Hydraulic Modeling of Deep Tunnel Provides Cost Savings

The City of Columbus, Ohio has approximately 5,000 acres of combined sewer area. There are 32 regulated combined sewer overflows (CSOs) that discharge into the Olentangy and the Scioto Rivers. The estimated annual CSO volume is 1,320 MG for a typical year. In 2003, the City of Columbus entered into a Consent Order with the Ohio EPA to develop a CSO Long Term Control Plan (LTCP) to mitigate these CSOs. The City submitted a plan in 2005 to manage wet weather flow which includes a list of proposed improvements to eliminate/mitigate CSOs. Critical components of the LTCP are the proposed relief to the Olentangy Scioto Interceptor Sewer (OSIS) by means of the OSIS Augmentation Relief Sewer (OARS), the construction of CSO High Rate Treatment, and a 10 MG storage facility to meet the desired level-of-control of zero overflow in a typical year. The total estimated cost of these three components in 2005 dollars was $396 million.

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 a deep tunnel eliminated the need for the proposed 10-MG storage tank to shave peak flow.

A hydraulic computer model was developed during the design phases of the OARS to optimize its size while examining the performance of the collection system to assure the anticipated level-of-control is attained. The computer model was employed to optimize the OARS tunnel which is reflected in cost savings as follows:

1. Assure the OARS tunnel would eliminate the need of the proposed 10-MG storage tank which represents a cost savings $72 million.
2. Size the OARS to eliminate the need for High Rate Treatment which represents a cost savings of $103 million.
3. Identify the optimum locations and elevations to intercept flow from the OSIS to minimize the required number of tunnel shafts. These structures are designed to relieve the OSIS during wet weather by diverting flow over a fixed crest weir. When flow levels exceed the weir elevation, flow will be directed into the OARS.
4. Eliminate the need of collector sewers from each CSO due to lowered hydraulic grade line in the OSIS.
5. Propose an operation strategy to reduce the required number of shafts from four (4) to three (3) due to numerous constructability risks.

After the construction of OARS (2014), the model estimates that the total CSO volume will be 50 MG, which is a reduction of 96% from 2005 using the typical year rainfall for Central Ohio. 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.