[Ord. No. 1598, 3-9-2015]
A.
Applicants proposing regulated activities in the Township that do
not fall under the exemption criteria shown in Section 106 shall submit
a stormwater management (SWM) site plan. The SWM criteria of this
Ordinance shall apply to the total proposed development even if development
is to take place in stages. Preparation and implementation of an approved
SWM site plan is required. No regulated activities shall commence
until the municipality issues written approval of a SWM site plan,
which demonstrates compliance with the requirements of this Ordinance.
B.
SWM site plans approved by the municipality, in accordance with Article
IV, shall be on site throughout the duration of the regulated activity.
C.
The municipality may, after consultation with the Department of Environmental
Protection (PADEP), approve measures for meeting the state water quality
requirements other than those in this Ordinance, provided that they
meet the minimum requirements of, and do not conflict with, state
law including but not limited to the Clean Streams Law.[1]
[1]
Editor's Note: See 35 P.S. § 691.1 et seq.
D.
For all regulated earth disturbance activities, erosion and sediment
(E&S) control best management practices (BMPs) shall be designed,
implemented, operated, and maintained during the regulated earth disturbance
activities (e.g., during construction) to meet the purposes and requirements
of this Ordinance and to meet all requirements under Title 25 of the
Pennsylvania Code and the Clean Streams Law. Various BMPs and their
design standards are listed in the Erosion and Sediment Pollution
Control Program Manual, No. 363-2134-008 (April 15, 2000), as amended
and updated.
E.
For all regulated activities, implementation of the volume controls
in Section 304 of this Ordinance is required.
F.
Impervious areas:
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 stages.
2.
For development taking place in stages, the entire development plan
must be used in determining conformance with this Ordinance.
3.
For projects that add impervious area to a parcel, the total impervious
area on the parcel is subject to the requirements of this Ordinance.
G.
Stormwater flows onto adjacent property shall not be created, increased,
decreased, relocated, or otherwise altered without written notification
of the adjacent property owner(s) from the developer. Such stormwater
flows shall be subject to the requirements of this Ordinance.
H.
All regulated activities shall include such measures as necessary
to:
1.
Protect health, safety, and property;
2.
Meet the water quality goals of this Ordinance by implementing measures
to:
3.
To the maximum extent practicable, incorporate the techniques for
low-impact development practices (e.g., protecting existing trees,
reducing area of impervious surface, cluster development, and protecting
open space) described in the Pennsylvania Stormwater Best Management
Practices Manual, Pennsylvania Department of Environmental Protection
(PADEP) No. 363-0300002 (2006). See Ordinance Appendix B for a summary
description.[3]
[3]
Editor's Note: Appendix B is included as an attachment to
this appendix.
I.
Infiltration BMPs should be spread out, made as shallow as practicable,
and located to maximize the use of natural on-site infiltration features
while still meeting the other requirements of this Ordinance.
J.
The design of all facilities over karst shall include an evaluation
of measures to minimize the risk of adverse effects.
K.
Storage facilities should completely drain both the volume control
and rate control capacities over a period of time not less than 24
and not more than 72 hours from the end of the design storm.
L.
The design storm volumes to be used in the analysis of peak rates
of discharge should be obtained from the Precipitation-Frequency Atlas
of the United States, Atlas 14, Volume 2, Version 3.0, U.S. Department
of Commerce, National Oceanic and Atmospheric Administration (NOAA),
National Weather Service, Hydrometeorological Design Studies Center,
Silver Spring, Maryland. NOAA's Atlas 14 can be accessed at http://hdsc.nws.noaa.gov/hdsc/pfds/.
M.
For all regulated activities, SWM BMPs shall be designed, implemented,
operated, and maintained to meet the purposes and requirements of
this Ordinance and to meet all requirements under Title 25 of the
Pennsylvania Code, the Clean Streams Law, and the Storm Water Management
Act.[4]
[4]
Editor's Note: See 35 P.S. § 691.1 et seq. and 32 P.S.
§ 680.1 et seq., respectively.
N.
Various BMPs and their design standards are listed in the Pennsylvania
Stormwater Best Management Practices Manual (PA BMP Manual).
[Ord. No. 1598, 3-9-2015]
Approvals issued and actions taken under this Ordinance do not
relieve the applicant of the responsibility to secure required permits
or approvals for activities regulated by any other code, law, regulation
or ordinance.
[Ord. No. 1598, 3-9-2015]
Within the Sawmill Run Watershed these additive requirements
must be applied.
