N.J. Admin. Code § 7:8-5.7
Current through Register Vol. 56, No. 24, December 18, 2024
Section 7:8-5.7 - Calculation of stormwater runoff and groundwater recharge(a) Stormwater runoff shall be calculated in accordance with the following: 1. The design engineer shall calculate runoff using the USDA Natural Resources Conservation Service (NRCS) methodology, including the NRCS Runoff Equation and Dimensionless Unit Hydrograph, as described in Chapters 7, 9, 10, 15, and 16, Part 630, Hydrology National Engineering Handbook, incorporated herein by reference, as amended and supplemented. This methodology is additionally described in Technical Release 55--Urban Hydrology for Small Watersheds (TR-55), dated June 1986, incorporated herein by reference, as amended and supplemented. Information regarding the methodology is available from the Natural Resources Conservation Service website at http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb1044171.pdf or at United States Department of Agriculture Natural Resources Conservation Service, 220 Davison Avenue, Somerset, New Jersey 08873.2. For the purpose of calculating curve numbers and groundwater recharge, there is a presumption that the pre-construction condition of a site or portion thereof is a wooded land use with good hydrologic condition. The term "curve number" applies to the NRCS methodology at N.J.A.C. 7:8-5.6(a)1 i. A curve number or a groundwater recharge land cover for an existing condition may be used on all or a portion of the site if the design engineer verifies that the hydrologic condition has existed on the site or portion of the site for at least five years without interruption prior to the time of application. If more than one land cover has existed on the site during the five years immediately prior to the time of application, the land cover with the lowest runoff potential shall be used for the computations. In addition, there is the presumption that the site is in good hydrologic condition (if the land use type is pasture, lawn, or park), with good cover (if the land use type is woods), or with good hydrologic condition and conservation treatment (if the land use type is cultivation).3. In computing pre-construction stormwater runoff, the design engineer shall account for all significant land features and structures, such as ponds, wetlands, depressions, hedgerows, or culverts, that may reduce pre-construction stormwater runoff rates and volumes.4. In computing stormwater runoff from all design storms, the design engineer shall consider the relative stormwater runoff rates and/or volumes of pervious and impervious surfaces separately to accurately compute the rates and volume of stormwater runoff from the site. To calculate runoff from unconnected impervious cover, urban impervious area modifications as described in the NRCS Technical Release-55, Urban Hydrology for Small Watersheds or other methods may be employed.5. If the invert of the outlet structure of a stormwater management measure is below the flood hazard design flood elevation as defined at N.J.A.C. 7:13, the design engineer shall take into account the effects of tailwater in the design of structural stormwater management measures.(b) Groundwater recharge may be calculated in accordance with the following: 1. The New Jersey Geological Survey Report GSR-32 A Method for Evaluating Groundwater-Recharge Areas in New Jersey, incorporated herein by reference as amended and supplemented. Information regarding the methodology is available from the New Jersey Stormwater Best Management Practices Manual; at the New Jersey Geological and Water Survey website at http://www.nj.gov/dep/njgs; or at New Jersey Geological and Water Survey, 29 Arctic Parkway, PO Box 420 Mail Code 29-01, Trenton, NJ 08625-0420.