A precast concrete stormwater treatment system brings new life to an historic NYC park.
By Joy LePree
The history of Van Cortlandt Park in the Bronx, N.Y., is an interesting one that began in the early 1700s when Jacobus Van Cortlandt built a dam across Tibbetts Brook to power his mills, thus creating Van Cortlandt Lake. More than a century later, New York City constructed an aqueduct, the city’s first major water supply project, now dubbed the Old Croton Aqueduct Trail. Then in 1895, the Van Cortlandt Golf Course became the centerpiece of the park, and holds the bragging rights as the oldest public golf course in the United States.
The story nearly ended tragically a few years ago when New York City engineers discovered that the lake had become the victim of the busy city that grew up around it. Once very scenic and beautiful, Van Cortlandt Park had the misfortune of being located adjacent to an area where several major New York City highways were subsequently built. Trash, debris, salt, sand and other runoff from these highways nearly choked the lake and resulted in a reduction in depth from an original 13 feet to just over 2 feet, which is not enough to support aquatic life. The pollutants also created potential noncompliance problems with the Environmental Protection Agency’s Clean Water Act of 1976.
In an effort to restore the lake to its original beauty and to comply with EPA’s regulations, city engineers decided to dredge the lake and install a stormwater treatment system to handle the runoff from the highways. While they knew working within the historic site to design a system large enough to treat the high volume of runoff would be no walk in the park, engineers opted for a sophisticated precast concrete collection system that would make the job much more efficient.
“The park is located between several highways, including the Deegan Expressway and the Henry Hudson Parkway, and a lot of that highway drainage was going either directly or indirectly into the lake,” says Steve DesNoyer, team leader of support services for the New York City Parks and Recreation Department. “We conducted a study in the ’80s and found that the silt was slowly depleting the oxygen from all the organic settlement on the bottom.”
DesNoyer says that the city was seeking a way to catch the highway runoff before it went into the lake and reroute it into the sewer system. However, the crews found that a lot of fresh water was entering the lake as well, and to separate the highway runoff from the spring water would be too complicated. “So we decided to set up a collection system that would catch and clean up the highway runoff before it entered the lake, but would allow the fresh water to enter the lake as well,” DesNoyer explains.
With the help of Matt O’Neill, a project engineer with Galvin Brothers Contracting Co. in Great Neck, N.Y., the lake was dredged and five separate precast concrete stormwater treatment systems were installed in the surrounding park area.
The systems were custom designed by Environment 21 LLC, an East Pembroke, N.Y.-based firm specializing in the design of these structures, and cast by Connecticut Precast Corp. of Monroe, Conn. Each of the five collection systems was customized to meet the various requirements for the amount of runoff and proximity to the lake. Despite the engineering differences, the chambers all work on the same principle.
Called the V2B1, the systems consist of two circular precast concrete structures: an upstream swirl-chamber/vortex separator and a floatables collection chamber with an outlet. Runoff from the highway first enters a catch basin placed along the roadway and is directed to the first chamber, where the contaminated water swirls around “like a washing machine during the rinse cycle,” says Peter Fenninger, sales manager with Connecticut Precast. “When this happens, the particles settle to the bottom and the cleaned water continues on its path to the floatables chamber.” Here, pollutants that don’t have the weight to settle to the bottom of the first chamber, such as oil from the roadway, are held back by a baffle wall. The clean water then drops below the baffle and continues on its way to the discharge point.
Each V2B1 used in the Van Cortlandt Park project varied in size due to different requirements for each location, but O’Neill says the average size was about 10 feet to 12 feet in diameter and about 10 feet high.
Casting the V2B1 systems
Obviously, casting something of this size can be complicated. The trickiest part of the job – engineering the holes for the pipes – had to be done first. “Setting the forms is a manual operation,” explains Fenninger. “Using hole formers and engineered drawings, our workers are responsible for placing the right-sized hole for the right-sized pipe in exactly the right place with the right elevation. It’s not always easy, but it can be done properly with expertise.”
Once that is accomplished, the precast structures can be manufactured. “Because of their size, the chambers have to be made in sections,” says Fenninger. The base section for each includes the base slab and a small amount of wall height. This base is done in a single pour to create a monolithic joint where the base and walls meet. Additional wall sections, or risers, that may be needed to obtain a certain height are cast separately. “Once the concrete is poured and cured, we strip the forms and ship (the products) out when they are needed. At the site, the base is lowered into the hole and the risers are placed on top and joined using a mastic seal.”
Despite the complexity of forming the pipe holes, precast concrete is still the material of choice for these applications. “There are various stormwater treatment systems on the marketplace, and they all contain precast concrete components for a reason,” says Fenninger. “It has proven itself as being an extremely durable product. Any stormwater treatment system not made from precast has a tough row to hoe in terms of proving itself.”
Besides being durable, O’Neill adds that it’s much less expensive to manufacture this type of structure in a precast plant under controlled conditions and assemble them at the job site than it is to cast it in place. Plus, it results in a better product. “When it comes to stormwater treatment systems, the inverts coming in and going out of the system have to be extremely accurate for it to work properly,” says DesNoyer. “You can only get that sort of accuracy in a controlled pouring environment. It’s too difficult to use poured-in-place when this level of engineering and accuracy is required.”
Installing the systems
To install these systems, accomplished in the spring of 2003, five locations around the park were excavated and the chambers that were specified for each area were dropped in by crane. While this is a standard process, it was difficult because of the park’s terrain. “Five tractor trailers had to back up about one mile along a narrow golf cart path before the crane could pick up the chambers and place them in the allotted holes,” says Fenninger. “Watching those trucks back up was a real nail biter.”
However, once that was done, things went smoothly with each system being placed in the appropriate location, risers added and sealed, and pipes inserted and sealed within a few hours.
“The main use of these systems is expected to take place during the fall and winter collecting leaf debris, salt and sand,” says DesNoyer. “We’ve done one cleaning since the installation and, at that point, they seemed to have a satisfactory amount of material in there. So thus far, we’ve been happy with what we’ve seen.”
Working quietly beneath the ground, precast concrete is doing wonders for the environment – and helping Van Cortlandt Park avoid a tragic ending to its rich history.
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