Stormwater Retrofitting to Protect Drinking Water Reservoirs from the Impacts of Urban Runoff - Part 2

June 26, 2002
Watershed Protection

About the author: James D. Benson, AICP, CPESC, is a supervisor for the NYC Department of Environmental Protection, 465 Columbus Avenue, Valhalla, N.Y. 10595, phone: 914-742-2034, e-mail: [email protected]. Melissa Beristain, CPESC, works with Benson in the New York City Department of Environmental Protection’s Bureau of Water Supplies.


Part 1 described the layout of the Kensico Reservoir and the components of the reservoir’s stormwater management program.

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 completed early in 2001. DEP has committed to monitoring and evaluating facility performance and maintaining the facilities.

Phase I: Watershed Assessment

The Kensico Reservoir watershed occupies approximately 13 square miles and includes four suburban towns in Westchester County, N.Y., and a small portion of Fairfield County, Conn. (Figure 2). 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 more than two-thirds of it contains slopes greater than 8 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 (Figure 2). The remaining land area is primarily active open space, farmland and commercial/business land. Since 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 that convey drinking water to the consumers. Impervious surface area in the reservoir’s subbasins ranges from 4 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: Stormwater Remediation Needs 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 initially were 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.


               Property ownership.

               Observed erosion.

Based on 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 evaluation criteria in combination with the results of detailed field investigations, maintenance considerations and physical site constraints.

Site Selection and Conceptual Facilities Evaluation Criteria

               Do 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?

               Are there property ownership/permission constraints that make implementation impractical or impossible?

               Does the existing condition warrant engineered improvements?

               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 that 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 was developed that included 57 stormwater management facilities. The final plan would reduce erosion and sedimentation; manage peak stormwater discharges; allow for settling of suspended solids, a reduction in turbidity and die-off of coliform; and ultimately reduce pollutant loads delivered to the reservoir (Figure 3). During the process of developing preliminary stormwater management facility designs, private property owners who initially consented to allowing DEP 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 material from the stormwater drainage system of Interstate 684 into the reservoir. In-reservoir containment booms are being installed to prevent any hazardous discharges of material from the 23 I-684 stormwater outfalls that discharge directly into the reservoir from migrating through it and affecting water quality. The booms will permit recovery and clean up 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: Implementing the Stormwater Management Plan in a Developed Area

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 gather 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.

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 preapplication 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 preapplication 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. The 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 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 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.

Constructing, Operating, Maintaining and Monitoring Facilities

Construction and Operation

Construction began in April 1999 a prioritized schedule based on 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 when construction was completed, 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 are 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 include the following

               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 preapplication 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 unforeseen 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 assist DEP in reevaluating 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, Incorporated, responsible for modeling and Shah Trans/Environ Engineering and Land Surveying, responsible for base map development.

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

The authors thank Catherine Durso for help producing this article.

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