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Is it possible to calculate the time required to empty the pipe network in WaterGEMS and WaterCAD?
One of the underlying assumptions for the EPANET solver that WaterGEMS and WaterCAD is based on is that the pipes are always pressurized. Because of that, you will not be able to directly model a pipe emptying or becoming partially full. As an indirect way of modeling this, you may be able to run an extended period simulation (EPS). If you have a tank in the model, you would be able to see how long it takes for the tank to drain, which will give you an idea of the timing for the pipes to drain.
A better way to model this type of situation is to use SewerGEMS. SewerGEMS allows for the modeling of partially-filled pipes that are running under gravity as well as pipes that are pressurized. In SewerGEMS, you would be better able to monitor the timing of when a pipe goes from the being under pressure to partially full, as well as how long it takes before the pipe is empty.
Modeling a case where an empty pipe is filling using Bentley HAMMER