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# 1114.3.1 Runoff Rate

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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

Where:

Where:

The following

*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 = C_{w} × I × A |
Equation 11-1 |

Where:

Q |
= | developed flow, cubic feet per second |

C_{w} |
= | weighted runoff coefficient |

I |
= | the rainfall intensity value, 5.95 in/hr |

A |
= | the total site area, acres (ac) |

C(_{w} = 1/A)Σ(A)_{K} × C_{k } |
Equation 11-2 |

Where:

C_{w} |
= | weighted runoff coefficient |

A |
= | The total site area, acres (ac) |

A_{K} |
= | The area of each surface coverage type, acres (ac) |

C_{K} |
= | 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:

Where:

Where:

Where:

Where:

- 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.

Q(_{inf} = FA)/43,200_{min} × i_{soil} |
Equation 11-3 |

Where:

Q_{inf} |
= | outflow rate due to infiltration in cubic feet per second |

FA_{min} |
= | minimum footprint or surface area of the stormwater disposal system |

i_{soil} |
= | soil infiltration rate in inches per hour |

QC_{o} = i_{WT} × × A_{T} |
Equation 11-4 |

Where:

Q_{o } |
= | the average outflow rate in cubic feet per second during the rainfall event |

C_{WT} |
= | the weighted runoff coefficient for the tributary area |

i | = | the average rainfall intensity in inches per hour for the event |

A_{T} |
= | the area tributary to the detention facility in acres |

t = 0.27 × (C_{V} × A_{WT} /Q_{t})_{DRR}^{0.5} — 15 |
Equation 11-5 |

Where:

t_{V} |
= | the duration of the storm in minutes, with a 10 year return frequency, requiring the maximum detention volume with a variable outflow |

C_{WT} |
= | the weighted runoff coefficient for the area tributary to the detention facility |

A_{t} |
= | the area tributary to the detention facility in square feet |

Q_{DRR} |
= | the detention facility maximum release rate in cubic feet per second |

V = (0.19 × C_{V} × A_{WT} /(t_{t} + 15) — 40 × Q_{V}) × t_{DRR}_{V} |
Equation 11-6 |

Where:

V_{V} |
= | the maximum required detention volume in cubic feet with a variable outflow |

C_{WT} |
= | the weighted runoff coefficient for the area tributary to the detention facility |

A_{t} |
= | the area tributary to the detention facility in square feet |

t_{V} |
= | the duration of the storm in minutes, with a 10 year return frequency, requiring the maximum detention volume with a variable outflow |

Q_{DRR} |
= | the detention facility maximum release rate in cubic feet per second |

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