(c) The precipitation depths of the current two-, 10-, and 100-year storm events shall be determined by multiplying the values determined in accordance with (c)1 and 2 below: 1. The applicant shall utilize the National Oceanographic and Atmospheric Administration (NOAA), National Weather Service's Atlas 14 Point Precipitation Frequency Estimates: NJ, in accordance with the location(s) of the drainage area(s) of the site. This data is available at: http://hdsc.nws.noaa.gov/hdsc/pfds/pfds_map_cont.html?bkmrk=nj; and2. The applicant shall utilize Table 5-5 below, which sets forth the applicable multiplier for the drainage area(s) of the site, in accordance with the county or counties where the drainage area(s) of the site is located. Where the major development lies in more than one county, the precipitation values shall be adjusted according to the percentage of the drainage area in each county. Alternately, separate rainfall totals can be developed for each county using the values in Table 5-5 below. Table 5-5: Current Precipitation Adjustment Factors
| County | Current Precipitation Adjustment Factors 2-Year Design Storm 10-Year Design Storm 100-Year Design Storm |
| Atlantic | 1.01 | 1.02 | 1.03 |
| Bergen | 1.01 | 1.03 | 1.06 |
| Burlington | 0.99 | 1.01 | 1.04 |
| Camden | 1.03 | 1.04 | 1.05 |
| Cape May | 1.03 | 1.03 | 1.04 |
| Cumberland | 1.03 | 1.03 | 1.01 |
| Essex | 1.01 | 1.03 | 1.06 |
| Gloucester | 1.05 | 1.06 | 1.06 |
| Hudson | 1.03 | 1.05 | 1.09 |
| Hunterdon | 1.02 | 1.05 | 1.13 |
| Mercer | 1.01 | 1.02 | 1.04 |
| Middlesex | 1.00 | 1.01 | 1.03 |
| Monmouth | 1.00 | 1.01 | 1.02 |
| Morris | 1.01 | 1.03 | 1.06 |
| Ocean | 1.00 | 1.01 | 1.03 |
| Passaic | 1.00 | 1.02 | 1.05 |
| Salem | 1.02 | 1.03 | 1.03 |
| Somerset | 1.00 | 1.03 | 1.09 |
| Sussex | 1.03 | 1.04 | 1.07 |
| Union | 1.01 | 1.03 | 1.06 |
| Warren | 1.02 | 1.07 | 1.15 |
(d) Table 5-6 below sets forth the change factors to be used in determining the projected two-, 10-, and 100-year storm events for use in this chapter, which are organized alphabetically by county. The precipitation depth of the projected two-, 10-, and 100-year storm events of a site shall be determined by multiplying the precipitation depth of the two-, 10-, and 100-year storm events determined from the National Weather Service's Atlas 14 Point Precipitation Frequency Estimates pursuant to (c)1 above, by the change factor in Table 5-6 below, in accordance with the county or counties where the drainage area(s) of the site is located. Where the major development and/or its drainage area lies in more than one county, the precipitation values shall be adjusted according to the percentage of the drainage area in each county. Alternately, separate rainfall totals can be developed for each county using the values in the table below. Table 5-6: Future Precipitation Change Factors
| County | Future Precipitation Change Factors |
| 2-Year Design Storm | 10-Year Design Storm | 100-Year Design Storm |
| Atlantic | 1.22 | 1.24 | 1.39 |
| Bergen | 1.20 | 1.23 | 1.37 |
| Burlington | 1.17 | 1.18 | 1.32 |
| Camden | 1.18 | 1.22 | 1.39 |
| Cape May | 1.21 | 1.24 | 1.32 |
| Cumberland | 1.20 | 1.21 | 1.39 |
| Essex | 1.19 | 1.22 | 1.33 |
| Gloucester | 1.19 | 1.23 | 1.41 |
| Hudson | 1.19 | 1.19 | 1.23 |
| Hunterdon | 1.19 | 1.23 | 1.42 |
| Mercer | 1.16 | 1.17 | 1.36 |
| Middlesex | 1.19 | 1.21 | 1.33 |
| Monmouth | 1.19 | 1.19 | 1.26 |
| Morris | 1.23 | 1.28 | 1.46 |
| Ocean | 1.18 | 1.19 | 1.24 |
| Passaic | 1.21 | 1.27 | 1.50 |
| Salem | 1.20 | 1.23 | 1.32 |
| Somerset | 1.19 | 1.24 | 1.48 |
| Sussex | 1.24 | 1.29 | 1.50 |
| Union | 1.20 | 1.23 | 1.35 |
| Warren | 1.20 | 1.25 | 1.37 |
N.J. Admin. Code § 7:8-5.7
Amended by 55 N.J.R. 1385(b), effective 7/17/2023