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

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:

1. Compute the runoff rate using Equations 11-1 and 11-2.
2. Calculate the outflow rate due to infiltration, in cubic feet per second, using Equation 11-3.
3. Calculate the outflow rate, in cubic feet per second per acre, of imperviousness using Equation 11-4.
4. Calculate the duration of the design storm in minutes using Equation 11-5.
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