Prior analyses of transient events in storm and combined sewer systems that have been referred to as geysers have been considered as created by inertial surge associated with a rapidly filling system. In previous laboratory studies and analyses of data from prototype systems, we have demonstrated the role of entrapped air pockets in lifting water through manholes or other vertical shafts. We have also suggested that if the vertical shaft undergoes a diameter expansion close to the tunnel crown, the magnitude of the liquid rise is reduced. This paper reports on the results of additional laboratory studies on these issues. In order to eliminate the effect of inertial surge associated with system filling, the experiments are performed by injecting air into a dead-end pipeline connected to a constant head reservoir at one end. The air propagates as a series of discrete pockets to a vertical riser of varying geometries.
We observed that the rise of air pockets through the riser sets up inertial oscillations as the air escape creates a sudden drop in water level in the riser. Analyses show that the observed period and magnitude of oscillation can be reasonably well predicted with a model in which the surge is created by the imbalance between the reservoir head and the water level in the riser following the air escape. The maximum vertical rise of water in the riser occurs during simultaneous conditions of high surge level plus the ejection of water ahead of the escaping air. Of the two processes, the water ejection by the air pocket provides the major contribution to the water rise in most cases. Results are presented for both a constant diameter vertical riser and one with a diameter expansion one tunnel diameter above the tunnel crown. We also vary the initial reservoir head which influences the amount of water in the riser prior to the air release. It is demonstrated that the diameter expansion reduces the vertical rise and that the vertical rise is greater with higher initial water levels in the riser.