Geysering in Rapidly Filling Stormwater Tunnels

Events that are referred to as geysers have been observed in stormwater or combined sewer systems and are associated with jets of water rising through manholes to a considerable distance above the ground surface. Visual observations suggest that air may be a significant component of the jet. The mechanisms of geyser occurrence have been previously assumed to originate in inertial oscillations that force water up through vertical ventilation shafts. Recent laboratory investigations indicate that geyser formation is associated with the release of trapped air pockets through partially filled vertical shafts. Pressure data from a stormwater tunnel subject to infrequent geyser events is presented to indicate that measured piezometric heads adjacent to the ventilation shaft never increase to levels approaching the ground surface during a geyser event suggesting that air interactions must be an important part of the process. It is concluded that system design to avoid geyser formation must include the consideration of trapped air within the tunnel system. CE Database Subject Headings: Combined sewers, stormwater management, underground storage, tunnels, surge, air-water interactions, buoyancy, geysering INTRODUCTION When stormwater or combined sewer overflow systems are filled rapidly, the phenomenon commonly referred to as “geysering” is of potential concern. Figure 1 is an image taken from a video recording of a geyser from a large diameter manhole in a stormwater collection system in Minneapolis, Minnesota. This particular image was from an event that occurred on July 13, 1997. Geysers have been observed in the operation of several combined sewer overflow storage tunnel projects and a design concern is how to avoid the occurrence. There appears to be some confusion in the literature about the origin of geysers, resulting in the possibility that the potential for geyser formation will not be properly accounted for in the design process and adequate measures for their mitigation will not be implemented. A discussion of geysering was presented by Guo and Song (1991) and a photograph of a geyser is included in that paper. A similar discussion was presented in Guo (1989). However, the analyses presented in these papers describe the problem of inertial surge in vertical risers connected to a nearly horizontal pipeline. In order for this process to generate the event depicted in Figure 1, the piezometric head in the pipeline would need to extend well above ground level and water velocities in the manhole would need to be on the order of 20 m/s in order to provide rise height on the order of an estimated 20 m (assuming that the rise height is approximately equal to the velocity head in the manhole). The image