Previous analysis of pressure and velocity data collected at a site experiencing stormwater geysers suggested a mechanism for the occurrence. Laboratory experiments attempting to reproduce the geysers were promising but inconclusive due to potential scale effects in the small scale experiments. Subsequent researchers have generally used this early work as the starting point for subsequent laboratory experiments. Over the years, a number of videos of geysers in actual systems have been obtained and a review of important features from these videos calls into question certain aspects of the original hypothesis. An alternative explanation for the phenomenon has been developed and an experimental system has been identified that may help resolve some questions. However, laboratory experiments may also be impacted by scale effects and several preliminary studies must be conducted to determine whether there is a fruitful path forward in advancing our knowledge of this phenomenon.

Deep storage tunnels are an alternative to mitigate combined sewer overflows, but they may develop transient flow problems during rapid filling conditions. Such issues include excessive surges, formation of pipe-filling bores, entrapped air pockets, and uncontrolled releases of air. Initial decisions on stormwater tunnel designs are generally based on steady-flow considerations but can have a significant impact on the transient hydraulic performance. However, tunnel designers rarely have an opportunity to systematically assess the interplay between geometric characteristics (e.g.,tunnel diameter, alignment, and junction areas) and inflow hydrographs. This work aims to provide an examination of the relationship between key design parameters and the performance of stormwater tunnels undergoing rapid filling. A flow regime transition model was used to simulate 216 different scenarios of rapid filling of tunnels, considering air-water interactions such as air pocket entrapment, compression, and expansion. Results indicate that peak surges and air outflows though vertical shafts correlated well with design parameters such as junction shaft plan areas and inflows at the time of tunnel pressurization. Other variables, such as entrapped air pocket volume, did not correlate well with geometric or inflow parameters selected in this study. While these results are not directly applicable to any existing stormwater tunnel geometry, the findings should help designers to understand potential transient flow issues and the input conditions that may lead to the most severe transient conditions.

Storm water and combined sewer systems are subject to rapid filling during intense rainfall events. These large-sized systems require a significant amount of air to be vented during filling. Several systems have experienced operational problems such as structural damages and geyser release through ventilation shafts. Observations from one storm water conveyance tunnel and laboratory experiments suggest that entrapment of large discrete volumes of air during rapid filling can explain geyser formation as well as large system pressures associated with the compression of trapped air. Only modest system pressures are required to produce geysers. Descriptions of several air–water interactions are presented to advance the hypotheses for the occurrence of the two detrimental phenomena. Requirements for numerical models to analyse rapidly filling conduits are discussed. A review of the existing modelling frameworks is presented. Some limitations to these models are discussed along with a discussion of possible model enhancements.

The performance and comfort of aircrew using stereoscopic displays viewed at a near distance over long periods of time are now important operational factors to consider with the introduction of aerial refueling tankers using remote vision system technology. Due to concern that the current U.S. Air Force vision standards and test procedures may not be adequate for accurately identifying aircrew medically fit to operate this new technology for long mission durations, we investigated performance with the use of a simulated remote vision system and the ability of different vision tests to predict performance and reported discomfort. The results showed that the use of stereoscopic cameras generally improved performance but that individuals with poorer vision test scores performed more poorly and reported greater levels of discomfort. In general, newly developed computer-based vision tests were more predictive of both performance and reported discomfort than standard optometric tests.

