Water quality treatment is provided for all proposed motor vehicle surfaces by the surface infiltration basins. A surface infiltration basin is considered green infrastructure and provides 80% TSS Removal. The basins will have a six inch sand layer and a maximum water quality storm depth of 0.61 and 0.70 feet which is below the maximum 2 foot depth allowed by the NJDEP BMP manual. The individual drainage areas to the systems for each lot are less than 2.5 acres and therefore meet the small scale requirements. Roof runoff which does not require water quality treatment, will be conveyed directly to the subsurface system.
Water quantity will be met by meeting the runoff reduction requirements to the point of analysis as demonstrated in the table below and the HydroCAD calculations in the appendix. Note that infiltration was included in the water quantity routing as allowed by the NJDEP BMP Manual. The roof runoff does not require pretreatment.
POA
| Storm Event | Existing Peak Runoff (cfs) | Required Reduction (%) | Allowable Peak Runoff (cfs) | Proposed Peak Runoff (cfs) |
|---|
| 2 | 1.08 | 50 | 0.54 | 0.43 |
| 10 | 1.71 | 75 | 1.28 | 0.67 |
| 100 | 3.02 | 80 | 2.42 | 2.36 |
Groundwater recharge is provided by infiltrating the increase in 2 year runoff from the site.
Existing 2 year runoff – 3,958 cf
Proposed 2 year runoff (post infiltration) – 1394 cf