Combined Sewer Overflow (CSO) events occur when heavy rainfall or snowmelt overwhelms combined sewer systems, causing untreated sewage to discharge into nearby water bodies.
The Clean Water Act of U.S. Environmental Protection Agency (EPA) requires municipalities to develop long-term control plans to reduce or eliminate CSO discharges.
The Allegheny County Sanitary Authority (ALCOSAN) maintains approximately 90 miles of interceptor sewers over a service area of 310 square miles conveying wastewater from 83 municipalities (including the city of Pittsburgh) to ALCOSAN’s 59-acre treatment plant. Each year, approximately 9 billion gallons of wastewater overflow into the region’s waterways leaving ALCOSAN non-compliant with the federal Act.
To address this, ALCOSAN initiated their Clean Water Plan to streamline conveyance of wastewater to the treatment plant. The Ohio River Tunnel (ORT) Project, the first of three CSO tunnel segments, features a 4.9-mile-long CSO storage and conveyance tunnel with new regulating structures diverting excess flow from existing sewers to deep tunnels via drop shafts. This project will provide a long-term solution for managing wet weather flows and reducing pollution in the Ohio River.
Design considerations were established through hydrological and hydraulic modeling using PCSWMM. Integrating the new structures with the existing sewer system presented several engineering challenges, particularly for site: ORT-A58-RG2. Limited space required a side connection to an existing box culvert, with additional challenges from the culvert’s steep slope and the river pool’s influence.
Design concepts were initially developed using one-dimensional hydraulic calculation methods. However, initial Computational Fluid Dynamics (CFD) modeling revealed that the required flow capture was not achieved across all flow scenarios. Through subsequent design iterations, a CFD-tested solution was identified. The CFD modeling outputs were promptly reviewed in a multi-disciplinary forum, enabling rapid design enhancements. This iterative process continued until the arrangement met site-specific hydraulic performance objectives while ensuring buildability.