To ensure attainment of the objectives of these regulations
and to ensure that performance standards will be met, the design,
construction and maintenance of drainage systems shall be consistent
with the following standards:
A. In the interest of reducing the total area of impervious surface,
preserving existing features, which are critical to stormwater management,
and reducing the concentration of stormwater flow, maximum use shall
be made of existing on-site natural and man-made stormwater management
facilities.
B. Innovative stormwater management facilities may be proposed (e.g.,
rooftop storage, underground storage structures and infiltration systems),
provided that they are accompanied by detailed engineering plans and
demonstrate performance capabilities that are acceptable to the Town
Engineer.
C. Stormwater management facilities shall be provided so the peak discharge
of the calculated post-development runoff to an adjacent property,
watercourse or water body does not exceed the peak discharge of the
pre-development runoff. Point discharge of stormwater runoff to an
adjacent property, watercourse or water body will not be allowed in
the post-development design if one did not exist in the predevelopment
condition. Point discharge is required to be returned to sheet flow,
or an easement will be required to be obtained from the adjoining
property owner if this condition cannot be met.
D. Runoff calculations for the pre-development and post-development
comparison shall consider the one-year, ten-year, twenty-five-year,
and one-hundred-year storm frequencies.
E. For pre-development computations, all runoff coefficients within
the study area shall be based on actual (present) land use conditions.
F. Retention and detention basins in compliance with NYSDEC standards
and guidelines and other approved alternatives shall be used to retain
and detain the increased and accelerated runoff and reduce pollutants
in runoff which the development generates. Water shall be released
from these areas at a rate equal to or less than the pre-development
conditions of the storm event. Measures shall be taken to protect
the outfall area from erosion. Water quality volume shall be addressed
by any proposed post-development design.
G. Retention/detention basins shall be designed to safely discharge
the peak discharge from the post-development one-hundred-year storm
frequency event through an emergency spillway in a manner which will
not damage the integrity of the basin or cause damage to adjoining
properties.
H. Retention/detention basins shall be landscaped in accordance with
current engineering practices and in accordance with the New York
Standards and Specifications for Erosion and Sediment Control, year
of latest revisions.
I. Retention/detention basins which may be used to collect sediment
during construction operations must have all sediment removed at any
time that it is at 60% of its original capacity. Upon completion of
all construction, any sediment in the basin must be removed, and the
basin shall be reshaped to the design dimensions and stabilized. A
maintenance schedule must be provided that indicates how often the
basin is to be cleaned thereafter and who is responsible for cleaning
it.
J. Retention/detention basins which are designed with a dam shall incorporate
the following minimum standards:
(1) The maximum water depth shall not exceed 10 feet unless approved
by waiver of the Town Board.
(2) The minimum top width of dams shall be eight feet.
(3) The side slopes of earth fill dams shall not be steeper than three
feet horizontal to one foot vertical on the downstream side of the
embankment.
(4) Basins without restricted access shall have impoundment areas with
side slopes no greater than five feet horizontal on one foot vertical.
(5) A cutoff trench of impervious material shall be provided under all
dams.
(6) All pipes and culverts through dams shall have properly spaced cutoff
collars or factory welded antiseep collars.
(7) A minimum of one foot freeboard, computed from the maximum water
surface elevation during the one-hundred-year storm event, shall be
provided in all basins.
(8) The minimum floor elevation of all structures that would be affected
by a basin or other water impoundments or open conveyance systems
where ponding may occur shall be two feet above the one-hundred-year
water surface elevation.
K. Runoff calculations for stormwater management facilities shall be
based upon the following methods:
(1) SCS TR-20 (latest revision) is the recommended and preferred method
for the study of watersheds with a drainage area greater than 100
acres. SCS TR-20 or SCS TR-55 Tabular Hydrograph Method (latest revision)
may be used for the study of watersheds with a drainage area greater
than 200 acres.
(2) SCS TR-55 Graphical Peak Method (latest revision) may be used in
lieu of the Tabular Hydrograph Method for sizing conveyance systems
or checking peak flows only. It shall not be used for basin muting
or subarea routing as it does not provide an adequate hydrograph.
(3) Other standard engineering models with approval of the Town Engineer.
