CSO 415 Stream Separation Planning

Combined Sewer Overflow (CSO) 415 is located along River Road west of downtown Cincinnati. A study was necessary to evaluate if wet weather overflows at CSO 415 into the Ohio River could be reduced, and whether CSO 415 activation resulted in greater overflow events and volume at CSO 416, east and upstream.

Gresham Smith engineers were selected to assess three alternatives using the SWMM5 Model deploying combinations of “green” and/or “grey” technologies for the separation of existing combined sewer flow, including:

• Elimination of existing stormwater runoff entering into sanitary sewers (e.g. downspout removal, use of localized natural storage, green roofs, rain gardens, etc.)
• Dispersion and dissipation of stormwater to facilitate its return to the hydrologic cycle
• Restoration of existing and/or previously existing natural drainage condition or stream corridors with consideration for increased flow resulting from separated stormwater

The planning analyses also addressed a previous Metropolitan Sewer District of Greater Cincinnati (MSDGC) recommendation to implement Alternate 2 from a previous CSO 416 overflow reduction study. The Business Case Evaluation process recommended Alternative 1 (grey/green solution) to relocate CSO 415 and separate combined flows. Key aspects included converting an existing 72-inch combined sewer into a separate sanitary sewer and tunneling/directional drilling a new 60-inch storm sewer under US 50 and several railroad tracks. Storm discharge will be into the Ohio River, requiring application of a nationwide permit for work below the normal river pool stage. Some limited green infrastructure improvements were considered in the design.

The project eliminates sending stormwater to the Muddy Creek Wastewater Treatment Plant and greatly reduces CSO overflows, with estimated annual overflow volume reduction estimated as 89% at CSO 415 (49 mg to 5.4 mg) and 21% at CSO 416 (54.2 mg to 42.9 mg). The combined affects of implementing both CSO 415 and 416 alternatives results in a predicted total combined overflow reduction of 87.3 mg, or 85% of the existing total wet weather overflow volume.