N.Y. Comp. Codes R. & Regs. tit. 6 § 646-4.9

Current through Register Vol. 46, No. 25, June 18, 2024

Section 646-4.9 - Design requirements and performance standards for stormwater regulation

Each municipality's stormwater regulatory program shall include design requirements and performance standards which shall be not less protective than the following:

(a) Minor projects. The following requirements shall apply to minor projects:

(1) Stormwater shall be managed on-site using stormwater control measures designed to afford optimum protection of ground and surface waters. Stormwater control measures shall be selected by giving preference to the best management practices for pollutant removal and flow attenuation. Stormwater may be calculated in accordance with the methodology in paragraph (b)(1) and subparagraph (b)(3)(i) of this section or, in the alternative, at a flat rate of 1.5 gallons of stormwater for every square foot of net increase in impervious area. Net increase is the difference between predevelopment and post-development conditions. All water from newly created impervious areas which would otherwise runoff the parcel shall be directed to an infiltration device. Location of the infiltration device shall be determined based upon soil test results.

(2) Stormwater control measures may include, but shall not be limited to, drywells of precast concrete, pits of crushed rock lined with geotextile fabric, and infiltration trenches. Such measures may also include natural and human made landscape features such as depressions, blind ditches, retention ponds, swales and others. Inlets to infiltration devices shall be protected from sediment at all times in order to maintain their capacity.

(3) Infiltration devices shall not be installed up gradient within 20 feet of the subsurface treatment system of a wastewater treatment system.

(4) Infiltration devices and buildings shall be located to maintain maximum attainable horizontal distance separation from wells, wetlands and waterbodies. Pumping stormwater shall not be permitted.

(5) The bottom of any infiltration device shall be a minimum of two feet above seasonal high ground water mark and two feet above bedrock.

(6) Temporary erosion controls shall be required to prevent siltation of waterbodies during construction.

(7) Stormwater control measures proposed to be installed at locations with slope > 15 percent before grading, soil percolation rate slower than 60 minutes per inch or which require placement of fill to meet vertical distance separations specified in this Subpart shall be designed by a qualified stormwater professional.

(b) Major projects. The following additional requirements shall apply to major projects:

(1) Methodologies for determination of runoff volume. Stormwater volumes and rates of flow shall be calculated using the following methods:

(i) for small watershed areas (up to 20 acres), any widely accepted method including the rational method may be used; and

(ii) for larger watershed areas any widely accepted method other than the rational method may be used.

(2) Design requirements for erosion controls.

(i) Erosion control shall be provided for all disturbed areas in accordance with with the most recently published New York State Standards and Specifications for Erosion and Sediment Control. The temporary erosion control measures shall be maintained continuously until permanent control measures are in service. Infiltration devices shall be protected from siltation during the period of construction and until the site is successfully revegetated by use of silt screens, inlet protection devices, sediment detention ponds or other suitable erosion control measures.

(ii) Staging of construction to facilitate erosion control shall be required. Only those areas where construction is actively occurring shall remain open and unvegetated. All areas that are not within an active construction area shall be mulched and stabilized or shall be mulched and revegetated. An active construction area is defined as one that has seen substantial construction within the past seven calendar days. Mulching or revegetation for erosion control shall be completed within 10 days following the last substantial construction activity.

(3) Design requirements for stormwater control measures.

(i) Stormwater control measures shall be designed so that there will be no increase in runoff volume from a 10-year frequency/24-hour duration storm event following development over the predevelopment volume.

(ii) For storm events exceeding the 10-year design storm, the stormwater control measures shall function to attenuate peak runoff flow rates for a 25-year/24 hour frequency storm to be equal to or less than predevelopment flow rates. For development greater than five acres, stormwater control measures shall function to attenuate peak runoff flow rates for a 100-year/24 hour, storm to be equal to or less than pre-development flow rates. Attenuation of the 100-year storm is intended to reduce the rate of runoff from development to prevent expansion of the 100-year flood plain so as to alleviate flooding of improved properties and roadways. The minimum requirement for peak flow attenuation can be waived for the 100-year storm event where it can be proven that downstream flooding is not a concern, such as where excess stormwater runoff is discharged to Lake George or to a regional stormwater facility designed to handle additional volume and peak discharge. The cumulative effect of all proposed development projects within the watershed should be considered in making this determination. Rainfall intensity curves for Lake George, NY shall be used in the design of these stormwater control measures.

(iii) Infiltration devices shall be designed such that the bottom of the system will be a minimum of two feet above the seasonal high groundwater level and bedrock to be realized following development. Where compliance with this requirement would prevent compliance with subparagraph (b)(3)(v) of this paragraph, compliance with this requirement may be waived. This provision shall not apply to wet ponds and similar stormwater control measures which are designed to be built in the saturated soil zone.

