Stormwater Retrofit Design Projects, Y-MU-40 Hallocks Mill Road

The project is part of the East of Hudson Watershed’s mission to reduce phosphorus levels in drinking water by retrofitting an area of the watershed that receives high phosphorus-content runoff in the Town of Yorktown. The retrofit is multi-faceted including a bioengineering infiltration practice and stream restoration. The stormwater treatment area intercepts flow from over sixty acres of residential area and diverts the water quality (90%) storm to a Focalpoint bioinfiltration unit. Bypass flows continue to the receiving stream. The stream is retrofitted to reduce phosphorus through the use of willow stake plantings, bank stabilization, and native stone bed material.

The Focalpoint system is unique in that it combines multiple forms of stormwater management and pollutant removal in one unit. The system is a three-layered treatment, with a modular open-cell underdrain system that promotes storage for infiltration to underlying soils, high performance soil media with a high conveyance rate and pollutant removal efficiency, and plantings chosen for maximum pollutant adsorption, high survival rates, pollination capabilities and aesthetic value for green space. The stormwater management area includes a diversion structure for treatment of low flows and bypass of larger events, the Downstream Defender, a hydrodynamic vortex separator, and a forebay plunge pool area for pretreatment before runoff enters the Focalpoint. As the prime consultant, HVEA provided design services and construction inspection services, including hydrologic watershed analysis, design of a complex stormwater management and drainage system, and compliance with environmental agencies during design and construction.

This retrofit design successfully removed 32.5 kg of phosphorus from the New York City drinking water supply through innovative bioengineering technologies.