Chasing Urban Runoff

To meet federal Surface Water Treatment Rule (SWTR) requirements and United States Environmental Protection Filtration Avoidance mandates, the New York City Department of Environmental Protection (DEP) developed a proactive program to manage and protect the Kensico Reservoir and its watershed. The prime components of the program are stormwater and waterfowl management, sewer system inspection and repair, an in-reservoir turbidity curtain, reservoir dredging and hazardous spill containment.

Protecting water quality in the Kensico Reservoir is particularly important because it is the final impoundment for over one billion gallons (90 percent) of New York City's (the city) unfiltered water supply before it enters the distribution system. This article summarizes the stormwater management element of the program, and its control of the two key pollutants regulated by the SWTR: fecal coliform bacteria and turbidity, which are conveyed to the reservoir by stormwater. The first phase of the project - watershed assessment, site selection and stormwater management facility screening and design - is complete. Facility construction began in the spring of 1999 and will be completed in 2001. DEP developed a protocol to assess the effectiveness of the program, collected baseline and storm event water quality data prior to facility construction and will collect data from representative stormwater facilities, once they are operational, to assess the effectiveness of the nearly $15 million program.


New York City has placed great emphasis on protecting and improving the quality of its drinking water supply through watershed protection and management programs. The DEP's Bureau of Water Supply developed and implemented one such program.

The older sections of the sewer line within the watershed were inspected using a video camera to locate potential sources of exfiltration.

The city's drinking water supply system is one of the largest in the world, supplying approximately 1.33 billion gallons of potable water each day to some nine million city and upstate residents. The entire watershed covers some 1,969 square miles on both sides of the Hudson River, and is comprised of 19 reservoirs, three controlled lakes and numerous wetlands, watercourses and intermittent streams. Land use, topography, hydrology and political climates in the system's three watersheds - the Delaware, the Catskill and the Croton vary dramatically.

One reservoir, the Kensico, is integral in managing unfiltered systems, because it serves as the final impoundment for water from the Catskill and Delaware watersheds before it enters the city's distribution system through Kensico's two effluent chambers. On average, approximately 1.3 billion gallons flow through the Kensico Reservoir each day, accounting for 90 percent of the system's daily demand. For this reason, it is critical to protect water quality in the Kensico. Controlling stormwater entering the reservoir is an important part of the city's Kensico Water Quality Protection Program.

The U.S. Environmental Protection Agency (EPA) recognizes the importance of the Kensico Reservoir and has required that DEP implement an aggressive watershed management and protection plan that reduces fecal coliform bacteria and turbidity inputs into the Kensico Reservoir. Elements of DEP's plan include aggressive stormwater and waterfowl management programs, sewer and septic system inspection and repair, an in-reservoir turbidity curtain, hazardous spill containment and a variety of other regulatory and nonregulatory nonpoint source pollution control programs. This article focuses on the stormwater management element. Other components of DEP's Kensico Program are summarized below.

Sewer Inspection and Repair - The sewer system within the watershed, including type and size of pipe and manhole locations, was mapped by the city. Of the 95,000 feet of sewer line in the watershed, 55,000 feet were installed before 1970 and are more prone to defects. The older sections of the sewer line within the watershed were inspected using a video camera to locate potential sources of exfiltration. The inspection program found 39 segments and three manholes in need of repair. The Town of Mount Pleasant and Westchester County completed the repairs under intermunicipal agreements and with city funding.

Stormwater Infrastructure Inspection/Sewer System Disconnection - To comply with the FAD requirement to track down discharges of wastewater into storm sewers, DEP is digitally mapping the storm sewer system in the Kensico watershed. Upon completion of the mapping, DEP will video inspect the storm sewer system in areas of the Kensico watershed served by sanitary sewers and locate any illicit wastewater connections that discharge into the storm sewer system. The results of the inspection will be referred to DEP and county enforcement officials for appropriate enforcement and remediation actions.

Waterfowl Management - The gull and waterfowl management program is designed to reduce the numbers of geese and gulls roosting and defecating in or near the surface water. Through hazing, shoreline meadow management, egg addling, physical barriers and Canadian geese egg depredation, the city has dramatically reduced the amount of bird waste that enters the reservoir. The program, implemented August 1 through March 31 each year, also includes research into new methods of bird control and ongoing assessments of program effectiveness. Although labor intensive, the waterfowl management program has been an overwhelming success in eliminating the greatest source of fecal coliform bacteria to the Kensico Reservoir.

