Current through Register Vol. 56, No. 18, September 16, 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.
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
Click here to view table.
TABLE 7.2
MANNING'S ROUGHNESS COEFFICIENTS
Click here to view table.
TABLE 7.3
CUMULATIVE AND INCREMENTAL RAINFALL DISTRIBUTIONS FOR THE
WATER QUALITY STORM
Click
here to view image.
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.
