By Jay Landers
In mid-August, an estimated $133 million project to restore a long-buried New York City stream cleared one of its final regulatory hurdles. Besides stream restoration, the planned daylighting of Tibbetts Brook in the New York City borough of the Bronx will add parkland and wildlife habitat in a heavily urban setting, decrease combined sewer overflows and environmental contamination, and restore the hydraulic connection between the stream and the Harlem River. At the same time, the project will increase recreational opportunities by extending an existing trail system for pedestrians and cyclists.
Buried brook
Tibbetts Brook originates in Yonkers, New York, and flows south into Van Cortlandt Park in the Bronx. In the 18th century, the stream was dammed to form Van Cortlandt Lake, which has since been renamed Hester and Piero’s Mill Pond.
Historically, Tibbetts Brook “was a meandering stream of approximately 1.8 mi from what is now Hester and Piero’s Mill Pond to the former Harlem River alignment, which has shifted south since the late 1800s,” says Pinar Balci, Ph.D., assistant commissioner for the Bureau of Environmental Planning and Analysis within the New York City Department of Environmental Protection. The DEP is overseeing the planned stream restoration and trail extension project along with the city’s Department of Parks and Recreation.
In 1912, discharges from Van Cortlandt Lake were directed into New York City’s combined sewer system, and the section of the brook south of the lake was filled in to facilitate development.
Because of this arrangement, approximately 4-5 mgd of water from Tibbetts Brook is conveyed to the city’s Wards Island Wastewater Resource Recovery Facility during dry periods, contributing to some 2.1 billion gal./year of relatively clean lake water undergoing treatment at the facility. During wet weather, higher volumes of stream water reduce capacity within the city’s overtaxed combined sewer system, exacerbating sewage overflows to the Harlem River and contributing to water quality impairments in the waterway.
‘Ambitious’ project
On Aug. 14, the Public Design Commission of the City of New York approved plans by the DEP and NYC Parks to direct flows from the pond into a new 1.6 mi long stream channel, according to the commission’s minutes from its meeting. Approximately 0.9 mi of the new channel will extend south above ground through Van Cortlandt Park and continue within a long, narrow strip of land that the city recently agreed to purchase for $11.2 million from the railroad freight company CSX Transportation. Near West 232nd Street, the stream will enter a closed conduit that will extend 0.7 mi to an outfall that discharges to the Harlem River.
All told, the project is expected to reduce the volume of CSOs entering the Harlem River by approximately 220 million gal./year, Balci says, making it one of New York City’s largest green infrastructure projects. For this reason, restoring the section of Tibbetts Brook is a critical component of the city’s consent order with the New York State Department of Environmental Conservation to reduce CSOs by means of green infrastructure.
Along with adding 4 acres of new parkland, the project will extend an existing trail known as the Putnam Greenway by 1.2 mi. The new trail section will run beside the restored stretch of Tibbetts Brook from Van Cortlandt Park to West 230th Street via the former CSX right of way.
“Rerouting this long-buried waterway above ground will reduce pollution going into the Harlem River, lessen flooding, connect greenways, and create even more recreational space for the Bronx,” said Rohit Aggarwala, Ph.D., New York City’s chief climate officer and the DEP commissioner, in a Jan. 18 news release announcing the city’s plans to buy the tract of land from CSX. “This is one of our most ambitious green infrastructure projects to date,” Aggarwala said.
Controlling flows
A new side weir will be constructed in Hester and Piero’s Mill Pond to divert flows into the new channel south of the pond. “This proposed side weir structure will be designed with two 8 in. tall slot weirs with a total weir length of 15 ft,” says Amy Motzny, the section lead for climate and equity within the Bureau of Environmental Planning and Analysis. “During dry flow, the side weir will release 7 cfs from the pond into the stream, and during wet-weather events the side weir will release up to 38 cfs.”
To remain in place to provide overflow relief, the existing weir in the pond “will be modified to include new weir plates that will allow for additional storage and underflow baffles to improve trash and floatables management,” Motzny says. “When the water level of the pond reaches the weir elevation, flow will overtop the weir and enter the existing combined sewer system.”
Within Van Cortlandt Park, the new channel will extend south from the lake for approximately 1,176 linear ft. “The open channel is designed to have an average slope of approximately 0.34% in the first 400 linear ft of the stream, followed by an average slope of 0.12% for the remainder of the channel,” Balci says.
“The width of the channel will vary within Van Cortlandt Park, but a typical width of the stream is 14 linear ft across, and the peak flow water surface elevation is approximately 33 linear ft wide,” Balci says.
Design constraints
Upon entering the CSX right of way, the stream width will constrict significantly. Within this stretch of the project, the proximity of existing buildings and other structures poses certain design challenges, Balci says. For much of its length, the new channel also will parallel the adjacent six-lane Major Deegan Expressway.
“Through consistent stakeholder and interagency coordination, we are designing a project that will daylight a stream and create a greenway in a narrow former railroad corridor that maintains 38 cfs flow in a dense urban environment while reducing the cut and fill required for grade change,” Balci notes. “The project will achieve that in an environment that has a very limited elevation change between Hester and Piero’s Mill Pond and the existing CSO outfall location that this project will connect into and in an area with existing building foundations and existing utilities that affect our project’s depths and foundations.”
Space limitations eventually dictate that the stream go underground. “Due to the constriction of the site around 232nd Street and adjacent development,” Motzny says, “the channel needs to be buried and continues underground through” property used by Metro-North Railroad, a rail line operated by the Metropolitan Transportation Authority.
To this end, Tibbetts Brook will enter the inlet leading to the closed concrete conduit, Motzny says. The 2.5 ft tall, 6 ft wide conduit will extend south and connect to an existing outfall that discharges to the Harlem River.
Multiple benefits
With the new stream channel and the Putnam Greenway extension, the city will create a “permanent recreational, ecological, and transportation resource,” Motzny says.
“Additionally, the project aims to establish and enhance wetland habitat and reduce urban heat island (effects) through landscape that will address the various microclimates of the site, from the rich forested areas of Van Cortlandt Park to the open, hot, dry, and exposed landscape of the CSX corridor along the Major Deegan Expressway,” Motzny says. “The plant palette was selected to thrive in these diverse conditions, providing shade, buffers, and habitat.”
For the project, consulting firm Hazen and Sawyer is providing engineering services, while Starr Whitehouse Landscape Architects and Planners PLLC is providing landscape architecture services. Although the city has selected a company to perform construction management on the project, a contract has not yet been awarded.
Construction on the project is expected to begin in 2025 or 2026. Before work starts, the city must obtain approval from the federal Surface Transportation Board to use the former rail line as a trail. The MTA also must complete a process known as a uniform land use review procedure to allow the DEP to construct and maintain the closed conduit on its property.
This article is published by Civil Engineering Online.
