Surge and air pockets release analysis of LOT and OARS tunnels in Columbus, Ohio

The Lower Olentangy Tunnel (LOT) is under design by the City of Columbus to provide adequate conveyance capacity and inline storage to significantly reduce combined sewer overflows from existing regulators located along the Olentangy River. LOT is approximately 3.3 miles in length and 12 feet inside diameter. Hydrologic and hydraulics analysis for LOT was conducted to ensure that all operational goals and Consent Order requirements will be achieved. LOT flows will be conveyed to the upstream end of the OSIS Augmentation and Relief Sewer (OARS) Tunnel located near Vine Street. Analysis consists of three main components; SWMM modeling to predict peak flow and volume in the two tunnels, HAST3 unsteady flow modeling to investigate potential surge conditions, and CFD modeling to evaluate the possibility of uncontrolled release of air pockets in the connecting Vine St shaft.

Through hydraulic analysis, it was determined that the upstream inflows to LOT is consistently much lower than OARS flows. Therefore, the design included adding a flap gate to prevent OARS back-ups into LOT and in order to reduce potential surge conditions in the shafts upstream the Vine shaft. Two extreme storm events, July 13, 2017 and August 11, 2018, with return period of 5-yr and 10-25yr respectively, were studied. In addition, 20 large events from 1995 to 2014 are selected for comprehensive system evaluation. Simulation results revealed the benefits of having large shaft plan areas and adit connections in OARS design. Simulations confirmed that the surge chambers at OARS shafts 4 and 6 will be used more frequently after LOT flows is added to OARS. The mechanism of air pocket formation and migration within the tunnels and the release of air pockets in the vertical structures were investigated. The Vine shaft CFD simulations revealed that air phase pressure that can develop at Vine shaft during a potential release of large air pockets could be large enough to pressure the Vine shaft covers. The finding is helping the effort to design the manhole covers and hatches at Vine Shaft to ensure they would not move during the large pocket release.

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