(iv) Stormwater recharge areas shall be located a minimum of 100 feet from the subsurface treatment system of a wastewater treatment system unless it is demonstrated that a lesser separation will not adversely affect the functioning of such leach fields.

(v) Infiltration devices shall be designed to extend a minimum of 10 percent of the infiltration surface area below the prevailing frost depth of four feet (whichever is greater) in order to provide infiltration during winter months.

(vi) The design of all infiltration devices shall depend on the infiltration capacity of the soils present at the project site. The design infiltration rate shall be based on the results of hydrogeologic studies performed by the applicant during preparation of the Stormwater Control Report. The studies shall include test pits or borings located to present a clear picture of geologic and hydrologic conditions existing at the site and the areas, both on and off the site, affecting, or to be affected by, the development. A minimum of three subsurface excavations shall be conducted and the results shall be included in the SCR. Interpretive logs of all excavations shall be submitted with the report. Hydrogeologic interpretations and conclusions shall be developed by qualified persons only. Following design of infiltration devices, additional subsurface investigations to confirm soil and groundwater conditions will be required in the areas proposed for infiltration devices. The design of any project or development shall ensure that the ability to manage stormwater is not affected by the placement of structures on those soils or locations best suited for stormwater management purposes.

(vii) All stormwater control facilities shall be designed to completely drain or return to design levels in accordance with the following: infiltration basin 5 days; infiltration trench 15 days; dry well 15 days; porous pavement 2 days; vegetated depression 1 day.

(viii) Pretreatment devices such as sediment traps, detention/stilling basins, filter strips, grassy swales, or oil/water separators shall be provided for runoff from paved areas or other areas subject to human-induced pollution including grease and oils, fertilizers, chemicals, road salt, sediments, organic materials and settleable solids, which shall be sufficient to remove pollutants from the runoff.

(ix) Stormwater control measures shall, at a minimum, incorporate the best available pollutant removal technology, which shall mean that which constitutes appropriate and cost effective means for removing pollutants from runoff so that the resulting treated stormwater will not degrade the water quality of any water body.

(x) Stormwater control measures shall be designed to preserve and maintain the base flow in all streams passing through, adjoining or receiving runoff from the site.

(c) General requirements for major and minor projects. The following requirements shall apply to major and minor projects:

(1) Projects shall comply with stream corridor standards at 646-5.

(2) Infiltration devices shall be located a minimum of 35 feet from Lake George and any downgradient drinking water supply or waterbody. Infiltration devices for high traffic areas shall be located a minimum of 100 feet from Lake George and any downgradient drinking water supply or water body. A separation of more than 100 feet may be required in cases where contamination of the water supply is possible due to highly permeable soils, shallow groundwater and similar situations. The separation distance for infiltration devices servicing high traffic areas shall be minimum of 50 feet from upgradient water supplies. Designs shall mitigate the possible adverse effects that groundwater recharge will have on adjacent wells, water supplies, wastewater treatment systems, buildings, roadways, properties and stormwater control measures. This design requirement shall not prohibit the creation of up to 400 square feet of pervious hardscape (i.e. porous pavers) for non-high traffic areas within 35 feet of Lake George and any downgradient drinking water supply or waterbody.

(3) Stormwater control measures shall include such other measures as are deemed necessary to prevent any increase in pollution caused by stormwater runoff from development which would otherwise degrade the quality of water in Lake George and its tributaries, render it unfit for human consumption, interfere with water-based recreation or adversely affect aquatic life.

(4) Emergency overflow provisions shall be made as necessary to prevent erosion, flooding, and damage to structures, roads and stormwater control measures.

(5) Stormwater control measures and development shall be designed so as to minimize adverse impacts to water bodies, minimize disturbance of water bodies, minimize land clearing, minimize the creation of impervious surfaces, and to maximize preservation of natural vegetation and existing contours.

(6) Development which involves the creation of areas subject to intensive landscape maintenance such as: golf courses, public parks and botanical gardens, shall require that a pest control and fertilizer management plan shall be prepared and included with the permit application.

(7) Development that involves structural components, such as retaining walls or dams, may require design and certification by a licensed professional engineer.

(8) On a site where pre-development exists, an applicant shall be required to undertake a stormwater retrofit project to control existing stormwater runoff discharges from the site in accordance with the major project standards of this Subpart to the maximum extent practicable. Such measures shall include, at a minimum, the infiltration of runoff from the first one-half inch of precipitation from all pre-development impervious areas within the site. The phased implementation of such controls for pre-development areas may be authorized. A variance will not be required for infiltration devices treating pre-development areas when conformance with the design standards for infiltration devices cannot be met. If the minimum one-half inch volume requirement cited herein cannot be met, the Commission may delegate the authority to issue a variance from this standard to staff.

N.Y. Comp. Codes R. & Regs. Tit. 6 § 646-4.9

Amended New York State Register March 17, 2021/Volume XLIII, Issue 11, eff. 3/17/2021