Numerical modeling indicates that high pressures can be expected when trapped air at the end of a pipeline is compressed as it stops a moving water column. In particular, the model results show that larger pressures are to be expected when compressed air volumes are small. Laboratory experiments where a trapped air pocket is compressed by an advancing filling front also confirm that small air volumes lead to the highest pressure rises. However, the model formulation essentially treats the moving water column as a vertical front with a trapped volume of air in front of it, an assumption that cannot be expected to hold for large diameter stormwater tunnels. The authors have been involved in the numerical modeling of rapidly filling flows in CSO storage tunnel systems. The Two-component Pressure Approach can predict the location where air can become entrapped and the associated volume, but the model framework is a single phase flow simulation and the air is not explicitly modeled. This leads to conceptual errors in the modeling of flow processes once the air becomes entrapped. The numerical model was reformulated to allowed the inclusion of the trapped air volume in the simulation in a simplified fashion. Simulations with the modified model for a specific application suggest that there are two types of flow conditions that can lead to trapped air pockets that are subsequently compressed although future investigations may define additional conditions. The simulation results suggest that only modest pressure rises should be expected in the particular application investigated and the physical explanations for this outcome are described.

The deep tunnel and reservoir project (TARP) in Chicago and several other combined sewer or stormwater systems have intermittently experienced events typically referred to as geysers which involve eruptions of water or water/air mixtures from vertical access or ventilation shafts that often spray tens of meters into the air. Geyser events can lead to public health or safety impacts. A fair amount of research has been conducted to understand the phenomenon and to develop methods for controlling or eliminating such occurrences. The literature on the subject is somewhat confusing due to an apparent lack of understanding of the circumstances that can lead to geyser formation. Much of the early work proceeded from an analysis of single phase flow (water) modeling under the premise that inertial surges in rapidly filling conduits were the essential mechanisms for geyser formation and the role of air in the process was neglected. Although this assumption leads to simpler model formulations, evidence from videos often fortuitously recording geyser events and limited quantitative data suggests that the assumption is often insufficient to account for observations. A brief review of some available data to support this contention will be presented. Results are also presented from controlled laboratory experiments involving the release of an entrapped pocket of air through a vertical shaft that can be interpreted as geysers. These experiments were carefully performed to eliminate inertial surges as a cause for the observed conditions. These results lead to the conclusion that a significant cause of geyser formation in at least some situations is the release of a trapped volume of air through a vertical shaft. This led to the formulation of a numerical modeling scheme that is capable of resolving the sharp filling fronts that can develop in a rapidly filling stormwater system. A discussion of model limitations and modifications that are being explored to more faithfully represent trapped air interactions is provided.

Large discrete air pockets trapped along the crown of a nearly horizontal stormwater tunnel have been shown in laboratory experiments to produce large vertical…

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) andWright 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

Increasing numbers of lines of evidence indicate that entrapment of large pockets of air during the rapid filling of stormwater and combined sewer systems may …

The rapid filling of closed conduits containing trapped air may result in severe pressure fluctuations associated with air compression; most numerical models of the filling process ignore the presence of the air. While some models have incorporated air-water interactions, most of these are based on lumped-inertia and assume well defined air-water interfaces. It is expected that air will be distributed along large diameter conduits and a relevant question is whether the assumption concentrated air at specific locations affects model predictions. This study compares predictions from two models that account for air pressurization, and from a model that disregards the air phase, with experimental results from rapid filling pipes with no ventilation. While the pressure peaks predicted by the two models that incorporate air pressure were similar, the discretized model incorporated more air-water interactions than the lumped inertia model. In contrast, peak pressures predicted by the model that disregards air phase were far larger than the ones observed in experiments and unrelated to the measurements.

