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In urban stormwater systems, a rapid increase in water inflow can cause the entrapment of air pockets and subsequent ejection of air/water mixture, commonly known as storm geysers. In this study, a three-dimensional computational fluid dynamics model was built based on a lab model, which consisted of an upstream pipe and a downstream pipe with different invert elevation (i.e. a drop), connected by a chamber with a riser on the top. The effects of the downstream pipe characteristics, location of the entrapped air pocket, and its volume on the pressure variation were examined. With the downstream pipe flowing in full, two mechanisms that possibly trigger geyser events were simulated and experimentally validated: (1) a rapid inflow front in the upstream pipe with the flow changing from free surface flow to pressurized flow, and (2) the air pocket releasing in a pressurized system. For the first mechanism, the pressure surge in the chamber was related to the capacity of the downstream pipe for the increased flow rate. For the second mechanism, a smaller air pocket in the upstream pipe can generate a higher pressure during the geyser event due to the reduced damping effect on the pressure variation. A larger pressure drop was observed for a larger volume of released air. Lower peak pressure was generated for an air pocket closer to the chamber due to the shorter duration for pressure to build up before the air pocket reached the chamber.

Numerical study on the mechanisms of storm geysers in a vertical riser-chamber system