Stormwater runoff from all development sites shall be calculated
using either the Rational Method or a Soil-Cover-Complex Methodology.
A. Any stormwater runoff calculations involving drainage areas greater
than 20 acres, including on- and off-site areas, shall use generally accepted calculation technique that
is based on the NRCS Soil Cover Complex Method. Table VIII-1 below
summarizes acceptable computation methods. It is assumed that all
methods will be selected by the design professional based on the individual
limitations and suitability of each method for a particular site.
The Plan Administrator may approve the use of the Rational Method
to estimate peak discharges from drainage areas that contain less
than 20 acres.
B. All calculations consistent with this chapter using the Soil-Cover
Complex Method shall use the appropriate design rainfall depths for
the various return period storms presented in Table B-1 in Appendix
B of this chapter. If a hydrologic computer model such as PSRM or HEC-1 is
used for stormwater runoff calculations, then the duration of rainfall
shall be 24 hours. The NRCS's curve, which is shown in Figure B-1
in Appendix B of this chapter, shall be used for the rainfall distribution.
C. For the purposes of predevelopment flow-rate determination, undeveloped
land shall be considered as "meadow" conditions, unless the natural
ground cover generates a lower curve number or rational "C" value
(i.e., forest).
D. All calculations using the Rational Method shall use rainfall intensities
consistent with appropriate times of concentration for overland flow
and return periods from the design storm curves from Pennsylvania
Department of Transportation Design Rainfall Curves (1986) (Figure
B-2). Times of concentration for overland flow shall be calculated
using the methodology present in Chapter 3 of Urban Hydrology for
Small Watersheds, NRCS TR-55 (as amended or replaced from time to
time by the NRCS). Times of concentration for channel and pipe flow
shall be computed using Manning's equation.
E. Runoff curve numbers (CN) for both existing and proposed conditions
to be used in the Soil-Cover Complex Method shall be obtained from
Table B-2 in Appendix B of this chapter.
Table VIII-1
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Acceptable Computation Methodologies for Stormwater Management
Plans
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Method
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Method Developed By
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Applicability
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TR-20 (or commercial package based on TR-20)
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USDA NRCS
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Applicable where use of full hydrology computer model is desirable
or necessary
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TR-55 (or commercial computer package based on TR-55)
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USDA NRCS
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Applicable for land development plans within limitations described
in TR-55
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HEC-1
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U.S. Army Corps of Engineers
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Applicable where use of full hydrologic computer is desirable
or necessary
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PSRM
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Penn State University
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Applicable where use of a hydrologic computer model is desirable
or necessary; simpler than TR-20 or HEC-1
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Rational Method (or commercial computer package based on Rational
Method)
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Emil Kuichling (1889)
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For sites less than 200 acres, or as approved by the Plan Administrator
and City Engineer
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Other methods
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Varies
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Other computation methodologies approved by the Plan Administrator
and City Engineer
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F. Runoff coefficients (c) for both existing and proposed conditions
for use in the Rational Method shall be obtained from Table B-3 in
Appendix B of this chapter.
G. Where uniform flow is anticipated, the Manning equation shall be
used for hydraulic computations and to determine the capacity of open
channels, pipes and storm sewers. Values for Manning's roughness coefficient
(n) shall be consistent with Table B-4 in Appendix B of this chapter.
H. The design of any stormwater detention facilities intended to meet
the performed standards of this chapter shall be verified by routing
the design storm hydrograph through these facilities using the Storage-Indication
Method. For drainage areas greater than 20 acres in size, the design
storm hydrograph shall be computed using a calculation method that
produces a full hydrograph. The City may approve the use of any generally
accepted full hydrograph approximation technique shall use a total
runoff volume that is consistent with the volume from a method that
produces a full hydrograph.
I. Outlet structures for stormwater management facilities shall be designed
to meet the performance standards of this chapter using any generally
accepted hydraulic analysis technique or method.
The methodology for determining required stormwater controls
for a regulated activity is shown in Figure VIII-1 and outlined below.
A. Compute:
(1) Predevelopment hydrograph at the site discharge point for the required
design storm.
(2) Post-development hydrograph at the site discharge point incorporating
any "nondetention" techniques such as pervious areas, swales, infiltration
trenches, etc.
Note: Hydrographs may be obtained from NRCS methods
such as TR-55, from use of the "modified" rational formulas.
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B. Compare: Post-development hydrographs with predevelopment hydrographs. If the peak rate of runoff and the shape of the hydrographs are nearly identical, stormwater management has been achieved. Detention will not be required. If not, proceed to Subsection
C.
C. Design: Detention/retention facilities, in conjunction with any nondetention
techniques, such that post-development peak rates from the site will
not exceed predevelopment levels for the required design storms.
Figure VIII-1
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Stormwater Control Determination Flow Chart
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