Innovative Design And Operational Features For A CSO Tunnel: Spiral Drop, Hydraulic Transient Control, And Operating Modes

The City of Richmond, Virginia is located at the head of tide on the James River and has a wastewater collection system comprised of separate and combined sewers. The area of the City served by combined sewers comprises about 12,000 acres, or about 32 percent of the City. During dry weather, sanitary wastes collected in the combined sewer system (CSS) are conveyed to the wastewater treatment plant (WWTP). During periods of rainfall, the capacity of a combined sewer is often exceeded and the excess flow is discharged directly to the James River or tributary waters. Combined Sewer Overflow (CSO) 4 (Hampton Street) and CSO 5 (McCloy Street) are two (out of 32 listed) combined sewer outfalls located on the northside of the James River.Over thirty alternatives, using a variety of control technologies, were developed to address CSOs in the City. For CSOs No. 4 and 5, the City evaluated twenty-four of these alternatives and narrowed the options to conveyance pipelines to the WWTP or a retention storage structure. In order to minimize the disruption to the riverfront environment, a deep tunnel retention structure was selected.Many hydraulic challenges were encountered during the design. The existing combined sewers were laid with steep slopes that generated supercritical flow with flow rates in excess of 1000 cfs during major rain events. Conveying the surface flow to the bottom of the tunnel (over a 70 foot drop at the McCloy Street end) also presented a problem that required a vortex drop to dissipate the energy and manage the airflow. Keeping the tunnel clean of sedimentation was another issue that was resolved with a self-cleaning flushing structure managed by a rather simple control sequence. Being an essentially closed system, careful consideration was also given to the negative effects of hydraulic transients. All of these challenges were considered during the design phase and resolved with easily maintainable and simple to operate solutions. The final result is a 5,900 linear foot retention tunnel that resolves a major CSO environmental concern and improves the water quality of the James River.