# Experimental Study on Geysers Induced by the Release of Trapped Air in Storage Tunnel Systems
## 1 Introduction  
The storage tunnel system has been widely applied to urban drainage systems to effectively increase stormwater storage capacity and prevent waterlogging disasters. Rapid inflows into the drainage network may lead to water-air interactions that give rise to the \"geyser\" phenomenon [1; 2; 3] which has been reported several times in the past few decades. Geysers can cause water to be ejected out of the ground and can give rise to strong instantaneous pressure variation thus negatively impacting the operation of urban network systems. Originally, [PERSON] and [PERSON] [4] believed that surges caused by pressure oscillations were the main reason for geysers. Subsequently, [PERSON] et al. [5] stated that pressure oscillations in the tunnel system alone were insufficient to give rise to geysers based on simple theoretical analysis. [PERSON] [6] also believed that the pressure oscillations induced by inertia could not fully explain the occurrence of the geyser phenomenon. [PERSON]'s experiment [7] indicated that the geyser phenomenon was closely related to the release of the trapped air pocket, and in the experiment, three types of geysers were observed: the air pocket released from the vertical shaft with initial water column, the water-air mixture jet, and the cross overflow between the air pocket and water. The first type was caused by the release of a large air pocket, and the last two types were caused by upstream surge. [PERSON] et al. [8] divided the geysers into three types based on flow regime and pressure characteristics: short column jets, column-breaking jets, and spray-like jets. These geysers are driven by system pressure and are dominated by the water-air mixture flow. [PERSON] and [PERSON] [9] first elaborated the mechanism of the kind of geyser induced by air pocket release. When the moving trapped air in the tunnel reaches the vertical shaft position, the air pocket rises and drives the surrounding fluid to move upwards and break at the free surface. After the release of the air, the water levels in the vertical shaft reduce considerably and sustain inertial oscillations for a long time. Subsequently, [PERSON] and [PERSON] [10; 11] carried out further research on this type of geyser, analyzing the influence of parameters, such as the water level of the shaft, air pocket pressure, and diameter of the shaft, and pointing out that the diameter is the key factor affecting the geyser strength. Similarly, [PERSON] et al. [12; 13] described the geyser mechanism and divided the geyser process into six stages. The authors presented gas mass flow rate as a significant parameter that affects geyser strength and provided the empirical formula for the maximum geyser height based on dimensional analysis and experimental statistics data. [PERSON] [14] analyzed the main driving factors of the geyser, including buoyancy, compressibility of air mass, inertia, and pressure, using two-dimensional numerical simulation and pointed out that the main factors affecting geyser strength are the volume of air pocket, the pressure in the main pipe, and the water level of the vent hole. [PERSON] et al. [15; 16] presented the relative relation between the water-air interface velocity and the free surface velocity using a large-scale experiment and computational fluid dynamics (CFD) simulation, and the results indicated that the velocities are approximately linearly proportional. [PERSON] et al. [17] divided the geyser process into three stages, namely, the occurrence of pressure bore, the entrapment and migration of compressed air pocket, and the eruption of water-gas mixed flow, and then developed a schematic diagram to illustrate the relations between dominant variables associated with geysers using the modified Preissmann slot model [7; 18] and the momentum equation. Recently, [PERSON] et al. [19] observed the release process of the air pocket by high-speed video camera technology.