The temporary belowground storage of stormwater is useful in urban areas to prevent flooding. During intense rain events the conduits can fill rapidly and completely undergo a transition from open channel flow to pressurized flow. This transition can create large discrete pockets of trapped air as the interface advances into the free-surface portions of the system. These pockets pressurize in the horizontal reaches of the system and then are released through vertical vents, potentially creating a geyser condition. There have been some experimental and numerical investigations of the geyser effect, but the complex nature of the physics and geometry make this a challenging problem. Numerical studies have treated the geyser as a lumped mass or rigid column, with no consideration to the multiphase flow dynamics. This work presents a preliminary finding of a multiphase flow numerical model to capture both the liquid and gas dynamics, as well as the necessary interaction to create the geyser.