[Ord. No. 1135, § 1, 9-27-1993; Ord. No. 1598, 3-9-2015]
A.
Mapping of stormwater management districts. In order to implement
the provisions of the Stony Creek/Saw Mill Run Stormwater Management
Plan, the Township is hereby divided into stormwater management districts
consistent with the Stony Creek/Saw Mill Run release rate map presented
in the plan. The boundaries of the stormwater management districts
are shown on an official map which is available for inspection at
the Township office. A copy of the official map at a reduced scale
is included in Appendix A of the plan for general reference.[1]
[1]
Editor's Note: Said map is on file in the Township offices.
B.
Release rate districts. Within a given district, the postdevelopment
peak rate of storm runoff must be controlled to the stated percentage
of the predevelopment peak rate of storm runoff in order to protect
downstream watershed areas. There are numerous release rate districts
that differ in the extent to which postdevelopment runoff must be
controlled. The release rates range from fifty (50) percent to one
hundred (100) percent.
[Ord. No. 1598, 3-9-2015]
A.
Release rate criteria. Any stormwater management controls required
by this ordinance and subject to release rate criteria (50 percent
to 100 percent) shall meet the applicable release rate criteria for
each of the two-, five-, ten-, 25-, 50- and 100-year return period
runoff events consistent with the calculation methodology specified
in Section 303.3.
B.
Location of district boundaries. The exact location of the stormwater
management district boundaries as they apply to a given development
shall be determined by mapping the boundaries using the two-foot topographic
contours provided as part of the drainage plan. The district boundaries,
as originally drawn, coincide with topographic divides, or in certain
instances are drawn from the intersection of the watercourse and a
physical feature such as the confluence with another watercourse or
a potential flow obstruction (road, culvert, bridge, etc.) to the
topographic divide consistent with topography.
C.
Criteria for downstream capacity analysis. Any downstream capacity
analysis conducted in accordance with this ordinance shall use the
following criteria for determining adequacy for accepting peak flow
rates:
1.
Natural or man-made channels or swales must be able to convey the
increased runoff associated with a two-year return period event within
their banks at velocities consistent with protection of the channels
from erosion. Acceptable velocities shall be based upon criteria included
in the DEP Soil Erosion and Sedimentation Control Manual (February,
1988).
2.
Natural or man-made channels or swales must be able to convey the
increased 25-year return period runoff peak within their banks or
otherwise not create any hazards to persons or property.
3.
Culverts, bridges, storm sewers or any other facilities that must
pass or convey flows from the tributary area must have sufficient
capacity to pass or convey the increased flows associated with the
25-year return period runoff event, except for facilities located
within a designated floodplain area that must be capable of passing
or conveying the 100-year return period runoff. Any facilities that
constitute stream enclosures per DEP's Chapter 105 regulations
shall be designed to convey the 100-year return period runoff.
D.
Runoff from site located in single release rate category area. For
a proposed development site located within a single release rate category
area, the total runoff from the site shall meet the applicable release
rate criteria. For development sites with multiple points of concentrated
runoff discharge, individual drainage points may be designed for up
to a 100 percent release rate, so long as the total runoff from the
site is controlled to the applicable release rate.
E.
Runoff from site located in two or more release rate category areas.
For a proposed development site located within two (2) or more release
rate category areas, the maximum peak rate of runoff that may be discharged
at any point is limited to the predevelopment peak rate of runoff
at that point, multiplied by the applicable release rate. The control
rates shall apply regardless of any grading modifications that may
change the drainage area that discharges at a given point.
F.
Drainage of sites located partially in release rate category area
and partially in no detention area. For proposed development sites
located partially within a release rate category area and partially
within a provisional no detention area, in no event shall a significant
portion of the site area subject to the release rate control be drained
to the discharge point(s) located in the no detention area.
G.
Individual lots within a subdivision. When a site is proposed to
be subdivided for the sale and development of lots on an individual
basis, either by the subdividing party or other party, the subdividing
party shall, as part of the subdivision proposal, show all provisions
for stormwater control, in accordance with the requirements of this
ordinance.
H.
Regional or subregional detention alternatives. For certain regions
within the watershed, it may be more cost effective to provide one
control facility for an entire subarea, group of subareas or portion
of a subarea incorporating more than one development site than to
provide an individual control facility for each development site.
The initiative and funding for any regional or subregional runoff
control alternatives are the responsibility of prospective developers.
