Stormwater Storage Tunnels Research Articles

Analysis of pressure surges for water filling in deep stormwater storage tunnels with entrapped air-pocket using a VOF model

Abstract The pressure surges for water filling in deep stormwater storage tunnels (DSSTs) with entrapped air-pockets seriously threaten the safety of pipeline structures and even lead to the destruction of urban water infrastructure. Hence, this paper develops a volume of fluid (VOF) model to study pressure surges in a two shafts and one tunnel system. Research works under different initial air-pocket parameters are carried out, while the VOF model is verified by the empty tunnel water filling experiment in the same system. The results show that the maximum pressure increases with an initial length/diameter ratio decrease or air volume fraction increase. Also, the extreme pressure with entrapped air-pocket at a certain scale can reach 1.6 times the static pressure (30 m). With the increase of initial air volume fraction, the frequency of pressure surges slows down, while the maximum pressure gradually increases and approaches a specific value within a fixed air volume fraction range (0.2–5.0%). The maximum pressure of air-pocket at different initial positions shows a downward trend from the middle of the tunnel to two sides, while the pressure of the air-pocket near a higher shaft will be slightly higher. The proposed model can be extended to systems with multiple shafts or air-pockets, and the conclusions have reference significance for structure design and water filling control in the DSST.

Structural behavior of segmental tunnel linings for a large stormwater storage tunnel: Insight from full-scale loading tests

The deeply-buried tunnels serve as an effective method for the sustainable management of stormwater to mitigate the waterlogging and sewer overflow problems of modern cities in rainy seasons. This paper presents a series of full-scale structural loading tests on the segmental lining of a large deeply-buried stormwater storage tunnel considering the operation conditions, in which the cycle of water inflow and drainage was repeated. The testing results show that water inflow has notable influences on the mechanical and structural behaviors of segmental lining. The bending moment increases slightly but the axial force decreases significantly with the increasing inner hydrostatic water head. The convergence deformation develops during water inflow and recovers in the process of drainage, following a parabola pattern. However, the deformation cannot be fully recovered after complete drainage, mostly due to the residual deformation of segmental joints. The structural responses of the segmental lining remain approximately constant subjected to a limited number of loading cycles. The effects of water inflow on the distribution of internal forces and deformation deviate from the elastic solutions for homogeneous ring subjected to isotropic inner pressure. The influences of bending moment and axial force on joint behavior are unified through eccentricity. Three distinct zones associated with the interaction condition between adjacent blocks at the joint are identified for the relationship between joint deformation and eccentricity, and also for the relationship between joint rotational stiffness and eccentricity. The observations made in the current study may shed light on the design and construction of similar projects in pilot cities committed for the Sponge City policy of China.

Anticipating Transient Problems during the Rapid Filling of Deep Stormwater Storage Tunnel Systems

AbstractDeep 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 expan...

TBM performance through the engineering geology of the Lee Tunnel

The Lee Tunnel is a 6·9 km long, 7·2 m dia. storm water storage tunnel, constructed in east London, at depths of up to 80 m below ground level. Ground investigation identified the three stratigraphic members of the Seaford Chalk Formation along the tunnel horizon, but with minimal variation of strength parameters between these during laboratory tests. A preconstruction geological model for the tunnel depicted a significant flint marker: the ‘Bedwell's columnar flint’, together with geological structures such as the Greenwich fault zone and Plaistow graben that indicated deterioration of ground conditions during construction. The model strongly influenced the choice of slurry tunnel-boring machine for excavation works and was also used to schedule tunnel face inspections. Monitoring of the tunnel-boring machine performance data during construction demonstrated its adaptability to the changes in ground mass quality brought about by the geological structures, as well as a pingo feature that had not been indicated on the original model.

Flow regime transition simulation incorporating entrapped air pocket effects

Flow regime transition is observed in closed conduit when there is a transition between free surface and pressurized flows. This condition is often observed in stormwater storage tunnels undergoing rapid filling. Such tunnels provide stormwater control inurbanized areas, relieving the stormwater collection system during intense storms. Operational issues have been reported in such tunnels during intense rain events, some of which linked to air pocket entrapment. These issues motivated development of flowregime transition models and more recent models attempt to improve predictions by accounting for air effects in formulations. However, no comparative study has been performed between recent models that account for air pockets and earlier models that don't include this feature. This work aims to address this gap, focusing on an actual tunnel geometry that was proposed for the city of Washington, DC. Results indicate that including air in model formulation yielded more realistic predictions of low pressures.

Kinematics of Entrapped Air Pockets Within Stormwater Storage Tunnels

During the rapid filling of stormwater storage tunnels, which is anticipated to occur during intense rain events, a number of different mechanisms may lead to …

Pressure Surges Following Sudden Air Pocket Entrapment in Storm-Water Tunnels

AbstractDeep storm-water storage tunnels may undergo pressurization during intense rain events. In the process, air pockets may become entrapped and pressurized, causing significant flow changes. Currently, the role of nearby surge relief structures is uncertain with respect to air-water interactions. This paper presents results from experimental and numerical investigations on air pocket entrapment and compression following reflection of inflow fronts on system boundaries. Steady flows were established in the pipe apparatus, with the upstream portion flowing in the pressurized regime while the downstream flowed in free surface conditions. Sudden flow obstruction caused by valve maneuvering at the downstream end generated unsteady conditions that were monitored by transducers and a MicroADV probe. Partial valve maneuver runs were performed, aiming to represent cases in which surge relief is provided during air pocket compression/expansion cycles. Whereas experiments performed without surge relief (complet...

Pressure Surges Following Sudden Air Pocket Entrapment in Stormwater Systems

Stormwater storage tunnels may undergo rapid pipe filling conditions during extreme rain events. Such conditions are relevant as adverse conditions may develop…

Geysering Generated by Large Air Pockets Released through Water-Filled Ventilation Shafts

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.

Investigation of rapid filling of poorly ventilated stormwater storage tunnels

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, 94mm 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.

Flow Regime Transition and Air Entrapment in Combined Sewer Storage Tunnels

The rapid filling of a stormwater storage tunnel is accompanied by a flow regime transition from free surface to pressurized flow with the presence of a hydrau…

Flow regime transition mechanisms in rapidly filling stormwater storage tunnels

An issue in the design of combined sewer overflow storage tunnels is to avoid “geysering” which is an air/water mixture blowing up through vertical shafts connected to the tunnel. Studies indicate that the origin of this phenomenon is the entrapment of large air pockets as the rapidly filling tunnel undergoes a transition between free surface and pressurized flow. Commonly implemented numerical models are of the shock fitting type that tracks the location of a pipe filling bore. However, the flow regime transition does not have to occur through a pipe filling bore. Another possibility involves a free surface bore with a following gradual transition to a full pipe condition. Large air volumes may be trapped in this situation following the bore reflection off a tunnel transition if this reflection closes the flow cross-section. Experimental observations are presented to demonstrate both types of flow regime transition. Traditional shock fitting methods are ill-equipped to accurately simulate gradual flow regime transitions. The shock capturing method proposed by Vasconcelos et al. (J Hydraul Eng 132(6):553–562, 2006) is demonstrated to be capable of resolving both types of observed bores.

Experimental Investigation of Surges in a Stormwater Storage Tunnel

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 14.6-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 filled 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.

Surges Associated with Filling of Stormwater Storage Tunnels

Surges may form during rapid filling incidents in stormwater storage tunnels as the tunnel undergoes a transition between a free surface flow and a surcharged …