The design of all stormwater management facilities shall incorporate
sound engineering judgment, principles and practices. The Borough
reserves the right to disapprove any design that would result in the
creation or continuation adverse hydrologic or hydraulic conditions.
The following general requirements shall apply to all regulated activities
within the Borough:
A. Impervious area.
(1) The measurement of impervious areas shall include all of the impervious
areas in the total proposed development, even if development is to
take place in phases.
(2) For development taking place in phases, the total proposed impervious
area within the SWM site plan must be used in determining conformance
with this chapter.
(3) For projects that add impervious area to a parcel, the total impervious area on the parcel is subject to the requirements of this chapter; except that the volume controls in §
136-17 and the peak rate controls of §
136-18 do not need to be retrofitted to existing impervious areas that are not being altered by the proposed regulated activity.
B. All calculations, assumptions, criteria and references used in the
design of the SWM site plan shall be provided.
C. Computations for determining stormwater runoff, and for the design
of stormwater management facilities, shall be based upon the NRCS
Soil-Cover-Complex Method (Technical Release 554 or Technical Release
20, as amended), or the Modified Rational Method. NRCS methods shall
be acceptable for watershed analyses of all sizes. The Modified Rational
Method shall be acceptable for watersheds up to 25 acres in size.
D. Precipitation intensities utilized for Rational Method runoff computations
shall be based on mean precipitation frequency estimates (Duration
equals 60 minutes) published in the Precipitation-Frequency Atlas
of the United States, Atlas 14, Volume 2, U.S. Department of Commerce,
National Oceanic and Atmospheric Administration (NOAA), National Weather
Service, Hydrometeorological Design Studies Center, Silver Spring,
Maryland 20910. NOAA's Atlas 145 can be accessed at http://hdsc.nws.noaa.gov/hdsc/pfds/.
E. Precipitation depths utilized for NRCS Method runoff computations
shall be based on mean precipitation frequency estimates (Duration
equals 24 hours) published in the Precipitation-Frequency Atlas of
the United States, Atlas 14, Volume 2, U.S. Department of Commerce,
National Oceanic and Atmospheric Administration (NOAA), National Weather
Service, Hydrometeorological Design Studies Center, Silver Spring,
Maryland, 20910. NOAA's Atlas 145 can be accessed at http://hdsc.nws.noaa.gov/hdsc/pfds/.
F. Runoff Curve Numbers (CN) utilized for runoff computations based
on the Soil-Cover-Complex Method shall be obtained from the NRCS TR-55
Manual 4.
G. Runoff coefficients (c) utilized for runoff computations based on
the Modified Rational Method shall be obtained from a source that
is widely accepted within the industry.
H. The stormwater management plan shall consider and provide provisions
for management of all stormwater runoff affecting the development
site, including safe conveyance of the 100-year design storm runoff
from offsite watersheds, if any.
I. Storm sewers, swales and other stormwater conveyance structures shall
be designed to convey post-development runoff from a twenty-five-year
design storm with adequate freeboard. Storm sewers shall not surcharge
above inlet grates. Six inches of freeboard shall be provided in swales
carrying less than 15 cfs. One foot of freeboard shall be provided
for in drainageways designed to carry flows of 15 cfs or greater.
J. The minimum pipe diameter permitted for use in storm sewer systems
and/or stormwater carrying culverts dedicated to the Borough shall
be 15 inches.
K. Stormwater drainage systems shall be provided in order to permit
unimpeded flow along natural watercourses.
L. Stormwater flows onto adjacent property shall not be created, increased,
concentrated or relocated without written consent by the adjacent
property owner(s). Such stormwater flows shall be subject to the requirements
of this chapter.
M. If existing diffused drainage discharge will be concentrated in the
post development condition and discharged onto an adjacent property,
the applicant must document that adequate downstream conveyance facilities
exist to safely transport the concentrated discharge, or otherwise
prove that no erosion, sedimentation, flooding or other harm will
result from the concentrated discharge.
N. Any stormwater management facilities regulated by this chapter that
would be located in or adjacent to waters of this Commonwealth or
wetlands shall be subject to approval by DEP and, if applicable, the
U.S. Army Corps of Engineers.
O. When there is a question whether wetlands or other regulated resources
may exist, it is the responsibility of the applicant or his agent
to demonstrate to the applicable regulatory agencies that the lands
or features in question cannot be classified as wetlands or other
regulated resources.
P. Any stormwater management facilities regulated by this chapter that
would be located in state highway rights-of-way shall be subject to
approval by the Pennsylvania Department of Transportation.
Q. A manhole, inlet or cleanout structure shall be provided at all horizontal
and vertical directional changes in storm sewer piping. They shall
install intermediate cleanout structures as required at spacing intervals
that will ease adequate maintenance of all storm sewer runs.
R. The stormwater management plan shall include calculations indicating
velocities of flow, grades, sizes and capacities of water carrying
structures, and retention and detention structures, as well as sufficient
design information to construct such facilities.
S. Proposed lots or buildings adjacent to stormwater management basins
and channels designed to convey a 100-year design storm shall have
a finished grade elevation or first floor elevation two feet above
the calculated 100-year storm event water surface elevation. First
floor and finished grade elevations shall be specified on the plan.
T. For all regulated activities, SWM BMPs shall be designed, implemented,
operated and maintained to meet the purposes and requirements of this
chapter and to meet all requirements under Title 25, Pa.Code, the
Clean Streams Law, 35 P.S. § 691.1 et seq., and the Storm
Water Management Act, 32 P.S. § 680.1 et seq. Special provisions
may be required for projects draining to, or located in, special protection
waters, which are high quality (HQ) or exceptional value (EV) by designation
in 25 Pa.Code, Chapter 93, or by statewide existing use as dictated
by DEP.