Surge analysis of OARS tunnel in Columbus, Ohio: Unsteady flow simulation and surge evaluation in rapid filling tunnel

A deep storage tunnel has been constructed by City of Columbus as an alternative to mitigate combined sewer overflows. However, deep tunnels in the collection system can experience surging or geysering when underwent rapid filling during intense rain event.

The life cycle of surge starts with air that is initially presented in collection system, which can be subsequently displaced or transported to ventilation points when storm water enters the system. However, different mechanisms can entrap air pockets in collection system and prevent air from fully ventilate, including 1) consecutive pressurization interfaces, and 2) surface waves within pipes. These air pockets then interact with water phase by 1) undergoing compression and expansion cycle; 2) spreading within tunnel reaches and be dragged by the flow; 3) be released in an uncontrolled fashion through vertical shafts. Uncontrolled air release through vertical structures can be very harmful and result in property and structural damage, including manhole lid displacement and drop shaft damage. Therefore, careful evaluation of the air water interaction in deep tunnels are essential for collection system operation and public safety.

In this study, we utilized an unsteady flow simulator, HAST, to investigate and evaluate potential surge conditions in the Olentangy Scioto Interceptor Sewer (OSIS) and the OSIS Augmentation Relief Sewer (OARS) in Columbus, Ohio. The model is capable of representing surges, bores, pressurization interfaces and other unsteady, single and two-phase flow conditions in large and below grade stormwater tunnels. Two recent extreme storm events, July 13 in 2017 and August 11 in 2018, which has a return period of 5-yr and 10-25yr respectively, are studied in detail. Through accurate representation of the collection system and advanced model physics, results of both events showed good agreement with field observations. 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 horizontal connections in OARS design. On the other hand, OSIS, which is shallower and smaller in size, tend to entrap air pockets. For potential surge events, we also explored the mechanism of air pocket formation and migration within the collection system. Finally, release of air pockets is evaluated in conjunction with characteristics of the vertical structures. This study will also discuss the advantages and limitations of unsteady flow simulation in collection system surge analysis.