From Milwaukee to DC: A Review of DC Water's Innovative CSO Tunnel Drop Shafts

Over the course of the next 15 years, the District of Columbia Water and Sewer Authority (DC Water) will be designing and constructing its DC Clean Rivers (DCCR) Project to reduce Combined Sewer Overflows (CSOs) to the District's receiving waters. Once completed, the $2.6 billion system will reduce CSO volume in an average year of rainfall by 96% overall, and by 98% in the Anacostia River alone. The Project consists of a series of large underground storage and conveyance tunnels, drop shafts, diversion sewers, diversion chambers, overflow structures, and a tunnel dewatering pump station located at DC Water's Blue Plains Advanced Wastewater Treatment Plant. The backbone of the Project is a 7 m (23 ft) diameter tunnel system that spans the length of Washington, DC, from the southwest to northeast quadrants of the city. The tunnels, constructed in soft-ground, will total over 21 km (13 mi) in length and be located approximately 30 m (100 ft) underground. In order to deliver the 19 billion L (5 billion gal) of captured CSO's to the tunnel system, DC Water has designed a series of innovative in-line drop shafts, ranging from 15 (50) to 23 m (75 ft) in diameter. The DC Water In-Line Drop Shaft design applies the hydraulic principles of Milwaukee-style deaeration (initially developed by S. C. Jain and J. F. Kennedy in the early 1980's as part of Milwaukee, Wisconsin's Water Pollution Abatement Program) and wrapped them around a circular shaft directly over the tunnel. The major goals of the design were to develop an in-line drop shaft that would deaerate flow to prevent air from being transferred into the tunnel, allocate space for surge dissipation and storage, and provide relief for trapped air to prevent geysers. In order to accomplish the design goals, a 1:15 scale physical model with a maximum flow rate of 48,180 L/sec (1,100 mgd), a drop height of 24 m (80 ft) and various tailwater increments was constructed. The prototype was studied under various flow conditions until acceptable hydraulic conditions within the shaft were met. Key parameters that were measured during the testing process included; flow rate into the drop structure and tunnel, air flow rate into the tunnel, air flow rate out of the deaeration channel, and the amount of air entrained in the tunnel due to water dropping into the tunnel. In addition to the physical model, the geometry of the drop shafts were also studied in the Surge and Hydraulic Analysis For Tunnels (SHAFT) model, which was used to evaluate the tunnel system's response to extreme events using multiple storms and initial fill levels. Once acceptable hydraulic conditions were met under both models, a design nomograph was developed to scale critical drop shaft dimensions based on the design diversion rate. This paper presents a detailed discussion of the design of DC Water's In-Line Drop Shaft, a critical element that will be instrumental in reducing overflows to the District's waterways.