N.J. Admin. Code § 5:21-7.2
Current through Register Vol. 56, No. 9, May 6, 2024
Section 5:21-7.2 - Stormwater calculations: runoff estimation techniques(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.
2. The equation for the Rational Method is: Q[p] = C I A
Where
Q[p] = the peak runoff rate in cubic feet per second
C = the runoff coefficient
I = the average rainfall intensity in inches per hour occurring at the time of concentration t[c] in minutes
A = the size of the drainage area in acres
i. Typical C values for 100-year frequency storm events appear in Table 7.1.ii. The Rational Method is most accurate when dealing with uniform drainage areas. Design engineers may divide nonuniform drainage areas into "uniform" sub-drainage areas and calculate the runoff from each of these areas separately, or they may use the weighted average technique for a composite drainage area. Design engineers also may use runoff coefficients from the following sources, incorporated herein by reference: (1)HEC-22 Urban Drainage Design Manual, Second Edition FHWA-NHI-01-021, August 2001, U.S. Department of Transportation, Federal Highway Administration, as supplemented or amended to date.(2) New Jersey Department of Transportation (NJ DOT) Roadway Design Manual, November 2001, as revised through March 28, 2003.3. Design engineers may estimate time of concentration (t[c]) with Figure 7.1, Time of Concentration nomograph, from Roadway Design Manual, NJ DOT, November 2001, as revised through March 28, 2003. Use of this figure is limited to the design of storm sewer systems. For other purposes, design engineers shall use the procedures outlined in Chapter 3 of Urban Hydrology for Small Watersheds, Technical Release No. 55 (TR-55), U.S. Department of Agriculture, NRCS. FIGURE 7.1
TIME OF CONCENTRATION
Data in image
TABLE 7.1
TYPICAL RUNOFF COEFFICIENTS (C VALUES) FOR 100 YEAR FREQUENCY STORM
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TABLE 7.2
MANNING'S ROUGHNESS COEFFICIENTS
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TABLE 7.3
CUMULATIVE AND INCREMENTAL RAINFALL DISTRIBUTIONS FOR THE WATER QUALITY STORM
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4. The National Engineering Handbook, Part 630 (Hydrology) and Part 650 (Engineering Field Handbook) also may be used.5. When using the Rational Method, rainfall intensity as a function of duration and storm frequency shall be based upon Figure 7.2, Rainfall Intensity Curves, below and/or local rainfall frequency data, where available, for the two-, 10-, 25-, and 100-year storms. Design engineers shall use the Cumulative and Incremental Rainfall Distributions in Table 7.3 for the water quality storm. Figure 7.2 shows rainfall intensity curves for Trenton, New Jersey. Design engineers may use this information for other parts of the State or they may substitute local rainfall frequency data, when available. More current data for Trenton and other areas of the State may be obtained from the National Oceanic and Atmospheric Administration's (NOAA) National Weather Service, which is part of the U.S. Department of Commerce. See http://www.nws.noaa.gov/ohd/hdsc. In all instances, design engineers shall use a minimum time of concentration of 10 minutes. For storm sewer design, a 10-year to 25-year storm frequency consistent with localized circumstances should be considered as a minimum, unless special circumstances are involved such as inadequate downstream stormwater facilities, lack of positive overland relief, or evidence of local flooding. In such special circumstances, design engineers shall design facilities to accommodate, as a minimum, the following storm frequencies: i. Ten-year storm for storm drain systems where excess flow, up to the 100-year storm, can continue downgrade in the street and not exceed the gutter capacity. Also, 10-year storms shall be used at low points in storm drain systems with overland relief that is routed through the stormwater quantity control structure.ii. Twenty-five-year storm where flow in a storm drain is totally carried by pipe when conditions under (c)5i above do not apply provided all overland relief up to the 100-year storm is routed through the stormwater quantity control structure.iii. Twenty-five-year storm for culvert design where the culvert will be located in streams shown on the New Jersey State Atlas or the United States Coast and Geodetic Survey maps. Culverts with an upstream drainage area of 50 acres or more shall be designed to accommodate a 100-year frequency storm in accordance with Flood Hazard Area Control Regulations, N.J.A.C. 7:13-2.16.iv. Twenty-five-year storms for open channels where the upstream drainage area is less than 50 acres. When the upstream drainage area is 50 acres or more, design engineers shall design open channels to accommodate the 100-year storm in accordance with Flood Hazard Area Control Regulations, N.J.A.C. 7:13-2.16. Data in image
6. The size of the drainage area shall include onsite and offsite lands contributing to the design point.7. Computer software adaptations of the Rational Method or the NRCS's TR-55 are acceptable, provided their data and graphic printout allow review and evaluation. (d) Design engineers shall use a consistent method to calculate peak rate of runoff and volume when computing runoff hydrographs. If TR-55, TR-20, HEC-HMS, or another recognized method is used to calculate peak rate of runoff, then the same method shall be used to determine volume. If the Rational Method is used for peak flow calculations, design engineers shall use the Modified Rational Method to calculate peak volume to be used for basin routing. 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 maximum drainage area of 20 acres shall be used for the Modified Rational Method.(e) In computing stormwater runoff from all design storms, the design engineer shall consider the relative stormwater runoff rates and/or volumes from 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 NRCS TR-55, Urban Hydrology for Small Watersheds or other approved methods may be employed.N.J. Admin. Code § 5:21-7.2
Administrative correction.
See: 29 N.J.R. 1296(a).
Amended by R.1999 d.374, effective 11/1/1999 (operative May 1, 2000).
See: 31 N.J.R. 477(a), 31 N.J.R. 3259(a).
In (b), substituted "size" for "necessity for, and sizing" in the second sentence; in (c), added a third sentence in 2i, and inserted new third and fourth sentences in the introductory paragraph of 4; in (d), inserted "when computing runoff hydrographs" at the end of the first sentence; and in Table 7.1, added a reference to Minor Streams.
Administrative correction.
See: 32 N.J.R. 684(b).
Amended by R.2004 d.35, effective 1/20/2004.
See: 35 N.J.R. 3981(a), 36 N.J.R. 447(a).
In (c)3, amended Figure 7.1.
Administrative correction.
See: 36 N.J.R. 1751(b).
Amended by R.2005 d.56, effective 2/7/2005.
See: 36 N.J.R. 4025(a), 37 N.J.R. 481(c).
Rewrote the section.
Amended by R.2007 d.177, effective 6/4/2007.
See: 38 N.J.R. 3698(a), 39 N.J.R. 2234(a).
Added (c)1vi.