CSO 410 Stream Separation Design

The Metropolitan Sewer District (MSD) of Greater Cincinnati estimated that combined sewer overflow #410 activates approximately 101 times during a typical hydrologic year, resulting in a total annual combined sewer overflow volume of 27 mg into the Ohio River west of downtown.

Gresham Smith was selected to assess five alternatives deploying combinations of “green” and/or “grey” technologies/BMPs for the separation of existing combined sewer flow. Recommendations were ranked using a Business Case Evaluation (Triple Bottom Line) process, and achieved combined overflow reduction by:

• Relocating CSO #410 to a new location north of River Road
• Reconnecting existing separate sanitary sewers and two combined sewers in River Road to the new CSO structure
• Jack and boring a new 10-inch sanitary sewer 200 feet under River Road and two railroad tracks to a new connection with the 15-inch East Branch Muddy Fork Interceptor Sewer
• Connecting the 10-inch interceptor as high above the interceptor invert as possible to maximize flow capacity and reduce surcharge in the interceptor
• Reusing existing 33-inch brick, 72-inch stone, and various other pipe  conduits  to  convey an intermittent stream flow on the north side of River Road
• Reusing the CSO 410 conduit and outlet structure to convey primarily stormwater and a greatly reduced combined sewer overflow to the Ohio River
• Maintaining at least two open lanes of traffic on River Road open at all times
• Coordinating with the City of Cincinnati, railroads, and obtaining private property easements

The proposed design solution is estimated to eliminate 22.3 mg (82.5%) of combined sewer overflow per year. Design services involved preparing construction drawings are to include:

• Elimination of existing engineered/incidental stormwater inlets into the combined sewer
• Dispersion and dissipation of stormwater to facilitate its return to the hydrologic cycle
• Collection and conveyance of stormwater to a natural or engineered  feature designed to retain and manage volume or detain peak flows prior to release to a nearby waterway or the combined sewer system
•  Restoration of existing and/or previously existing natural drainage conditions or corridors with consideration for increased loading resulting from separated stormwater