(4) Stormwater runoff shall be based on the following twenty-four-hour
storm events with a Type III distribution:
|
Storm Event
|
Inches of Rainfall
|
---|
|
1-year
|
2.9
|
|
10-year
|
5.5
|
|
100-year
|
8.0
|
|
Source: SCS Technical Release 55 (year of latest revision).
|
|
(5) Use of other criteria, assumptions, references, calculation methods
and computer programs may be utilized, provided that detailed design
information and programming, with references, are submitted to and
found acceptable by the Town Engineer prior to submission of the SWPPP.
L. The design plan and construction schedule shall incorporate measures
to minimize soil erosion and sedimentation.
M. Consideration shall be given to the relationship of the subject property
to the drainage pattern of the watershed.
N. Stormwater shall not be transferred from one watershed to another
unless one of the following shall apply:
(1) The watersheds are subwatersheds of a common watershed which join
together within the perimeter of the property.
(2) The effect of the transfer does not alter the peak discharge onto
adjacent lands, watercourse or water bodies at any point.
(3) Easements from the affected landowners are provided.
O. Technical references. For the purpose of this chapter, the following
documents shall serve as the official guides and specifications for
stormwater management. Stormwater management practices that are designed
and constructed in accordance with these technical documents shall
be presumed to meet the standards imposed by this chapter:
(1) The New York State Stormwater Management Design Manual (New York
State Department of Environmental Conservation, most current version
or its successor, hereafter referred to as the "Design Manual").
(2) New York Standards and Specifications for Erosion and Sediment Control,
(Empire State Chapter of the Soil and Water Conservation Society,
2004, most current version or its successor, hereafter referred to
as the "Erosion Control Manual").
P. Technical standards.
(1) For all swales and gutters, the Manning's roughness coefficient "n"
factors used to determine capacity and velocity shall be based on
accepted engineering practices.
(2) Corrugated metal pipe will not be allowed to be used in any drainage
system or facility without the approval of the Town Superintendent
of Highways and the Town Engineer. If allowed, corrugated metal pipe
shall be fully asphalt coated with paved inverts, and the "n" factor
shall be 0.022 for annular pipe and 0.017 for helical pipe.
(3) The "n" factor for concrete pipe shall be 0.013.
(4) The "n" factor for polyethylene pipe shall be 0.010 for smooth interior
pipe and 0.019 for corrugated interior pipe.
(5) Manufacturer's specifications may be submitted to the Town Engineer
for acceptance if other types of pipes or sizes of pipes indicate
that another value of "n" should be used.
(6) Catch basins shall be designed with a sump of 16 inches.
(7) Catch basin inlet capacity shall be based on design data provided
by the manufacturer.
(8) Any existing drainage structures within 200 feet of the subdivision/site
shall be included in the SWPPP. Structures that convey streams shall
be checked to determine if they have the capacity to carry the fifty-year
storm flows, and all other structures shall be checked for capacity
to carry the twenty-five-year storm flows.
(9) A tabulation of flows through all drainage systems shall be submitted
with the plans.
(10)
Culvert design shall consider inlet/outlet control at each structure
or hydraulic losses shall be calculated through the system. These
calculations are to be submitted as part of the plans. At a minimum,
when pipe sizes change, the tops of the pipes shall match in elevation.
(11)
All culverts having diameters of 24 inches to 48 inches shall
have a removable inlet grating of five-eighths-inch-diameter (minimum)
reinforcing bars spaced approximately six inches on center. All design
calculations are to reflect this inlet control condition.
Q. Catch basins shall be located in the swale along open-section roadways.
The calculated depth of flow in the swale shall not exceed l/2 of
the total depth of the swale before placing a catch basin. The catch
basin shall be capable of accepting 100% of the flow in the swale,
based on a twenty-five-year-return-frequency storm event. On closed-section
roadways, catch basins shall be located along the curbline and are
not permitted along the curb radius at intersections. For the purpose
of catch basin placement, the depth of flow along the curb and across
intersections shall not exceed two inches.
R. Manholes and catch basins shall not be spaced more than 300 feet
apart. Structures shall be placed at all points of changes in horizontal
or vertical direction.