Transient analyses of filling stormwater or combined sewer systems are often conducted to determine the potential for water returning to grade through vertical manhole or ventilation shafts. Although several studies have investigated interactions with air trapped during the filling process (Li and McCorquodale, 1999; Vasconcelos and Wright, 2005, 2006 and 2009; Wright et al, 2008; Zhou et al, 2002a and 2002b;), numerical models to simulate filling transients almost exclusively consider only the water phase (Capart et al, 1997; Cardle and Song, 1988; Politano et al, 2007; Vasconcelos et al, 2006). With these models, the mechanism for the return of water to grade is an inertial surge associated with the filling process, as discussed by Guo and Song (1990). The existence of air interactions allows for the possibility of other mechanisms for large water rises in vertical shafts. Previous research by the authors has indicated the ability of trapped air pockets entering vertical shafts to eject water ahead of the rising air (Lewis et al, 2010; Wright et al, 2007).This chapter presents the results of field measurements during geyser events in a stormwater tunnel system that indicate that the hydraulic grade line remained over 20 m below grade during events in which water was ejected 15 m to 20 m into the air. This chapter also presents the results of laboratory experiments that were performed in a setup that involved the release of air pockets but was intended to minimize the development of inertial surges. These experiments were intended to investigate the effectiveness in mitigating surges of a configuration involving an expansion in the riser diameter. Observations of the water level in the riser attached to a horizontal pipeline with initial stagnant water and air injection were analyzed to determine the maximum water level in the riser. The observations indicated a fluctuating water level within the riser, with additional rise that was characterized as splash as the air burst through the water surface. Subsequent investigation indicated that the fluctuating water levels were created by inertial oscillations set up by pressure drops within the riser as the air was expelled.Considering these laboratory observations, a relevant question is whether these inertial oscillations would also be relevant in prototype applications. This question was addressed using the numerical model by Vasconcelos et al. (2006) in an idealized scenario that was intended to reproduce initial conditions similar to those associated with the release of a large mass of air through a vertical shaft. The simulation results suggest that the occurrence of inertial oscillations driven by air release is relevant to full scale systems.

One potential problem affecting below-grade storm-water storage tunnels is the occurrence of geysering, which is defined as the return of conveyed water to grade. Most investigations to date have linked this occurrence with inertial oscillation of the water within vertical shafts. Another mechanism that can lead to geysering is the release of air and water through ventilation towers. This study presents a systematic investigation on geysering caused by the release of large air pockets through partially water-filled ventilation towers. Parameters considered in the study included the water level in the ventilation tower, air-phase pressure head, and ventilation tower diameter. An important parameter in geysering was the diameter of the ventilation tower. A simplified numerical model was developed to simulate the experiments; it was able to reproduce the essential features of the experiments.

The discussers would like to congratulate the authors for a very interesting paper, in which the gradual flow regime transition i.e., in the absence of a pipe-filling bore front —an outstanding issue in interface tracking models for flow regime transition—was addressed. The proposed model is in this regard an improvement on the family of models presented by Song et al. 1983 and Cardle and Song 1988 , among others. Based on the discussers’ experience in modeling tunnel systems, gradual-flow regime transitions should probably be more common than pipe-filling bores, even though some extreme inflow conditions, which could be very important for design purposes, may require pipe-filling bore modeling. The discussers would like to make a few comments and raise some issues: 1. Geysering events are not necessarily characterized solely by water jets, as investigated by Vasconcelos 2005 . Experiments and field evidence indicate that such phenomena could also be triggered by air pocket expulsion through water-filled vertical shafts in stormwater systems. Careful consideration of the air pocket entrapment is required when dealing with flow-regime transition events. 2. There is already available an alternative to model flowregime transition other than Preissmann slot or interface tracking-based models, namely the TPA model of Vasconcelos et al. 2006 . The TPA model overcomes the limitations of Preissmann-slot models of modeling subatmospheric flows, while retaining a single set of equations to simulate both free-surface and pressurized flows. Indeed as the authors mentioned, flow-regime transition models constructed with shock-capturing approaches may develop numerical oscillations in the vicinity of pipe-filling bores. However, Vasconcelos et al. 2006 propose an approach to attenuate such oscillations. More recently, Vasconcelos et al. 2009 address this issue, and other techniques to attenuate these numerical oscillations are proposed. 3. The discussers doubt that the proposed model would be able to handle a depressurization scenario in which pressurized flows are succeeded by free-surface flows. In such cases, the flow regime interface would be characterized by air intrusion

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.

