Private on-site stormwater disposal systems shall comply with the provisions of Section 1114.
1114.1.1 When Permitted
The use of private on-site stormwater disposal systems shall be permitted only in the following circumstances:
Pursuant to a certification issued by the New York City Department of Environmental Protection that a public storm or combined sewer is not available or that connection thereto is not feasible in accordance with Section 107.6.2.2, Item 1(i);
Pursuant to a certification submitted by the applicant to the New York City Department of Environmental Protection that a public storm or combined sewer is not available or that connection thereto is not feasible, in such cases where the availability and feasibility of connection to a public storm or combined sewer are allowed to be certified by the applicant pursuant to rules of the New York City Department of Environmental Protection, in accordance with Section 107.6.2.2, Item 1(ii);
Pursuant to a certification submitted by the applicant to the New York City Department of Environmental Protection authorizing on-site stormwater disposal in accordance with Section 107.6.2.1, Item 1;
For enlargements less than 1000 square feet (93 m2) in accordance with Section 107.6.2, Exception 2;
Stormwater chambers that facilitate infiltration; and
Alternate method of on-site disposal as approved by the New York City Department of Environmental Protection.
1114.1.3 Minimum Setbacks
On-site stormwater disposal systems shall be located at least 5 feet (1524 mm) from all lot lines except where the lot line abuts a public right of way and 10 feet (3048 mm) from all foundations or walls existing on the date of application for a building permit or proposed under the application to construct the on-site stormwater disposal system. Systems shall be located 20 feet (6096 mm) from disposal fields and 20 feet (6096 mm) from seepage pits. On-site stormwater disposal systems shall not be located within the building footprint.
1114.2 Field Investigation
The size of an on-site stormwater disposal system shall be predicated on a field investigation performed prior to construction document approval that is performed at the site of a proposed on-site stormwater disposal system to assess the suitability of the soil and site. The investigation shall conform to Sections 1114.2.1 and 1114.2.2 and shall occur prior to approval of construction documents for the system. The field investigation shall be subject to special inspection in accordance with Section 1704.21 of the New York City Building Code.
1114.2.1 Classification of Soil Based on Borings and Test Pits
At least one boring and one test pit shall be made at the approximate site of each proposed on-site stormwater disposal system. Soil borings and sampling procedures shall in accordance with ASTM D 1586 and ASTM D 1587, and generally accepted engineering practice. Soil and rock samples shall be classified in accordance with Section 1802.3 of the New York City Building Code.
1114.2.2 Soil Infiltration Capabilities
The suitability of the subsurface soils must be verified in place by either a percolation test or a permeability test. Where testing determines that the infiltration rate of the subsurface soils is less than 1/2 inch (12.7 mm) per hour, private on-site stormwater disposal systems shall not be permitted. Such tests shall conform to Section 1114.2.2.1 or 1114.2.2.2, as applicable.
1114.2.2.1 Percolation Tests and Procedures
The infiltration rate of subsurface soils shall be verified with a percolation test. Percolation tests shall be performed in accordance with Sections 1114.2.2.1.1 through 1114.2.2.1.3 under the supervision of a special inspection agency in accordance with Section 1704.21.1 of the New York City Building Code. At least one percolation test in each system area shall be conducted. The holes shall be spaced uniformly in relation to the bottom depth of the proposed absorption system. More percolation tests shall be made where necessary, depending on system design. The results of the percolation tests shall be filed with the department stating the suitability of the site and the capacity of the subsoil for the proposed use.
1114.2.2.1.1 Percolation Test Hole
The test hole shall be dug or bored. The test hole shall have vertical sides and a horizontal dimension of 4 inches to 8 inches (102 mm to 203 mm). The bottom and sides of the hole shall be scratched with a sharp-pointed instrument to expose the natural soil. All loose material shall be removed from the hole and the bottom shall be covered with 2 inches (51 mm) of gravel or coarse sand.
