A River Runs Through It: Application of an Integrated Combined Sewer and Stream Channel Model to Support a Sustainable CSO Control Plan

Imagine if you will an interceptor within a combined sewer system (CSS) that runs underneath a channel. Now imagine the channel is concrete; constructed by the U.S. Corps of Engineers to mitigate flood damage in the 1930s. Finally, imagine the channel and interceptor connected through a series of grated inlets on the interceptor and combined sewer overflow (CSO) discharges into the channel. What you have imagined is a reality for the Metropolitan Sewer District of Greater Cincinnati (MSDGC). The West Fork Branch Interceptor conveys combined sewage from an approximately 25 km2 (10 mi2) watershed. The West Fork Branch (WFB) stream conveys stormwater from this watershed along with overflows from with 15 combined sewer overflows (CSOs). 10,000 LF of the WFB channel is rectangular concrete channel constructed by the Corps of Engineers in the 1930s to manage flooding through the 100-year event. Grate structures installed in the bottom of the concrete channel allow stream flow and combined sewage to pass between the channel and the interceptor.To support the implementation of sustainable CSO controls that have the overall objective of using source control techniques for reducing or removing stormwater inflow into the combined sewer system, MSDGC expanded the existing WFB model to integrate the concrete channel portion of the WFB with the interceptor. Using additional in-system flow monitoring data along with detailed channel cross-section geometry and stream rating curves at key locations in the WFB channel, the model was calibrated to predict both flows and grade lines within the interceptor and the complex interaction of the channel and the interceptor. Because MSDGC was well along in the implementation of many green infrastructure and other sustainable projects to address many of the 15 CSOs within the WFB watershed, the calibrated model was updated to reflect the implementation of these projects.Using this fully calibrated and integrated tool, MSDGC conducted a rigorous evaluation to understand the complex interaction of the channel and the interceptor, evaluate the impact of the proposed sustainable projects on the WFB channel and interceptor hydraulics, and ultimately help size a new interceptor to convey flows without any connection to the WFB channel. In addition, the integrated model was used to help size and evaluate returning the concrete portion of the WFB channel to a natural stream channel. The model helped assess the potential impacts on downstream flooding, appropriately size the stream channel, and determine the appropriate interceptor size to provide the targeted CSO reductions required in MSDGC's consent decree. The resulting plan included:• 6,000 linear feet of 48-inch interceptor, and • Combined total of over 2 million gallons of CSO storage.• Restoration 6,000 linear feet of the WFB stream channel,• Additional stormwater detention adjacent to the West Fork,• “Express” storm sewers at three CSOs, and• Storm sewer separation, stormwater detention, and stream restoration at CSO 130This project provided MSDGC with a balanced plan for CSO control that also provided significant environmental benefits by returning the channel to a more natural state and provided a significant public amenity for the adjacent communities. The integrated approach of upstream sustainable projects, cost-effective gray solutions, and significant stream restoration supported MSDGC's “triple-bottom line” approach to implementing its wet weather improvement plan. This paper will provide other communities with a unique approach for evaluating complex stream and sewer interactions and lessons learned for effectively implementing green technologies to meet CSO control objectives.