A below-grade vertical stormwater storage system is one of the solutions to reduce the volume of sewer overflows released into the environment. The system is submerged most of the time during filling, which can result in hydraulic problems. This research intent to provide some insight on potential hydraulic problems that can occur in a vertical storage system during intense rain events. An experimental study was conducted using a physical scale model that consists of two vertical storage shafts, a horizontal tunnel and an inflow drop shaft. The results showed that both entrapped air in the system and mass flow oscillation in the system can cause a rapid rise of water level, or a geyser, at the drop shaft. The predictions of a modified version of HAMMER compared well with the experimental result while the InfoWorks CS model was unable to simulate vertical momentum in the drop shaft.