradients and any force ventilation. If a pocket gets trapped and compressed in a storm/sewer, it can move up or down stream, and when it reaches a manhole, can blow the manhole cover off.
For surge/transient applications, our separate HAMMER product can model local air and vapor pockets, but it only works with pipes that are flowing full during the initial conditions (normal operating conditions).
One possible approach would be to observe the animation of the profile in SewerGEMS/CivilStorm and see which parts of the pipeline become surcharged last, which may reveal spots where air pockets may be trapped/compressed.
Consider also seeking local guidelines / "rules of thumb", and reviewing past research and related papers/articles on this subject - see below.
(Paper) Hydraulics of corrosive gas pockets in force mains (Walski, Barnhart, Driscoll, Tencha - Water Environment Research, volume 66, number 6)
(Forum Discussion) Hydraulic Tunnel - Sewer ventilation
(Wiki article) Can the storm-sewer products perform a transient analysis in the sewer force mains?