Stormwater runoff shall be calculated in accordance with the
following:
A. The design engineer shall calculate runoff using one of the following
methods:
(1) The USDA Natural Resources Conservation Service (NRCS) methodology,
including the NRCS Runoff Equation and Dimensionless Unit Hydrograph,
as described in the NRCS National Engineering Handbook Section 4 —
Hydrology and Technical Release 55 — Urban Hydrology for Small
Watersheds; or
(2) The Rational Method for peak flow and the Modified Rational Method
for hydrograph computations.
B. For the purpose of calculating runoff coefficients and groundwater recharge, there is a presumption that the preconstruction condition of a site or portion thereof is a wooded land use with good hydrologic condition. The term "runoff coefficient" applies to both the NRCS methodology at §
330-55A(1) and the Rational and Modified Rational Methods §
330-55A(2). A runoff coefficient 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).
C. In computing preconstruction 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 preconstruction stormwater runoff rates and volumes.
D. 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 and other methods may be employed.
E. 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.
Groundwater recharge may be calculated in accordance with the
following: The New Jersey Geological Survey Report GSR-32 A Method
for Evaluating Ground-Water 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 http://www.state.nj.us/dep/njgs/; or at New Jersey
Geological Survey, 29 Arctic Parkway, P.O. Box 427, Trenton, New Jersey
08625-0427; 609-984-6587.