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The introduction of non-linear, shock-capturing schemes has improved numerical predictions of hydraulic bores but significant numerical oscillations have been reported in the predictions of pipe-filling bore fronts associated with the transition between open-channel and pressurized flow regimes. These oscillations can compromise the stability of numerical models. A study of these oscillations indicates that the strength of the numerical oscillations is associated with the sharp discontinuities in the flow parameters across the jump, particularly the wave celerity. Approaches to attenuate oscillations by artificially reducing acoustic wave speeds may result in the loss of simulation accuracy. Two new techniques to attenuate the oscillation amplitudes are presented, the first based on numerical filtering of the oscillations and the second based on a new flux function that judiciously introduces numerical diffusion only in the vicinity of the bore front. Both approaches are effective in decreasing the strength of the numerical oscillations.

Below grade storage tunnels in stormwater systems are usually designed to operate in a free-surface flow regime. However, intense rain events may trigger flow regime transition to pressurized flow, which results in operational problems. To date, little guidance is available as to the considerations necessary to properly design a system undergoing flow regime transition. In this investigation, an experimental apparatus consisting of a 14.3 m long, 94 mm diameter acrylic pipe was used to observe the nature of flows in such conditions. It was noticed that the air near the pipe crown may pressurize and influence the flow dynamics. Qualitative observations regarding the interactions between the air and water phases during the filling events are included in this study. Generally the surge intensity was maximized when a hydraulic bore propagating towards the surge riser just fills the pipe cross-section, and it increased with the pressure head behind the pressurization front. Furthermore, the results indicate that the effects of air phase pressurization should be properly included in numerical simulations if ventilation conditions are limited.

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One potential problem affecting below-grade stormwater 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. It was found that the one important parameter in the geysering occurrence was the diameter of the ventilation tower. A simplified numerical model was developed to simulate the experiments it was capable of reproducing the essential features of the experiments.

Events that are referred to as geysers have been observed in stormwater or combined sewer systems and are associated with jets of water rising through manholes to a considerable distance above the ground surface. Visual observations suggest that air may be a significant component of the jet. The mechanisms of geyser occurrence have been previously assumed to originate in inertial oscillations that force water up through vertical ventilation shafts. Recent laboratory investigations indicate that geyser formation is associated with the release of trapped air pockets through partially filled vertical shafts. Pressure data from a stormwater tunnel subject to infrequent geyser events is presented to indicate that measured piezometric heads adjacent to the ventilation shaft never increase to levels approaching the ground surface during a geyser event suggesting that air interactions must be an important part of the process. It is concluded that system design to avoid geyser formation must include the consideration of trapped air within the tunnel system.

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The entrapment and compression of air in closed conduits is a relevant problem in pipeline systems that experience unsteady flow regimes. Severe surging resulting from large air compressibility leads to failures, structural damage and other operational issues.Various studies have been performed on the topic, most of which simplified the flow equations by adopting a lumped inertia approach to simulate the unsteady water flow, an implementation of the ideal gas law, momentum equation and air-water continuity equations. The modeling benefits of a discretized approach, such as the method of characteristics (MOC), to simulate the water phase have not been sufficiently investigated. To address this knowledge gap, this paper compares an MOC and a lumped inertia model in adverse pipe slope conditions involving sudden air pocket compression caused by the closure (partial or total) of a downstream knife gate valve. In laboratory experiments air pressures and flow rates were measured during various sudden air compression events, serving to assess the accuracy of each modeling approach. Results of the comparison indicate that the two hydraulic models have comparable accuracy for partial valve closure cases. For total valve closures, the models are comparable for smaller surge events but significantly diverge when maximum H/D values exceed 60 to 80. This feature did not depend on the pipe L/D ratio for the proposed experiments, but the magnitude of error was affected by this ratio, specifically when the air pocket volume was less than unity. Additional experiments are needed to better assess the effects of the pipeline L/D ratio and other geometrical parameters, such as slope, on peak surge predictions.