Physical Processes Resulting in Geysers in Rapidly Filling Storm-Water Tunnels

Geysers, which involve the explosive release of water through vertical shafts connected to a nearly horizontal pipeline, have been attributed to either pipeline surge or the release of air. Laboratory experiments involving the release of a large entrapped pocket of air through a surcharged vertical riser indicate that the air can force water upward in the shaft but that a jet such as seen in video records of prototype systems does not form. This difference is attributed to processes that cannot be scaled down to the laboratory experiments. Data from a storm-water tunnel in Minneapolis that experienced a series of observed geyser events were analyzed. Measurements included pressures and velocity within the tunnel that can be correlated with observations on a videotape of the geysers. The pressure records do not indicate surge pressures sufficient to lift the water to the ground surface. Features of the pressure records can be interpreted to indicate the release of large air pockets through the manhole shaft similar to the laboratory experiments. 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 air effects.