The rapid filling of a stormwater storage tunnel is accompanied by a flow regime transition from free surface to pressurized flow. Nearly all previous experimental investigations of the flow regime transition have involved the study of pipe-filling bores; this seems to be a consequence of the experimental setup implemented as opposed to a necessarily common occurrence in prototype systems. In implementing a numerical model for the analysis of transient filling conditions in a large combined sewer overflow storage tunnel proposed for Washington DC, we became aware that for more typical filling conditions, the flow regime transition more commonly occurs as a gradual flow regime transition which involves a regular free surface bore followed by a gradually sloping free surface up to the pipe crown. This unsteady process can result in air pockets being trapped and pressurized in the system. As these air pockets reach a ventilation shaft, they can be quickly released upward in an air-water mixture. This phenomenon known as “geysering” potentially results in the untreated wastewater returning to the surface. Laboratory experiments were used to observe the formation of pressurized air pockets and geysering events associated with gradual flow regime conditions. Geysers that occur due to the release of these air pockets do not necessarily correspond to high pressures in the system. The consequences of these observations for the requirements of numerical models are discussed. The researchers also show that the geysering phenomenon can be significantly reduced by adjusting the geometry of the ventilation shafts.