[Amended 9-26-2024 by L.L. No. 8-2024]
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 100-year storm frequency event through an emergency spillway with principal spillway not accessible 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 distribution curve in compliance with NYSDEC SPDES Permit # GP-0-20-001 or as amended and revised:
(a) NYSDEC 2015 Stormwater Design Manual or as amended and revised.
(b) Cornell Education Extreme Participation.
(c) National Oceanic and Atmospheric Administration (NOAA) precipitation data.
(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, 2016, 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.