Hydraulic Energy Flushing Of Inverted Siphons

The City of Richmond, Virginia is located at the head of tide on the James River. The older central part of the City is served by a combined sewer system (CSS) that comprises about 12,000 acres. The largest basin in the system is the Shockoe Creek Basin area that is comprised of about 8,000 acres, about 2/3 of the overall system. The City began addressing Combined Sewer Overflow (CSO) control in the late 1970s by constructing a 38 million gallon “first flush” retention system on the Shockoe Creek Basin in the early 1980s. The Shockoe Creek Basin discharges into a diversion structure at the retention system through an arch sewer. A crest dam in the Outfall Structure, down stream of the Diversion Structure, prevents dry weather flow from overflowing into the river. Flows into this structure are transferred to the Wastewater Treatment Plant (WWTP), across the James River using two 66-inch inverted siphon pipelines. The siphons have a firm capacity of 200 mgd. Flows feeding the river crossing vary from a dry weather flow of approximately 30 mgd to a wet weather flow of about 60 mgd. These wet weather CSO flows are very turbulent and sediment laden. Wet weather CSO events after the winter driving season contribute large quantities of sand and grit. Over time, deposition of solids in the twin river crossing can reduce the flow capacity of the inverted siphons pipelines, reducing transfer capacity to the wastewater treatment plant. This plugging causes operations personnel to modify standard operating procedures and maintenance personnel to perform extra duties.This paper will describe a mechanical flushing system located in the confined space of the Outfall Structure. The mechanical flushing system was designed to store and release a large volume of wastewater to flush the twin river crossing with a high volume, high velocity stream while avoiding accidental releases of dry weather overflow to the river. The design also allows safe operation under extreme high flow conditions. The flushing system will be utilized during dry flow however; flows can increase rapidly thus requiring safe external operation.