Kensico Reservoir Stormwater Basin Improvements


The Kensico Reservoir is part of a series of aqueducts, shafts, tunnels, and dams that rely on gravity to transport clean water collected in upstate New York to New York City. Covering 2,145 acres near Mount Pleasant, 15 miles northeast of the Bronx, the reservoir has a 30.6 billion gallon capacity. An extensive stormwater management system uses best practices to prevent untreated runoff from carrying sedimentation and pollution into the reservoir. As designed, the system reduces total suspended solids in runoff by an estimated 80 percent and total phosphorus by 40 percent. It also prevents flooding and downstream erosion.

A joint venture between Arcadis and Gannett Fleming provided engineering design to improve the system’s extended detention facility, which manages runoff at Malcolm Brook–a reservoir tributary. The project had three main goals: provide access to the main detention basin for sediment removal, prevent erosion around the discharge outlet pipe and a forebay spillway, and ensure accurate flow measurement at the culvert outlet.

What We Did

To facilitate maintenance of the basin and provide access for dredging, the team’s design features a 120-foot-long, 12-foot-wide gravel access ramp built along the western slope of the main detention basin down to the water. The roadway allows the basin to be regularly cleared of sedimentation that can limit its effectiveness. To reinforce earthen embankments, reduce water velocity, and prevent erosion, the team added stone riprap around the basin’s outlet pipe and the spillway from one of the facility’s two forebays.

Because the angle of the main basin’s existing culvert outlet did not allow accurate flow measurement, the team installed a stainless steel V-notch weir at the outlet, providing a straightforward way to calculate volumetric flow. These measurements help workers monitor the basin’s effectiveness. Together, the improvements keep the stormwater facility in top operating condition, allowing it to protect the water in the Kensico Reservoir, as it was designed to do.

Key Features

  • Gravel access ramp facilitates sedimentation removal in the main basin.
  • Riprap at basin and forebay outlets stabilizes embankments and prevents erosion.
  • V-notch outlet weir provides accurate flow measurements.

Awards & Recognition

  • Special Recognition Award, 2017, New York City Public Design Commission, 35th Annual Awards for Excellence in Design.


  • Improved effectiveness of stormwater management system.
  • Access to detention basin for sedimentation removal.
  • Accurate flow calculations enabled by V-notch weir.


New York City Department of Environmental Protection


Mount Pleasant, N.Y.



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