The concept of a bottom slot outlet was developed to serve as a combined sewer overflow diversion structure to convey flow to deep tunnel storage under free discharge conditions and pass the remaining volume once storage capacity is exceeded with minimum backwater effects. In order to test the concept, a 1:19.5 scale model was constructed and tested over a range of discharges in order to determine the required slot length to pass a certain discharge. Different bottom slopes and slot widths were tested. For subcritical approach flow, the flow in the structure passed through critical depth at the upstream end of the slot, eliminating the possibility of backwater effects. The results were found to be nearly independent of the bottom slope for typical small slopes in sewer systems, and a simple dimensionless relation was developed to relate the slot length to other parameters. A mathematical model combining continuity and energy relations for the remaining flow in the structure with an orifice relation for flow through the slot was capable of reproducing the observed experimental results.

The discussers would like to congratulate the authors for a very interesting paper, in which the gradual flow regime transition i.e., in the absence of a pipe-filling bore front —an outstanding issue in interface tracking models for flow regime transition—was addressed. The proposed model is in this regard an improvement on the family of models presented by Song et al. 1983 and Cardle and Song 1988 , among others. Based on the discussers’ experience in modeling tunnel systems, gradual-flow regime transitions should probably be more common than pipe-filling bores, even though some extreme inflow conditions, which could be very important for design purposes, may require pipe-filling bore modeling. The discussers would like to make a few comments and raise some issues: 1. Geysering events are not necessarily characterized solely by water jets, as investigated by Vasconcelos 2005 . Experiments and field evidence indicate that such phenomena could also be triggered by air pocket expulsion through water-filled vertical shafts in stormwater systems. Careful consideration of the air pocket entrapment is required when dealing with flow-regime transition events. 2. There is already available an alternative to model flowregime transition other than Preissmann slot or interface tracking-based models, namely the TPA model of Vasconcelos et al. 2006 . The TPA model overcomes the limitations of Preissmann-slot models of modeling subatmospheric flows, while retaining a single set of equations to simulate both free-surface and pressurized flows. Indeed as the authors mentioned, flow-regime transition models constructed with shock-capturing approaches may develop numerical oscillations in the vicinity of pipe-filling bores. However, Vasconcelos et al. 2006 propose an approach to attenuate such oscillations. More recently, Vasconcelos et al. 2009 address this issue, and other techniques to attenuate these numerical oscillations are proposed. 3. The discussers doubt that the proposed model would be able to handle a depressurization scenario in which pressurized flows are succeeded by free-surface flows. In such cases, the flow regime interface would be characterized by air intrusion

Various rapid filling scenarios lead to the formation of large discrete air pockets in below ground storage tunnel systems. The release of trapped air pockets …

A deep, large-diameter underground shaft to provide detention storage for combined sewer overflow control may be advantageous in urban environments, where space limitations require solutions with a small footprint. An underflow baffle wall is provided at the center of the treatment shaft to prevent short-circuiting of the flow. An additional objective is to maintain low headlosses through the structure. A physical model study was conducted to determine the effect of the bottom elevation of the baffle wall on the headloss and breakthrough curve for dye injected to the inflow. It was found that there is a considerable range of elevations for which the structure behaves acceptably in providing adequate contact time for disinfectant while maintaining small headlosses.

Below-grade stormwater storage tunnels are designed to provide relief for collection networks during intense rain events. Air must be evacuated as the tunnel fills and, depending on the system geometry, the air may become pressurized. This research presents an investigation on various ways that the air pressurization influences the water flow. The investigation used an experimental apparatus consisting of a horizontal 14.8m long, 94 mm diameter acrylic pipeline with various degrees of air ventilation. The experiments were primarily conducted to explore two features of the flow termed pre-bore motion and the interface breakdown. Experimental measurements include flow velocity, air and water pressure, and flow depth. The experimental results were compared to the predictions of a numerical model based on the Saint-Venant equations that handles the flow regime transition and the possibility of air pressurization. The numerical predictions agree well with the experimental observations and provide an explanation for the interface breakdown occurrence.

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.