Surge Analysis for the Proposed OSIS Augmentation Relief Sewer Tunnel
The City of Columbus, Ohio, submitted a wet weather management plan in July 2005 (City of Columbus, 2005) in response to two consent orders from the State of Ohio. The second consent order, The Combined Sewer Overflow (CSO) Consent Order, mandated the development of a long term control plan (LTCP) in compliance with the requirements of the U. S. Environmental Protection Agency CSO Control Policy.
A critical component of the LTCP is the proposed Olentangy–Scioto Interceptor Sewer (OSIS) Augmentation Relief Sewer (OARS). The OARS will relieve the Olentangy Scioto Interceptor Sewer (OSIS) which is the principal trunk sewer that transports combined flow from the downtown Columbus sewersheds to the downstream wastewater treatment plants (WWTPs). The intent of OARS is to intercept and convey a significant amount of the wet weather flow from the OSIS in a separate conduit to the WWTPs for biological treatment
which would eliminate or reduce overflows to the Scioto River from the downtown regulators in the OSIS system. Most significantly, the overflows from the Whittier Street storm standby tank (WSST) located on the OSIS system south of downtown Columbus, which cause 85% of all annual overflows, will be eliminated at the Whittier Street site for the typical-year (a standardized precipitation year developed in the LTCP for continuous model simulation). Also, there will be no CSO discharges anywhere in the system for the typical-year except at a new CSO farther south near the Jackson Pike WWTP, where four overflows during the typical-year are permitted. This new CSO will have a high rate treatment (HRT) plant associated with it, to provide treatment and disinfection. In addition to the above typical-year requirements required by the LTCP, the City has requested that OARS be designed to capture or convey the 10-yr flow event (generated using recurrence-interval analysis of single events, Gheith et al., 2007) so that there are no overflows in the downtown regulator even for the 10-y flow event.
The most significant component of the OARS project is a 23 000 ft (7 010 m) tunnel approximately 150 ft (45.8 m) deep that is to carry the combined flow through downtown Columbus to the downstream project components. Flows from the OSIS are to be diverted to the OARS tunnel at three locations through a system of drop shafts, relief structures, and appurtenant structures such as tangential inlets and approach channels. The initial design of the OARS project, including the tunnel elements, was accomplished by means of a city-wide SWMM 4.4H model which had been developed over many years to evaluate the impacts of various LTCP projects. However, there was concern that the SWMM 4.4H model could not adequately simulate the surges that could occur in the OARS tunnel as a result of rapid filling of the tunnel. Significant surges could lead to elevated hydraulic grade lines (HGLs) at the connecting relief structures which in turn could cause unanticipated CSOs at the adjacent downtown regulators. In addition, there was need to evaluate the potential for air entrapment in the tunnel and to devise methods for releasing the air safely in order to prevent geysering. Recognizing the need to conduct a surge-ventilation analysis for the OARS tunnel, the project design team selected the Transient Analysis Program (TAP) for this purpose. TAP is a computer program that calculates steady-state, gradually-varied, and rapidly-varied flow conditions in a system of open channels and closed conduits. TAP has been previously used to analyze CSO tunnel systems (Ridgway, 2008). This chapter details how TAP was applied to the OARS project, the interaction with the SWMM 4.4H model, and comparison of TAP results with those of SWMM 4.4H.
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