Turbidity Curtain - A turbidity curtain installed at the mouth of Malcolm Brook in the southwest section of the Kensico Reservoir successfully directs turbidity and fecal coliform bacteria conveyed to the reservoir by two watercourses away from the Catskill Upper Effluent Chamber. Maintaining high quality water in the effluent chamber is critical, as water is conveyed directly to the distribution system from the chamber. Water entering the chamber is constantly monitored to determine compliance with the SWTR. Because of the curtain's effectiveness, the city will maintain it indefinitely.

Reservoir Dredging - The channels leading to the reservoir's two effluent chambers were dredged in 1999 to eliminate the potential for accumulated sediments to be resuspended during storms and impact the quality of water entering the effluent chambers. Some 1,777 cubic yards of sediment were excavated, dewatered and removed from the Kensico Reservoir, significantly reducing the potential for elevated turbidity in the reservoir.

Failing Septic System Detection and Remediation - To ensure that failing septic systems are promptly identified and remediated, DEP is conducting a house to house survey to identify any watershed residences with septic systems and determine if any systems are failing. DEP conducted a similar survey in 1991 and is applying a May 1998 "Methodology for Prioritizing Routine Inspections to Detect Septic System Failures," to stream and reservoir water quality monitoring data to identify potential septic failures. DEP also routinely patrols the watershed for potential water quality threats, including failing septic systems. Potential septic failures are thoroughly investigated and promptly remediated.

Stormwater Management

The Kensico Reservoir Stormwater Management Program is designed to reduce fecal coliform bacteria and turbidity delivered to the reservoir by controlling and treating stormwater. The first phases of the project, assessment of the watershed, site selection and the screening and design of stormwater control and treatment facilities were completed in July 1998. Facility construction began in the spring of 1999 and is to be completed early in the year 2001. DEP has committed to monitoring and evaluating facility performance and maintaining the facilities.

DEP also routinely patrols the watershed for potential water quality threats, including failing septic systems.

Phase I: Watershed Assessment

The Kensico Reservoir watershed occupies approximately 13 square miles and includes four suburban towns in Westchester County, New York, and a small portion of Fairfield County, Connecticut. To assess stormwater pollutant loadings in the Kensico watershed, the reservoir basin's physical characteristics, including land use, soils, topography, vegetation and reservoir tributaries, were inventoried and digitally mapped.

The watershed's topography is hilly and rolling, and over two-thirds of it contains slopes greater than eight percent. Almost one-third of the land area is used as passive open space, and approximately one-fifth of the land area is developed with low-density residential uses. The remaining land area is primarily active open space, farmland and for commercial/business uses. As water quality is, in part, a function of the amount of impervious surfaces in the watershed, the greatest concern is runoff from developed land directly adjacent to the effluent chambers, which convey drinking water to the consumers. Impervious surface area in the reservoir's subbasins ranges from four percent in the Whippoorwill basin to 45 percent in the Malcolm Brook basin (adjacent to the Catskill Upper Effluent Chamber) and averages 19 percent.

Phase II: Assessment and Management Plan Development

A preliminary assessment of stormwater remediation needs in the Kensico watershed was conducted by evaluating tributary water quality data, land use and impervious surfaces, model predictions of stormwater runoff quantity and quality and field observations of existing erosion. That evaluation concluded that 73 of the watershed's 148 drainage basins have a relatively high potential to contribute fecal coliform bacteria and suspended solids to the reservoir. As turbidity is a function of suspended solids in the water column, the stormwater program targets sources of suspended solids. Using the criteria listed below and field observations, 19 of the 73 reservoir drainage basins were initially selected for stormwater remediation measures.

Preliminary Stormwater Remediation Evaluation Criteria:

  • proximity to reservoir effluent chambers;
  • known or potential sources of pollutants;
  • quality and quantity of stormwater runoff;
  • presence of wetlands;
  • topography;
  • property ownership; and
  • observed erosion.

Based upon these criteria, conceptual designs were prepared for 88 stormwater management facilities within the 19 drainage basins. The conceptual designs were the basis for an environmental impact statement required under New York State's Environmental Quality Review Act (SEQRA).

Once the initial screening process and SEQRA process were completed, but before the engineering design phase began, the conceptual stormwater management plan was refined by applying the following criteria, in combination with the results of detailed field investigations, maintenance considerations and physical site constraints.

Site Selection and Conceptual Facilities Evaluation Criteria:

  • Does the site and the facility meet the intent of reducing pollutant loads?
  • Does the facility minimize impact to environmental resources and achieve measurable water quality benefits?
  • Does the existing condition warrant engineered improvements?
  • Are there property ownership/permission constraints, which make implementation impractical or impossible?
  • Have any watershed and land use conditions or assumptions changed since issuance of the final environmental impact statement that affect the appropriateness of the facility and/or the site?
  • Are there likely to be permit or property ownership issues which will compromise the viability of the practice?
  • Are the maintenance and/or operation requirements of the practice, as applied, so burdensome as to effectively make the practice inappropriate?