1114.2.2.1.2 Test Procedure, Sandy Soils
The hole shall be filled with clear water to a minimum of 12 inches (305 mm) above the bottom of the hole for tests in sandy soils. The time for this amount of water to seep away shall be determined, and this procedure shall be repeated if the water from the second filling of the hole seeps away in 10 minutes or less. The test shall proceed as follows: Water shall be added to a point not more than 6 inches (152 mm) above the gravel or coarse sand. Thereupon, from a fixed reference point, water levels shall be measured at 10-minute intervals for a period of 1 hour. Where 6 inches (152 mm) of water seeps away in less than 10 minutes, a shorter interval between measurements shall be used, but in no case shall the water depth exceed 6 inches (152 mm). Where 6 inches (152 mm) of water seeps away in less than 2 minutes, the test shall be stopped and a rate of less than 3 minutes per inch (7.2 s/mm) shall be reported. The final water level drop shall be used to calculate the percolation rate. Soils not meeting the above requirements shall be tested in accordance with
Section 1114.2.2.1.3.
1114.2.2.1.3 Test Procedure, Other Soils
The hole shall be filled with clear water, and a minimum water depth of 12 inches (305 mm) shall be maintained above the bottom of the hold for a 4-hour period by refilling whenever necessary or by use of an automatic siphon. Water remaining in the hole after 4 hours shall not be removed. Thereafter, the soil shall be allowed to swell not less than 16 hours or more than 30 hours. Immediately after the soil swelling period, the measurements for determining the percolation rate shall be made as follows: Any soil sloughed into the hole shall be removed and the water level shall be adjusted to 6 inches (152 mm) above the gravel or coarse sand. Thereupon, from a fixed reference point, the water level shall be measured at 30-minute intervals for a period of 4 hours, unless two successive water level drops do not vary by more than 1/16 inch (1.59 mm). At least three water level drops shall be observed and recorded. The hole shall be filled with clear water to a point not more than 6 inches (152 mm) above the gravel or coarse sand whenever it becomes nearly empty. Adjustments of the water level shall not be made during the three measurement periods except to the limits of the last measured water level drop. When the first 6 inches (152 mm) of water seeps away in less than 30 minutes, the time interval between measurements shall be 10 minutes and the test run for 1 hour. The water depth shall not exceed 5 inches (127 mm) at any time during the measurement period. The drop occurs during the final measurement period shall be used in calculating the percolation rate.
1114.2.2.2 Permeability Tests
Soil shall be evaluated for estimated percolation based on a permeability test performed in place, in accordance with procedures established by the New York City Department of Environmental Protection and accepted engineering practice.
1114.3 Design
The design of on-site stormwater disposal systems shall comply with the provisions of Section 1114.3.1.
1114.3.1 Runoff Rate
The runoff rate shall be calculated using the rational method, Equation 11-1. The calculation shall incorporate the total site area with a rainfall intensity value of I = 5.95 inches per hour. The weighted runoff coefficient shall be calculated using Equation 11-2 and shall incorporate the different combinations of surfaces using the C values listed below.
Q = Cw × I × A
Equation 11-1
Where:
Q
=
developed flow, cubic feet per second
Cw
=
weighted runoff coefficient
I
=
the rainfall intensity value, 5.95 in/hr
A
=
the total site area, acres (ac)
Cw = (1/A)Σ(AK × Ck )
Equation 11-2
Where:
Cw
=
weighted runoff coefficient
A
=
The total site area, acres (ac)
AK
=
The area of each surface coverage type, acres (ac)
CK
=
The runoff coefficient associated with each surface coverage type
The following C-values shall be used for calculating a sites weighted runoff coefficient:
0.95
=
roof/concrete
0.85
=
asphalt
0.7
=
porous asphalt/concrete or permeable pavers
0.7
=
green roof with four or more inches of growing media
0.65
=
gravel parking lot
0.3
=
undeveloped areas
0.2
=
grass areas
0.2
=
rain gardens, vegetated swales and other surface green infrastructure practices
1114.3.1.1 Storage Volume
The storage volume of an on-site stormwater disposal system shall be measured 3 feet (610 mm) above the level of the water table. The location of the water table shall be verified at the time of the field investigation conducted in accordance with Section 1114.2. Unless otherwise approved by the New York City Department of Environmental Protection, the storage volume of the on-site stormwater disposal system shall accommodate the total stormwater volume calculated in this section. The stormwater volume shall be calculated as follows:
Compute the runoff rate using Equations 11-1 and 11-2.
