Stormwater Retrofitting to Protect Drinking Water Reservoirs from the Impacts of Urban Runoff - Part 2
Part 1 described the layout of the Kensico Reservoir and the
components of the reservoir’s stormwater management program.
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
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
to reservoir effluent chambers.
or potential sources of pollutants.
and quantity of stormwater runoff.
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
the site and the facility meet the intent of reducing
the facility minimize impact to environmental resources and achieve measurable
water quality benefits?
there property ownership/permission constraints that make implementation
impractical or impossible?
the existing condition warrant engineered improvements?
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?
there likely to be permit or property ownership issues that will compromise the
viability of the practice?
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
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
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.
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,
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
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.
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.
preapplication reviews are key to expediting the potentially time-consuming
approval process. These should be conducted during the conceptual stage of
quality modeling results and available sampling data should support the
selection and prioritization of sites and facility types.
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.
designs should maximize water quality benefits and minimize environmental
designers should be experienced in watershed assessment, application of the
remediation programs likely to be warranted in the area and local codes.
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
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.
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
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