Hydraulic Energy Flushing Of Inverted Siphons

摘要

The City of Richmond, Virginia is located at the head of tide on the James River. The olderrncentral part of the City is served by a combined sewer system (CSS) that comprises about 12,000rnacres. The largest basin in the system is the Shockoe Creek Basin area that is comprised of aboutrn8,000 acres, about 2/3 of the overall system. The City began addressing Combined SewerrnOverflow (CSO) control in the late 1970s by constructing a 38 million gallon “first flush”rnretention system on the Shockoe Creek Basin in the early 1980s. The Shockoe Creek Basinrndischarges into a diversion structure at the retention system through an arch sewer. A crest damrnin the Outfall Structure, down stream of the Diversion Structure, prevents dry weather flow fromrnoverflowing into the river. Flows into this structure are transferred to the Wastewater TreatmentrnPlant (WWTP), across the James River using two 66-inch inverted siphon pipelines. Thernsiphons have a firm capacity of 200 mgd. Flows feeding the river crossing vary from a dryrnweather flow of approximately 30 mgd to a wet weather flow of about 60 mgd. These wetrnweather CSO flows are very turbulent and sediment laden. Wet weather CSO events after thernwinter driving season contribute large quantities of sand and grit. Over time, deposition of solidsrnin the twin river crossing can reduce the flow capacity of the inverted siphons pipelines, reducingrntransfer capacity to the wastewater treatment plant. This plugging causes operations personnel tornmodify standard operating procedures and maintenance personnel to perform extra duties.rnThis paper will describe a mechanical flushing system located in the confined space of thernOutfall Structure. The mechanical flushing system was designed to store and release a largernvolume of wastewater to flush the twin river crossing with a high volume, high velocity streamrnwhile avoiding accidental releases of dry weather overflow to the river. The design also allowsrnsafe operation under extreme high flow conditions. The flushing system will be utilized duringrndry flow however; flows can increase rapidly thus requiring safe external operation.