NBC Sewer Separation / Flow Modification

A vital component of the Narragansett Bay Commission’s Phase II CSO program, the objective of this project was to provide flow reduction and capacity modifications to reduce combined sewer overflows as mandated by the RIDEM Consent Order.

BETA completed a hydrologic and hydraulic analysis for the combined sewer shed utilizing SWMM and evaluated various alternatives including I/I reduction, partial separation, flow slipping, flow restriction, and complete separation. The model was an integral part of the alternative evaluation to determine the optimal solution.

Based on the results of the analysis, BETA recommended complete separation of the combined sanitary system within the 250-acre tributary area. Recommendations were also made to install additional regulators within the remaining combined storm sewer system to maximize available capacity within the system.

The project included:

• Complete separation of the combined sanitary system, requiring the design/construction of 48,850 feet of new 12-inch to 60-inch storm sewer system
• New drainage outfall to the Moshassuck River
• Installation of two new regulators within the remaining combined sewer system to maximize available storage capacity within the upstream system and decrease the peak flow at the downstream CSO regulator
• Modifications to four (4) existing regulators
• Rehabilitation of 5,300 feet of sewer via cured-in-place pipe (CIPP) liner.
• Elimination of 10 “common manholes” servicing both sewer and drain
• Relocation of 29,000 feet of 6-inch to 12-inch DICL water main
• Relocation of  8,000 feet of gas main
• Relocation of electric, telephone, cable and fiberoptic
• New infrastructure and roadway resurfacing impacting a total of 42 streets, including a major arterial (North Main Street, Route 1)

The restoration work included curb setting, installation of sidewalks, ADA-compliant wheelchair ramps, and curb-to-curb roadway resurfacing/pavement rehabilitation. Roadway restoration included 2,500 linear feet of cement concrete base.

Extensive coordination was required with local and state agencies including the City of Providence, RIDEM, RIDOT, various utility companies, and Miriam Hospital. The final design resulted in four construction contracts for sewer separation.