Having applied these criteria, a final plan that included 57 stormwater management facilities was developed. The final plan would reduce erosion and sedimentation, manage peak stormwater discharges, allow for settling of suspended solids, a reduction in turbidity, die-off of coliform and ultimately reduce pollutant loads delivered to the reservoir.

During the process of developing preliminary stormwater management facility designs, private property owners who initially consented to allowing DEP to construct facilities on their property required that five facilities be redesigned and denied permission to construct the facilities at three sites. Ultimately, 44 engineered facility designs were completed. Facility types included 10 extended detention basins, 14 segments of stream channel stabilization, 13 outlet stilling basins, one area of parking lot stabilization and one sand filter system. Additional road stabilization and drainage improvements to reduce erosion were incorporated into the stilling and detention basins and sand filter designs.

The final Kensico Stormwater Management Plan also addresses potential releases or spills of hazardous materials from the stormwater drainage system of Interstate 684 into the reservoir. In-reservoir containment booms are being installed to prevent any hazardous discharges of materials from the 23 I-684 stormwater outfalls that discharge directly into the reservoir by migrating through it and impacting water quality. The booms will allow for recovery and cleanup of hazardous substances and other material.

Having met the final siting and facility type criteria, each facility was engineered to minimize environmental impact on and off the site, without sacrificing water quality benefits. For example, each design incorporated existing topography, avoided wetland encroachment, included landscaping and wetland plantings and features necessary for long-term maintenance and discouraging waterfowl. The effort to minimize disturbance and subsequent on- and off-site impacts was a crucial component of enlisting the support of the community, regulatory agencies and private property owners.

Phase III: Implementation Gaining Community Support

Immediately after proposing the 88 conceptual facilities in 1995, DEP identified the owners of property where the facilities would be sited and launched an outreach campaign to explain the project and engender support for it. The ultimate goal of the campaign was to secure local support and legal permission to gain access to design, construct and maintain the facilities on private property. Securing permission to construct 18 facilities on private land holdings from 32 landowners was a challenging aspect of the project. Alternate sites located within the same basin were pursued where access to private property was denied during the facility siting and design phases of the project.

As turbidity is a function of suspended solids in the water column, the stormwater program targets sources of suspended solids.

Peer Review

An expert advisory panel was enlisted to review conceptual plans and facility designs planned for the highest priority drainage basin, Malcolm Brook, which discharges in the direct proximity to the Catskill Upper Effluent Chamber. Panel members were engineering and health professionals from academia and government agencies that are actively involved in public water supply protection and stormwater management projects. The panel supported the project and offered comments that helped to shape the designs and gain community support for certain aspects of the plan.

Regulatory Approvals

In addition to approval from private landowners, municipal support for the project and regulatory approvals to construct the facilities were needed. Initially, this involved a series of briefings with town supervisors, engineers and planners. Once support for the conceptual project was obtained, applications for local permits and approvals were submitted. A similar process of "pre-application" meetings followed with federal and New York State permitting agencies. The pre-application meetings set the stage for the relationship between DEP, the municipalities and regulatory agencies and allowed the agencies to comment on the designs before they were finalized and permit applications were officially submitted. The goal of the pre-application process was to minimize the need for design revisions and to avoid delays during the regulatory approval process.

Modeling Water Quality Benefits

Water quality modeling predictions provided valuable supporting information when developing the stormwater management plans. EPA's Stormwater Management Model (SWMM) was used to simulate runoff characteristics and turbidity and fecal coliform bacteria loading in select drainage basins of the Kensico Reservoir. The model was used to predict stormwater pollutant loads delivered to the reservoir by a drainage basin's tributary under existing conditions and future build-out conditions in the year 2010, with and without the stormwater management facilities. Model results estimated that projected increases in impervious surfaces (in the year 2010) would increase future loads of turbidity and fecal coliform bacteria in stormwater by 16 percent and 21 percent, respectively. The model further estimated that construction and operation of the 44 stormwater facilities will reduce future inputs of turbidity and fecal coliform bacteria by 23 percent and 15 percent, respectively, when compared to future loads without the stormwater controls. Model predictions of anticipated water quality benefits in individual basins are listed in Table 1. The model predicts that the water quality benefits of the plan will be substantial.

Table 1. SWMM Model Predictions of Tributary Loads of Turbidity and Fecal Coliform Bacteria.



