DC Water’s Clean Rivers Project is planning a 4.2-million-gallon underground storage tunnel to capture sewage combined with stormwater, known as combined sewer overflow (CSO), that would otherwise overflow into Piney Branch and ultimately Rock Creek during heavy rains. This project will reduce overflow volume from CSO 049 by 96 percent in an average rainfall year when completed.

The Piney Branch Tunnel Project was first approved as part of DC Water’s Long Term Control Plan federal consent decree in 2005 and revised in 2020.  It will be constructed in the northwest part of Washington, DC, south of the existing CSO 049 and Piney Branch Parkway. The project’s pupose is to intercept the overflow before it reaches Piney Branch Stream, temporarily store the captured flow until the rain event subsides and then and send it to Blue Plains WWTP for treatment, as required by the revised consent decree. This will reduce the overflow frequency from 25 to one per average rainfall year and reduce the overflow volume from 39.7 million gallons to 1.4 million gallons – a reduction of 96 percent.  

The project includes construction of a diversion structure at the Piney Branch outfall, known as CSO 049, to redirect sewer flows to the storage tunnel and eventually convey the combined sewage to DC Water’s Blue Plains Advanced Wastewater Treatment Plant for treatment when the pipes and system can handle the volume. This project is needed to reduce CSOs to improve water quality in Piney Branch, Rock Creek, and ultimately the Potomac River and the Chesapeake Bay; and to comply with the terms of the Federal Consent Decree.