New Jersey Administrative Code › Title 7 - ENVIRONMENTAL PROTECTION › Chapter 8 - STORMWATER MANAGEMENT › Subchapter 5 - DESIGN AND PERFORMANCE STANDARDS FOR STORMWATER MANAGEMENT MEASURES › Section 7:8-5.7 - Calculation of stormwater runoff and groundwater recharge

Current through Register Vol. 56, No. 14, July 15, 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.