S. Stormwater collection systems shall have a minimum diameter of 15
inches and shall be designed to have a minimum velocity of three feet
per second. However, at the terminus of the system, the flow velocity
at the discharge point shall not exceed four feet per second prior
to the flow entering a natural watercourse, water body or adjacent
property.
T. The maximum swale, gutter or curb velocity of stormwater runoff shall
be maintained at levels which result in a stable condition both during
and after construction. Swales shall be designed and stabilized in
accordance with New York Standards and Specifications for Erosion
Control, year of latest revision. Swales shall be designed to allow
for infiltration of stormwater runoff and removal of pollutants from
the runoff whenever possible. This can be accomplished by keeping
the swale at as flat a slope as possible, stabilizing the swale with
a water-tolerant erosion-resistant grass that will not be mowed close
to the ground, increasing the percolation ability of the swale by
tilling the soil before establishing vegetative cover and installing
check dams with riprap on the downstream side to prevent scouring.
U. Drainage facilities not located within public rights-of-way shall
be located within easements.
V. When plan applications are submitted in sections, each section shall
control stormwater runoff and sedimentation as though it were a separate
entity. If temporary facilities are required for construction of a
section, they shall meet all of the requirements of these regulations.
A construction or phasing schedule shall be submitted with each plan
and shall demonstrate the methods to be used to minimize stormwater
runoff and soil erosion and sedimentation.
W. Stormwater management facilities shall not be constructed within
or discharge to NYDEC-regulated wetland areas, wetland buffer areas
or water bodies unless either:
(1) The appropriate permits from applicable regulatory agencies have
been obtained; or
(2) A letter from said agencies has been obtained stating that a permit
is not required for the proposed work. Copies of the permits or letters
shall be submitted to the Town Planning Board for review by the Town
Engineer prior to the final approval of the plan.
X. Individual lots, buildings and dwellings shall be provided with drainage
facilities to assure proper runoff from roofs, driveways, paved areas
and footing drains. Footing drains shall discharge to free-flowing
outlets. The installation of such facilities shall be in accordance
with these regulations and the Town of Wallkill Road Specifications
and are required prior to the issuance of a certificate of occupancy.
Y. Any land development activity shall not cause an increase in turbidity
that will result in substantial visible contrast to natural conditions
in surface waters of the State of New York.
Z. Stormwater discharges should be consistent with the thermal criteria
found in Part 704 of the Water Quality Regulations, Title 6, Chapter
X, New York State Codes, Rules and Regulations.
SWPPPs must demonstrate that the proposed development or activity
has been planned and designed and will be constructed and maintained
to meet each of the following standards:
A. Ensure that after development, runoff from the site maintains the
rate of flow and quality of runoff that would have occurred following
the same rainfall under existing conditions.
B. Maintain the existing hydraulic and characteristics of the watershed.
C. Protect the quality of ground and surface waters.
D. Protect groundwater levels.
E. Protect the beneficial functioning of wetlands as areas for the natural
storage of surface waters and the chemical reduction and assimilation
of pollutants.
F. Prevent increased flooding and damage that result from improper location,
construction and design of structures in areas which are presently
subject to an unacceptable danger of flooding.
G. Minimize injury to flora and fauna and adverse impacts to fish and
wildlife habitat.
H. Otherwise further the objectives of these regulations.
The applicant shall reserve easements or drainage rights-of-way
within the subdivision where stormwater or surface water drainage
facilities are existing or proposed, whether man-made or natural.
The easement shall conform as closely as possible to the lines of
such course and shall also meet the following criteria:
A. Easements shall have a minimum width of 20 feet and shall be adequately
designed to provide area for the collection and discharge of water,
the maintenance and repair of the facility and the passage of equipment
for such work.
B. In the case of streams or open-channel drainage, the easement shall
encompass the one-hundred-year storm event or the flood of record,
plus one foot freeboard above that elevation. Calculations acceptable
to the Town Engineer supporting those elevations shall be submitted
with the plans.
C. When a proposed drainage system carries water onto adjacent lands
where no discharge point existed in the pre-developed condition, stormwater
will be returned to a sheet flow condition prior to exit from the
site or an easement must be obtained from the downstream property
owners(s).
D. The plans shall clearly indicate who has the right of access and
the responsibility of maintenance for all facilities.