New Jersey Administrative Code › Title 5 - COMMUNITY AFFAIRS › Chapter 21 - RESIDENTIAL SITE IMPROVEMENT STANDARDS › Subchapter 7 - STORMWATER MANAGEMENT › Section 5:21-7.2 - Stormwater calculations: runoff estimation techniques

Current through Register Vol. 56, No. 14, July 15, 2024
(a) Drainage area
				stormwater management requires the determination of a watershed runoff hydrograph
				that displays the peak discharge rate and volume over time. The hydrograph shall
				compare pre-and post-development conditions. 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. For
				the purpose of calculating runoff coefficients 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 "runoff coefficient"
				applies to both the Natural Resources Conservation Service (NRCS) of the United
				States Department of Agriculture (USDA) methodology of the TR-55 program (see
				(c)1iii below) and the Rational and the Modified Rational Methods (see (c)1i and
				(c)1ii, respectively, below). Both the Rational and Modified Rational methods are
				described in "Appendix A-9 Modified Rational Method" in the Standards for Soil
				Erosion and Sediment Control in New Jersey at N.J.A.C. 2:90. A runoff coefficient or
				a groundwater recharge land cover for an existing condition may be used on all or a
				portion of a 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 immediately 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 is cultivation).
(b) Design engineers shall use the runoff
				hydrograph peak rate to determine the configuration and sizes of pipes, channels,
				and other routing or flow-control structures. They shall use the hydrograph to
				determine the size of stormwater management facilities.
(c) For the runoff peak rate of discharge
				calculation, design engineers shall have the option to choose the methodology to
				estimate peak rate of discharge.
1. Design
				engineers shall calculate peak rate of runoff in accordance with the following
				procedures and methods, incorporated herein by reference:
i. For relatively small drainage areas of up to
				one-half square mile (320 acres), the peak rate of runoff may be calculated by the
				Rational Method, its derivatives, or the referenced methods that follow.
ii. Where the project necessitates reductions in
				the rate of runoff or the calculation of runoff volume in accordance with
				N.J.A.C.
				5:21-7.5, the Modified Rational Method must be
				used. The use of the Modified Rational Method is limited to drainage areas of 20
				acres or less.
iii. NRCS's
Urban
				Hydrology for Small Watersheds, Technical Release No. 55 (TR-55)
.
iv. NRCS's
Computer Program for Project
				Formulation--Hydrology, Technical Release No. 20 (TR-20).
v.
HEC-HMS Hydrologic Modeling
				System
, version 2.2, May 2003, Hydraulic Engineering Center, U.S. Army
				Corps of Engineers, used in appropriate conditions with appropriate
				values.
vi. Runoff calculations derived
				from NRCS methods (TR-20 and TR55) shall be done in accordance with NRCS New Jersey
				Bulletin No. NJ210-3-1, September 8, 2003 (application of the Delmarva unit
				hydrograph in the coastal plain region of New Jersey) and NRCS New Jersey Bulletin
				No. NJ210-4-1, September 8, 2004 (average county rainfall data), incorporated herein
				by reference. The coastal plain region is shown on the map below. New Jersey
				Bulletin No. NJ210-3-1 from the NRCS calls for the use of the Delmarva hydrograph
				only in coastal plain areas that have a flat topography (average watershed slope
				less than five percent), low relief, and significant surface storage in swales and
				depressions. (For more information on NRCS methods in New Jersey see:
http://www.nj.nrcs.usda.gov/
.)
Data in
				  image
Source: NRCS, NJ Supplement, Dimensionless Unit Hydrograph,
				  September 2003.