Air-Water Interactions that Generate Large Water Lift through Vertical Shafts in Stormwater Conduits
Storm and combined sewer collection systems can fill rapidly during large precipitation events leading to a dynamic condition referred to as geysering in which an intense upward movement of a mixture of air and liquid rises through a vertical shaft well above the grade elevation. In some instances, the untreated liquid mixture jets tens of meters above the ground surface posing safety risks and flooding hazards. To date, most discussions of the rapid filling of stormwater systems present analyses of inertia-induced surges in the system. Numerical modeling such as that presented by Cardle and Song (1988) and Guo and Song (1991) have only considered single phase (water) flow phenomena to simulate the occurrence of geysers. Lewis et al. (2011) and Wright et al. (2011) present pressure data for a geyser event in a stormwater tunnel in Minneapolis, Minnesota that indicates the geyser could not have been produced by an inertial surge since the hydraulic grade line remained more than 20 m below grade during a series of geysers that jetted 15 m to 20 m into the air.
Laboratory studies conducted by the authors and others (Wright et al., 2003; Vasconcelos and Wright, 2005a; Wright et al., 2007) pointed to the role of discrete pockets of entrapped air in geyser formation. During the course of the study by Vasconcelos and Wright (2005b), observations were made of the interaction of a large trapped pocket of pressurized air migrating along the 9.4 cm diameter pipe crown with a 2.5 cm diameter ventilation shaft approximately 50 cm high and surcharged to an initial water depth of about 25 cm. Two distinct jets of water were observed, first as the front of the air pocket arriving at the shaft forced water out the top of the shaft ahead of it and then as the trailing end of the air pocket left the pipe and water refilled the ventilation shaft. The second jet of water reached over 1 m above the top of the ventilation shaft and was stronger than the initial jet. Since the small scale laboratory experiments do not dynamically reproduce all phenomena associated with water ejection at the scale of stormwater tunnels, it was unclear which of the two water jets was most closely associated with the geyser phenomenon seen in videos of actual sewer systems. This question led to the current investigation during which a number of different types of experiments were performed in order to develop a more comprehensive understanding of the processes that result in geyser formation. Several of these studies have been reported (Wright et al., 2007; Lewis et al., 2010; Lewis et al., 2011). This chapter aims to summarize these studies in an attempt to clarify the physical processes that may occur in rapidly filling pipeline systems to generate geyser events. To gain more insight into the processes involved in the observed laboratory surges associated with air, it was decided to perform additional experiments that dealt separately with the issue of the air arrival at the vertical shaft and the displacement of air from the system by pressurized water. More different types of experiments were performed, but only two sets of experiments are reported in this chapter. The dissertation by Lewis (2011) describes the entire experimental program and results. An interpretation of the results is provided to indicate the most likely processes involved in geysering and the critical variables that control the strength of the geyser. As mentioned above, the small scale laboratory experiments do not reproduce all dynamic effects so an interpretation is required, but the experiments suggest an explanation for the geyser formation.
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