A Tale of Two Capitals: Modeling Helps Designers Manage Strong Surge and Pneumatic Forces in Deep Combined Sewer Storage Tunnels

摘要

Rapid advancements in tunneling technology and the need to avoid surface disruptions have made deep tunnels popular options for dense urban areas to store and transport large volumes of untreated combined storm and waste water. The District of Columbia and Thames Water Utilities (London) have selected large branching tunnels to capture combined sewage for treatment. Rapid filling is a given for these types of systems and presents many design challenges due to the massive forces involved. Wastewater geysers, blown manholes- even structural failures can result if not adequately considered. Advanced computer modeling used to help designers avoid these problems in Washington and London is showcased. The District of Columbia Water and Sewer Authority (DCWASA) and Thames Water Utilities (TW) are each designing large tunnel systems to meet pollution control requirements and reduce flooding potential. Plans for both national capitals are to rapidly design, construct and put the tunnel systems in operation. An innova?tive computer model, based on research by Vasconcelos and Wright, has been simulating rapid filling, transient surges and pneumatics as an integral part of the evaluation of the tunnel designs, particularly in managing or controlling adverse conditions. The tunnel model, called SHAFT, simulates both open-channel and pipe-filling bores, the flows and forces at work and predicts locations and volumes of air entrapment and displacement. The model uses a novel shock capturing technique to simulate both flow regimes and the transitions from open channel to closed conduit flow. The model further predicts where air will be trapped in otherwise unexpected portions of the interconnected tunnels due to the piston-like action of hydraulic surges, the timing of different surges and wave reflections. Model results for a matrix of hydrographs, initial fill levels, control alternatives and runnel profiles have been used to adjust the location and sizing of drop shafts and vents to avoid dramatic sewage geysers and loud air releases in both capitals. Some general lessons learned are presented and discussed that will be informative to tunnel designers elsewhere.