How big should my tunnel be?

The City of Columbus has approximately 5,000 acres of combined sewer runoff. There are 32 regulated CSO relief locations, which include two storage facilities. The Whittier Street Storm Standby Tanks (WSST) facility accounts for the majority of the CSOs. The estimated annual CSO volume is 1,320 MG for a typical year. The WSST facility alone is responsible for a CSO volume of 1,130 MG (86%).

In 2003, the City of Columbus, Ohio entered into a Consent Order with the Ohio EPA that called for a substantial reduction in CSOs by the year 2010. The City undertook a Wet Weather Management Plan (WWMP) that included a CSO Long Term Control Plan (LTCP) in which fourteen different alternatives were developed and analyzed. A critical component of the LTCP is the proposed relief of the Olentangy Scioto Interceptor Sewer (OSIS) into the Olentangy Augmentation Relief Sewer (OARS).

The OARS project will consolidate the majority of CSOs by intercepting and conveying them to the City’s two wastewater treatment plants for biological treatment and future high rate treatment (HRT). The proposed OARS will be sized to provide adequate conveyance and in-line storage for a “typical year” continuous flow simulation. To eliminate activation of the WSST, it was initially proposed in the LTCP to construct OARS as 10-ft x 16-ft conduit for 17,000 feet, plus a 10-ft x 13-ft conduit for 7,800 feet. In addition, it was proposed to install an additional 10 MG storage tank to offset overflows during peak discharges. During the course of the OARS project, the OARS conduit initially changed into two 11-ft or two 12-ft near surface diameter pipes and then finally into an 18-ft deep tunnel. The benefit of OARS was examined based on the performance of the collection system at key future dates (2010, 2017, 2025, and 2047) that correspond to completion of major improvements.

A Value Engineering team evaluated three primary construction methods for the OARS – near surface conduit, deep tunnel, and shallow tunnel. An alternatives evaluation matrix was developed with input provided by the City of Columbus regarding capital costs, right of way acquisition, constructability, ease of operation, etc. The results of the matrix favored the deep tunnel option. In addition, the use of an 18-ft deep tunnel eliminated the need for an additional 10-MG storage tank, initially proposed.

The model results for the proposed 18-ft deep tunnel show a CSO volume of 330 MG (a 75% reduction) from all CSOs by year 2010 running the typical year rainfall. After full implementation of the CSO LTCP by 2025, the estimated annual total CSO volume will be reduced to 32 MG (97% reduction).

The modeling revealed significant flexibility to implement different operation strategies to appropriately control flow in the collection system by adjusting gate settings, operational rates of pump stations and different tunnel sizes. It also identified constraints of the WWTP capacities and the limitations of the existing collection system.