Physical Processes Resulting in Geyser Formation in Rapidly Filling Stormwater Tunnels

A significant concern in the design of large, underground conduits for storm or combined sewer systems is the avoidance of geyser formation, a phenomenon which involves the explosive release of water through vertical shafts. Current practice for transient analyses of these systems involves application of numerical models to simulate the rapid filling processes. Since these models do not consider interactions between the inflows and air being displaced during the filling process, it is felt that these models cannot predict geyser formation. Data from a large diameter stormwater tunnel in Minneapolis, Minnesota has been analyzed to determine the events that led to a series of observed geyser events. Data include pressure and velocity measurements within the tunnel that can be correlated with a videotape of the geyser events themselves. The pressure records are inconsistent with hydrostatic pressure lifting water to the ground surface. Features of the pressure records can be interpreted to indicate the release of large air pockets through the dropshaft. A qualitative comparison of these results with smaller scale laboratory experiments indicates similarities in the pressure responses. These results suggest that the entrapment of large air pockets is an important component to the geysering process and that tunnel design procedures need to properly account for these effects.