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.
Where:
Where:
The following C-values shall be used for calculating a sites weighted runoff coefficient:
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) |
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 onsite 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 1114.2.1. Unless otherwise approved by the New York City Department of Environmental Protection, the storage volume of the onsite 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 detention volume using Equation 11-6.
Where:
Where:
Where:
Where:
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 detention volume using Equation 11-6.
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 |
Where:
| Qo | = | restricted flow rate in cubic feet per second per acre of imperviousness |
| Qinf | = | outflow rate due to infiltration cubic feet per second |
| Cw | = | weighted runoff coefficient for the area tributary to the stormwater system |
Where:
| t | = | duration of the storm in minutes |
| Qo | = | restricted flow rate in cubic feet per second per acre of imperviousness |
Where:
| v | = | maximum required detention volume |
| t | = | duration of the storm in minutes |
| Qo | = | restricted flow rate in cubic feet per second per acre of imperviousness |
| A | = | Area tributary to the detention facility in acres |
| Cw | = | weighted runoff coefficient for the area of tributary to the stormwater system |