Calculate the outflow rate due to infiltration, in cubic feet per second, using Equation 11-3.
Calculate the outflow rate, in cubic feet per second per acre, of imperviousness using Equation 11-4.
Calculate the duration of the design storm in minutes using Equation 11-5.
Calculate the maximum required retention volume using Equation 11-6.
Qinf = (FAmin × isoil)/43,200
Equation 11-3
Where:
Qinf
=
outflow rate due to infiltration in cubic feet per second
FAmin
=
minimum footprint or surface area of the stormwater disposal system
isoil
=
soil infiltration rate in inches per hour
Qo = CWT × i × AT
Equation 11-4
Where:
Qo
=
the average outflow rate in cubic feet per second during the rainfall event
CWT
=
the weighted runoff coefficient for the tributary area
i
=
the average rainfall intensity in inches per hour for the event
AT
=
the area tributary to the detention facility in acres
tV = 0.27 × (CWT × At /QDRR)0.5 — 15
Equation 11-5
Where:
tV
=
the duration of the storm in minutes, with a 10 year return frequency, requiring the maximum detention volume with a variable outflow
CWT
=
the weighted runoff coefficient for the area tributary to the detention facility
At
=
the area tributary to the detention facility in square feet
QDRR
=
the detention facility maximum release rate in cubic feet per second
VV = (0.19 × CWT × At /(tV + 15) — 40 × QDRR) × tV
Equation 11-6
Where:
VV
=
the maximum required detention volume in cubic feet with a variable outflow
CWT
=
the weighted runoff coefficient for the area tributary to the detention facility
At
=
the area tributary to the detention facility in square feet
tV
=
the duration of the storm in minutes, with a 10 year return frequency, requiring the maximum detention volume with a variable outflow
QDRR
=
the detention facility maximum release rate in cubic feet per second
1114.4 Required Components
On-site stormwater disposal systems shall be designed to provide adequate storage, support the use at the surface, and allow for operation and required maintenance. Systems shall be constructed with all necessary components and materials required by the manufacturers specifications. Drywell design shall incorporate a grit chamber, and where required, a sand column constructed in accordance with Figures 1114.4(1) and 1114.4(2), respectively.
All drywells shall contain a grit chamber as part of the drywell system. Grit chambers shall be constructed in accordance with the following requirements:
Solid access cover with a minimum diameter of 15 inches (381 mm).
Grit chamber designed to support the maximum anticipated load.
Outlet invert elevation shall be a minimum of 1 inch (25 mm) lower than the lowest inlet elevation.
The sump shall be a minimum of 18 inches (450 mm) or two times the largest inlet pipe diameter, whichever is greater, as measured to the outlet invert elevation.
The interior dimensions shall be a minimum of 18 inches (450 mm) or four times the largest inlet pipe diameter whichever is greater.
1114.4.2 Reserved
1114.5 On-Site Stormwater Disposal System Installation
On-site stormwater disposal systems shall be installed in accordance the manufacturer's recommendations and shall conform to Sections 1114.5.1 through 1114.5.3.
1114.5.1 Support of Excavation
When an on-site stormwater disposal system installation requires an excavation deeper than 5 feet (1524 mm), the sides of the excavation shall be protected and maintained in accordance with Section 3304.4 of the New York City Building Code.
1114.5.2 Sand Column Installation
Where the installation of an on-site stormwater disposal system requires the installation of a sand column, measures shall be taken to ensure the sand column is installed without contamination by impervious materials.
