A.
In complying with the design and performance standards in Article IV, the design engineer shall calculate stormwater runoff using one of the following methods:
(1)
The USDA natural resources conservation service (NRCS)
methodology, including the NRCS Runoff Equation, NRCS Dimensionless Unit Hydrograph,
and appropriate NRCS twenty-four-hour design storm, as described in the current
NRCS National Engineering Handbook Part 630 - Hydrology, and the current Technical
Release 55 - Urban Hydrology for Small Watersheds or superseding document;
or
(2)
The Rational Method for peak stormwater runoff rate calculations
and the Modified Rational Method for stormwater runoff hydrograph calculations.
Use of the Rational Method and Modified Rational Method are limited to drainage
areas of 20 acres or fewer. Neither the Rational Method nor Modified Rational
Method shall be used to calculate runoff volumes for groundwater recharge
or stormwater runoff infiltration purposes.
B.
When selecting or calculating runoff coefficients for
pre-developed project site conditions using any of the above methods, the
project site's land cover shall be assumed to be woods. However, another land
cover may be used to calculate runoff coefficients if:
(1)
Such land cover has existed at the site or portion thereof
without interruption for at least five years immediately prior to the time
of application; and
(2)
The design engineer can document the character and extent
of such land cover through the use of photographs, affidavits, and/or other
acceptable land use records. If more than one land cover other than woods
has existed on the site during the five years immediately prior to the time
of application, the land cover with the lowest runoff potential (including
woods) shall be used for the computations. All pre-developed land covers shall
be assumed to be in good hydrologic condition and, if cultivated, shall be
assumed to have applied appropriate conservation practices.
C.
In calculating pre-developed site stormwater runoff,
the design engineer shall include the effects of all land features and structures,
such as ponds, wetlands, depressions, hedgerows and culverts, that reduce
pre-developed site stormwater runoff rates and/or volumes.
D.
In calculating stormwater runoff using the NRCS methodology,
the design engineer shall use appropriate twenty-four-hour rainfall depths
as developed for the project site by the National Oceanic and Atmospheric
Administration.
E.
In calculating stormwater runoff using the NRCS methodology,
the design engineer shall separately calculate and then combine the runoff
volumes from pervious and directly connected impervious surfaces within a
drainage area.
F.
Calculation of stormwater runoff from unconnected impervious
surfaces shall be based, as applicable, upon the two-step methodology as described
in the Department's current Stormwater Best Management Practices Manual or
the NRCS methodology described in the current Technical Release 55 - Urban
Hydrology for Small Watersheds.
A.
In complying with the design and performance standards in Article IV, the design engineer may calculate groundwater recharge in accordance with the New Jersey Groundwater Recharge Spreadsheet (NJGRS) computer program as described in the Department's current Stormwater Best Management Practices Manual. Alternative groundwater recharge calculation methods may be used upon approval by the Municipal engineer.
B.
In complying with the design and performance standards in Article IV, the design engineer shall calculate stormwater runoff infiltration volumes in accordance with the USDA Natural Resources Conservation Service (NRCS) methodology, including the NRCS Runoff Equation, as described in the current NRCS National Engineering Handbook Part 630 - Hydrology and the current Technical Release 55 - Urban Hydrology for Small Watersheds. In addition, the design engineer shall use appropriate two-year, twenty-four-hour rainfall depths as developed for the project site by the National Oceanic and Atmospheric Administration.
C.
When selecting or calculating runoff coefficients for
pre-developed project site conditions for groundwater recharge or stormwater
runoff infiltration calculations, the project site's land cover shall be assumed
to be woods. However, another land cover may be used to calculate runoff coefficients
if:
(1)
Such land cover has existed at the site or portion thereof
without interruption for at least five years immediately prior to the time
of application; and
(2)
The design engineer can document the character and extent
of such land cover through the use of photographs, affidavits, and/or other
acceptable land use records. If more than one land cover other than woods
has existed on the site during the five years immediately prior to the time
of application, the land cover with the lowest runoff potential (including
woods) shall be used for the computations. All pre-developed land covers shall
be assumed to be in good hydrologic condition and, if cultivated, shall be
assumed to have conservation treatment.
