Current through Register Vol. 56, No. 18, September 16, 2024
(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;
and
2. 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 |
