The requirements of this section apply to all watersheds. In the event that individual watershed volume requirements listed in §§
134-15,
134-16, and
134-17 of this chapter result in a required volume that differs from the volume-control requirements of this section, BMP design must comply with the greater volume control requirement, as applicable. The green infrastructure and low-impact development practices provided in the BMP Manual shall be utilized for all regulated activities wherever possible. Water volume controls shall be implemented using the Design Storm Method in Subsection A or the Simplified Method in Subsection B below. For regulated activity areas of one acre or less that do not require hydrologic routing to design the stormwater facilities, this part establishes no preference for either methodology; therefore, the applicant may select either methodology on the basis of economic considerations, the intrinsic limitations on applicability of the analytical procedures associated with each methodology and other factors.
A. The Design Storm Method (CG-1 in the BMP Manual) is applicable to
any size of regulated activity. This method requires detailed modeling
based on site conditions.
(1) Do not increase the post-development total runoff volume for all
storms equal to or less than the two-year, twenty-four-hour duration
precipitation.
(2) For modeling purposes:
(a)
Existing (predevelopment) nonforested pervious areas must be
considered meadow in good condition.
(b)
One hundred percent of existing impervious area, when present,
shall be considered meadow in good condition in the model for existing
conditions.
B. The Simplified Method (CG-2 in the BMP Manual) provided below is
independent of site conditions and shall be used if the Design Storm
Method is not followed. This method is not applicable to regulated
activities of one acre or greater or for projects that require design
of stormwater storage facilities. For new impervious surfaces:
(1) Stormwater facilities shall capture at least the first two inches
of runoff from all new impervious surfaces.
(2) At least the first one inch of runoff from new impervious surfaces
shall be permanently removed from the runoff flow, i.e., it shall
not be released into the surface waters of this commonwealth. Removal
options include reuse, evaporation, transpiration, and infiltration.
(3) Wherever possible, infiltration facilities shall be designed to accommodate
infiltration of the entire permanently removed runoff; however, in
all cases, at least the first 0.5 inch of the permanently removed
runoff must be infiltrated.
(4) This method is exempt from the requirements of §
134-12, Stormwater management districts - peak rate control.
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 shall use generally accepted calculation
technique that is based on the NRCS Soil Cover Complex Method. Table
134-21 summarizes acceptable computation methods. Method must be selected
by the applicant based on the individual limitations and suitability
of each method for a particular site. The Rational Method may be used
to estimate peak discharges from drainage areas that contain less
than 200 acres. The Rational Method is recommended for drainage areas
under 100 acres.
Table 134-21. Acceptable Computation Methodologies for Stormwater
Management Plans
|
---|
Method
|
Method Developed By
|
Applicability
|
---|
TR-20 (or commercial computer package based on TR-20)
|
USDA - NRCS
|
Applicable where use of full hydrology computer model is desirable
or necessary
|
TR-55 (or commercial computer package based on TR-55)
|
USDA - NRCS
|
Applicable for land development plans within limitations described
in TR-55
|
HEC - 1, HEC-HMS
|
U.S. Army Corps of Engineers
|
Applicable where use of full hydrologic computer model is desirable
or necessary
|
PSRM
|
Penn State University
|
Applicable where use of a hydrologic computer model is desirable
or necessary simpler than TR-20 or HEC-1.
|
Rational Method (or commercial computer package based on Rational
Method)
|
Emil Kuiching (1889)
|
For sites less than 200 acres or as approved by the Municipal
Engineer
|
Other methods
|
Various
|
Other computation methodologies approved by the Municipal Engineer
|
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 according to the National Oceanic
and Atmospheric Administration (NOAA) Atlas 14 rain data corresponding
to the Doylestown rain gauge. The SCS Type 11 rainfall curve from
NOAA is found on Figure A-1 in Appendix A of this chapter. This data may also be directly retrieved
from the NOAA Atlas 14 website: https://hdsc.nws.noaa.gov/hdsc/pfds/pfds_map_cont.html?bkmrk=pa.
If a hydrologic computer model such as PSRM or HEC-1/HEC-HMS is used
for stormwater runoff calculations, then the duration of rainfall
shall be 24 hours.
C. Runoff curve numbers (CN) for both existing and proposed conditions
to be used in the Soil Cover Complex Method shall be obtained from
Table A-4 in Appendix A of this chapter.
D. All calculations using the Rational Method shall use rainfall intensities
consistent with appropriate times of concentration for overland flow
and return periods from NOAA Atlas 14, Volume 2, Version 2.1 (Figure
A-1). Times of concentration for overland flow shall be calculated
using the methodology presented in Chapter 3 of Urban Hydrology for
Small Watersheds, NRCS, TR-55 (as amended or replaced from time to
time by NRCS). Times of concentration for channel and pipe flow shall
be computed using Manning's equation.
E. Runoff coefficients (C) for both existing and proposed conditions
for use in the Rational method shall be obtained from Table A-7 in
Appendix A of this chapter.
F. For the purposes of existing conditions flow rate determination,
undeveloped land and existing impervious surfaces shall be considered
as meadow in good condition, unless the natural ground cover generates
a lower curve number (CN) or rational C value (e.g., forest), as listed
in Table A-4 or A-7 in Appendix A of this chapter. Wooded areas shall use a ground cover
of woods in good condition. An area shall be considered wooded if
there is a contiguous canopy of trees existing over an area of 1/4
acre or more.
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 A-8 in Appendix
A of this chapter.
H. 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 accepted by the Township.
I. The design of any stormwater management facilities intended to meet
the performance standards of this chapter shall be verified by routing
the design storm hydrograph through these facilities using the Storage
Indication Method. The design storm hydrograph shall be computed using
a calculation method that produces a full hydrograph.
J. The Township has the authority to require that computed existing
runoff rates be reconciled with field observations and conditions.
If the design professional engineer can substantiate through actual
physical calibration that more appropriate runoff and time-of-concentration
values should be utilized at a particular site, then appropriate variations
may be made upon review and recommendations of the Municipal Engineer.
Calibration shall require detailed gauge and rainfall data for the
particular site in question.
K. Township reserves the right to require stormwater management calculations
and analyses related to existing downstream drainage facilities, in
order to ascertain whether the stormwater management design for the
regulated activity adversely effects existing downstream facilities
by increasing flooding potential and/or frequency from the predevelopment
condition.