Future Load without Plan

Future Load with the Plan


Fecal Coliform Bacteria


Fecal Coliform Bacteria

Malcolm Brook

5% increase

6% increase

95% reduction

72% reduction


104% increase

95% increase

91% reduction

60% reduction


9% increase

11% increase

81% reduction

41% reduction


14% increase

10% increase

63% reduction

38% reduction


103% increase

106% increase

90% reduction

52% reduction


30% increase

23% increase

84% reduction

54% reduction


107% increase

not modeled

68% reduction

not applicable

Bear Gutter Creek 5

60% increase

59% increase

77% reduction

59% reduction

Bear Gutter Creek 8

76% increase

73% increase

95% reduction

64% reduction


11% increase

not modeled

11% increase

not applicable


0% change

0% change

96% reduction

70% reduction

Constructing, Operating, Maintaining and Monitoring Facilities

Construction and Operation

Construction began in April 1999, following a prioritized schedule based upon erosion potential, water quality benefits, proximity to the effluent chambers and permitting and property owner constraints. During construction, issues such as private property owner demands, permit conditions and utility locations necessitated design revisions. Cooperation from municipal officials and DEP's construction contractor made it possible to make the revisions and construct the facilities within the constraints of the city's contracting process. While each facility began functioning immediately upon completion of construction, in many instances, final landscaping and stabilization was not completed until weather permitted.

Inspection and Maintenance of Facilities

DEP recognizes the need for an aggressive inspection and maintenance program to ensure that the stormwater facilities function in perpetuity as designed. Prior to construction, arrangements to carry out inspection and maintenance were established.

Monitoring Facility Performance

Facility designs incorporated water quality monitoring stations needed for DEP's five-year performance evaluation studies.

DEP developed a monitoring program to evaluate the performance of the treatment facilities, to determine if those basins were functioning as designed and the effectiveness of the Stormwater Management Program as a whole. The monitoring program targets pollutants of interest in the drinking water supply: turbidity, suspended solids, fecal coliform bacteria and total and dissolved phosphorus. Monitoring began in the spring of 2000.

Conclusions, Recommendations and Challenges

The Kensico Reservoir watershed stormwater management plan will improve water quality in the public drinking water supply reservoir by eliminating sources of contamination and controlling and treating stormwater runoff from priority tributaries. Incorporating an aggressive public outreach campaign, designing the facilities to minimize site and resource disturbances, providing for proper long term maintenance of stormwater controls and monitoring effectiveness were high priorities for DEP. The stormwater management plan is being used as a template for DEP watershed management programs in other urban reservoir watersheds and in DEP's overall stormwater management, mitigation and cost-sharing programs. Program recommendations are as follows:

  • An advisory panel should be formed to review conceptual plans and facility designs. The panel should be fully informed of watershed conditions, jurisdictional constraints and agency capabilities.
  • An aggressive outreach campaign should be launched early in the program to secure support for the project, permission to include privately owned land and approvals from regulatory agencies. The campaign should begin during conceptual plan development and continue through facility construction, operation maintenance and monitoring.
  • Regulatory pre-application reviews are key to expediting the potentially time-consuming approval process. These should be conducted during the conceptual stage of project planning.
  • Water quality modeling results and available sampling data should support the selection and prioritization of sites and facility types.
  • Facility inspection, maintenance and monitoring requirements should be identified and incorporated into the designs, and resources to carry out the requirements should be secured prior to construction.
  • Facility designs should maximize water quality benefits and minimize environmental impacts.
  • Facility designers should be experienced in watershed assessment, application of the remediation programs likely to be warranted in the area and local codes.
  • The construction contractor must be experienced in implementing erosion and sediment control plans, working in and adjacent to critical resources (wetlands and streams) and constructing the types of facilities in a stormwater remediation program.
  • Facility construction contracts should include contingency items to allow the contractor to promptly implement revisions that address unforseen conditions at a site, or conditions imposed by regulators or property owners.
  • The contracting agency must closely monitor and strictly enforce the provisions of erosion and sediment control plans. In addition, facility contracts should contain provisions for independent erosion control inspection and enforcement.


Roy F. Weston Inc. was contracted to develop the Kensico Water Quality Control Program and conceptual stormwater management plan.

Hazen and Sawyer, P.C. was contracted to reevaluate the conceptual stormwater management plan and prepare engineering designs for stormwater facilities. Hazen and Sawyer headed an excellent team which included the Center for Watershed Protection responsible for facility planning and design, HydroQual Inc., was responsible for modeling, and Shah Trans/Environ Engineering and Land Surveying was responsible for base map development.

Thalle Construction Co. constructed the facilities. Thalle was sensitive to environmental concerns (erosion and sediment control and natural resources), flexible, accommodating and provided a quality product that incorporated numerous design revisions.

This article originally appeared in the September 2001 issue of Water & Wastewater Products, Volume 1, Number 2, page 14.

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