New York project focus of study of vortex technologies
The solution to controlling CSOs discharging into Flushing Bay involved a project that has been entered in ASCE's 2001 Outstanding Civil Engineering Award Program competition.
A project that is expected to be the focus of a major, long-term, evaluation of vortex technologies is one of the entries this year in the American Society of Civil Engineer's 2001 Outstanding Civil Engineering Achievement Awards Program.
The project is one developed by the New York City Department of Environmental Protection in order to control combined sewer overflows occurring during wet weather, which were discharging into Flushing Bay causing a major contribution of floatable and settleable solids.
The project-the Corona Avenue Vortex Facility-is being monitored to determine whether it is a cost-effective method to control CSO discharge from many of the other 450 CSO outfalls in the rest of the city. It is expected that the results of this high profile test will be cited as the basis for selection of CSO control methods in other parts of the country, too.
It was imperative, therefore, that the test methodology be comprehensive and objective, and that the sample data correctly represent the actual conditions which were being measured.
The NYCDEP and its consultants determined that a peer review of the test protocols, and of the sampling program, would ensure that the conclusions are credible, and could be relied upon as the basis for future wet weather planning. To accomplish this, they looked to the Water Environment Research Foundation to undergo peer review, which is underway.
CAVF services recreational areas
Flushing Bay and the area surrounding it encompass many significant recreational resources. In addition to the marinas located along its southern and western shorelines, the US Tennis Center, Shea Stadium and Flushing Meadows-Corona Park lie immediately to the south of the Bay. Historically, wet weather discharges to Flushing Bay from combined sewer overflows (CSO) have resulted in drifting masses of floating debris and deposits of odor-producing sediments along its shoreline.
NYCDEP is responsible for the City's water quality and combined sewer system. One of the largest CSOs discharging to Flushing Bay, and a major contributor of floatable and settleable solids to the Bay, is the CS-3 outfall which drains an area of 3000 acres to the south and west of the Bay. This outfall also had chronically experienced dry weather overflows (DWO) due to the limited access to regulators located under the Long Island Expressway. The DWO contributed enormously to the dissolved oxygen deficit in Flushing Bay which renders water quality significantly below standards. This outfall was selected as the location for a high-rate prototype facility, to be designed to test the ability of vortex technologies to remove pollutants from CSO.
The NYCDEP project team, led by URS Corporation, determined that the prototype facility would comprise three vortex units operating in parallel. The three units would include vortex designs following the USEPA swirl concentrator design; the British designed Hydrodynamic Separator; and the German Fluidsep design. The facility would include provisions for diverting all dry weather flow through the facility and would be designed to accommodate wet weather flows up to 600 efs.
Extensive sampling and monitoring equipment included in design
Equipment would be included in the facility to support an extensive sampling and monitoring program, including flow monitoring and sampling pumps at 10 locations; level monitoring at 5 locations; sampling screens to capture floatables at the overflow from each of the vortex units and in a sampling channel to represent the influent quality; and automated sluice gates to allow flow to be routed to each of the vortex units individually or in combinations. A SCADA system would provide automated control and monitoring. URS included HydroQual, Inc. and Moffa and Associates in the design team for their expertise, respectively, in floatables sampling and vortex hydraulics.
The design engineers confronted significant challenges in siting, planning and constructing the prototype facility. The City faced regulatory pressure to expedite the abatement of CSOs discharging to Flushing Bay. Securing a site for a major facility in congested central Queens is a time-consuming process. The project team determined that the implementation time could be significantly reduced by constructing the proposed facility completely underground within a street right-of-way'. The eastern-most block of Corona Avenue presented an almost ideal siting solution. This site abuts the alignment of the CS-3 outfall at a location where Corona Avenue is approximately 100 feet wide.
Constructing a major CSO facility under street was challenge
However, constructing a major CSO facility under a street presented the project team with a new set of design challenges. The location of a Nursing Home to the south and the Special Children's Playground to the north of Corona Avenue required that the design allow construction with a minimum of noise and vibration, and that safe access be maintained.
Japanese technology used was fairly new to U.S.
The design initially called for sheet piling as support for the 40-foot deep excavation, however, the noise and vibration would have presented a significant hardship for the residents. Furthermore, the soils in the area are highly unstable and the driving of sheet piling could result in settling, With input from the contractor, Perini Construction, design engineers decided to utilize a combination of soil-mix wall and slurry-wall construction, with tight sheeting placed within the wall. This Japanese technology was relatively new to the United States, and this was its first application in New York City.
The CS-3 outfall at this location is a double-decked conduit, with the lower deck consisting of two 9.5-feet H x 15-feet W barrels. To allow flow to be diverted to the vortex facility, new openings were required in the structural wall between the two barrels, and in the exterior wall of the lower deck. This raised significant structural issues pertaining to the support of the upper barrel, with the reduced structural capacity of the walls of the lower deck. Supporting beams framed into new external columns were installed to provide the required structural support.
Elevation of facility was critical
The elevation of the facility was critical to the hydraulic requirement to maintain gravity flow, passing it through the vortex units, and returning it to the conduit at essentially the same elevation. Weirs were constructed in the existing outfall to divert flow to the vortex facility, separating influent flow from treated flow. The CS-3 outfall is tidally affected, limiting the capacity of the outfall at peak high tide, causing surcharging of the outfall when peak tides and heavy wet weather flow coincide. Motorized sluice gates were provided to exclude the tide, however the facility was also designed to withstand flooding from extreme conditions, with water submersible equipment, elevated local switches and panels, and electrical components located in a separate control room.
Microtunneling provided one solution
A means for construction of a new sewer to carry foul waste underflow from the facility to the existing 108" Street Pumping Station also presented a challenge to design engineers. The routing of the new sewer would follow the heavily-traveled service road for the Long Island Expressway. This constraint was overcome through the use of micro-tunneling to construct the 1800 feet of 48-inch sewer.
The Corona Avenue Vortex Facility began operation in November 1998, eliminating all dry weather discharges and treating a significant portion of the CSO discharged to Flushing Bay. Currently, the City is rebuilding a promenade along the waterfront to provide enhanced access to the new cleaner waterway.
A panel of distinguished judges, including Public Works Online Content Developer Joyce Everhart, is determining the ASCE award-winning project, which will be revealed in April. Until then, PWOL is featuring various entrants whose projects are of interest to the public works community.
Source: American Society for Civil Engineers