Rochester Combined Surge And Dropshaft Model Studies

Abstract

The City of Rochester, New York, is developing the combined sewer overflow and abatement plan (CSOAP) to handle sanitary sewage and storm water. The west side system contains 40 dropshafts along a 26 mile long tunnel. The function of these dropshafts is to transport the water from one elevation and energy level to a lower elevation and energy level. At several locations along the tunnel it is proposed to construct surge shafts to attenuate surge pressures in the system. To minimize construction disturbances at the surface, reduce the amount of boring and excavation, and consequently reduce the construction costs, the surge shafts have been combined with conventional dropshafts. These combined surge and dropshaft structures would have dual functions of conveying water from the ground surface to the underground tunnels and relieving surge pressures in the system. Conduits at the ground surface collect the water and convey it to a quarter cylinder elbow which deflects the water 90 degrees into a verticalrectangular shaft. The vertical shaft is located inside and at the upstream side of the surge shaft. One wall of the vertical shaft is slotted to provide for air re-entrainment and also pressure relief in the dropshaft. The surge shaft is a large excavated and lined cylinder which extends from the tunnel level to the ground surface. A boot is attached to the surge shaft just below the dropshaft. Attached to the downstream side of the surge shaft is a deaeration chamber containing a slotted weir, false crown with air slots, and a bellmouth entrance to the exit conduit. An air vent is provided along the downstream side of the surge shaft. The water falling through the elbow and vertical shaft entrains considerable air while gaining kinetic energy. The falling water-air mixture impinges on the floor of the surge shaft. The boot, surge shaft, and deaeration chamber dissipate some of the energy, remove and collect the entrained air, and direct the water at a reduced velocity into the exit conduit. Some of the entrained air is released in the surge shaft and rises directly to the surface. The remaining air rises to the false crown in the deaeration chamber, passes through the air slots into the chamber above, and returns to the upper part of the surge tank through a vent shaft. Part of this return air is then recirculated via the dropshaft air slots and air ramps. A definition sketch (Chart 1) indicates the various shaft components.