The design of any regional control basins must incorporate reasonable
development of the entire upstream watershed. The peak outflow of
a regional basin would be determined on a case-by-case basis using
the hydrologic model of the watershed consistent with protection of
the downstream watershed areas. "Hydrologic model" refers to the calibrated
Stony Creek/Saw Mill Run version of the Penn State Runoff Model as
developed for the stormwater management plan.
I.
Capacity improvements. In certain instances, primarily within the
provisional no detention and provisional 100 percent release rate
category areas, local drainage conditions may dictate more stringent
levels of runoff control than those based upon protection of the entire
watershed. In these instances, if the developer could prove that it
would be feasible to provide capacity improvements to relieve the
capacity deficiency in the local drainage network, then the capacity
improvements could be provided by the developer in lieu of development
controls on the development site. Any capacity improvements would
be designed based upon development of all areas tributary to the proposed
improvement and the capacity criteria specified in Section 303.2C.
In addition, all new development upstream of a proposed capacity improvement
shall be assumed to implement the applicable runoff controls consistent
with this ordinance, except that all new development within the entire
subarea(s) within which the proposed development site is located shall
be assumed to implement the developer's proposed discharge control,
if any.
Capacity improvements may also be provided as necessary to implement
any regional or subregional detention alternatives or to implement
a modified "no harm" option which proposes specific capacity improvements
to document the validity of a less stringent discharge control that
would not create any harm downstream.
J.
Runoff control based on minimum additional impervious cover waiver.
Any proposed regulated activity, except those defined in Sections
105.8 and 105.9, which would create 5,000 square feet or less of additional
impervious cover would be exempt from meeting the runoff control provisions
of this ordinance. For developments that are to take place in stages,
the entire development plan must be used in determining conformance
to these criteria. Additional impervious cover shall include, but
not be limited to, any roof, parking or driveway areas, and any new
streets and sidewalks constructed as a part of or for the proposed
development. Any areas that may be designed initially to be semipervious
(e.g., gravel, crushed stone, etc.) shall be considered impervious
areas for the purposes of waiver evaluations.
No waiver shall be provided for any regulated activities as
defined in Sections 105.8 and 105.9.
[Ord. No. 1598, 3-9-2015]
A.
Methods of calculating runoff. Stormwater runoff from all development
sites shall be calculated using either the Rational Method or a soil
cover complex methodology.
B.
Verification of design of detention basin. The design of any detention
basin intended to meet the requirements of this ordinance shall be
verified by routing the design storm hydrograph through the proposed
basin. For basins designed using the modified rational method technique,
the detention volume shall, at minimum, equal the volume derived from
the approximate routing process as contained in SCS Technical Release
Number 55 (TR 55).
C.
Minimum freeboard. All stormwater detention facilities shall provide
a minimum one foot freeboard above the maximum pool elevation associated
with the two- through 25-year runoff events. An emergency spillway
shall be designed to pass the 100-year runoff event with a minimum
one foot freeboard.
D.
Calculations using Soil Cover Complex Method. All calculations using
the Soil Cover Complex Method shall use the Soil Conservation Service
Type II 24-hour rainfall distribution. The 24-hour rainfall depths
for various return periods to be used consistent with this ordinance
are taken from the PennDOT Intensity-Duration-Frequency Field Manual
(May 1986) for Region 5, as follows:
|
Return Period
|
24-Hour Rainfall Depth
| |
|---|---|---|
|
2 year
|
3.3 inches
| |
|
5 year
|
4.2 inches
| |
|
10 year
|
5.0 inches
| |
|
25 year
|
5.8 inches
| |
|
50 year
|
6.4 inches
| |
|
100 year
|
7.2 inches
|
E.
Calculations using Rational Method. All calculations using the Rational
Method shall use rainfall intensities consistent with appropriate
times of concentration and return periods and the intensity-duration-frequency
curves.
F.
Runoff curve numbers. Runoff curve numbers (CNs) to be used in the
Soil Cover Complex Method shall be based upon the matrix presented
in the plan.
G.
Runoff coefficients. Runoff coefficients for use in the Rational
Method shall be based upon the table presented in the plan.
H.
Manning Equation. The Manning Equation shall be used to calculate
the capacity of watercourses. Manning 'n' values used in
the calculations shall be consistent with the table presented in the
plan. Pipe capacities shall be determined by methods acceptable to
the Township Engineer.
I.
Design of detention basins requiring dam safety permits. Any detention
basin intended to meet the requirements of this ordinance which requires
a dam safety permit from DEP shall be designed consistent with the
provisions of the Dam Safety and Encroachments Act[1] and the DEP Chapter 105 rules and regulations.
[1]
Editor's Note: See 32 P.S. § 693.1 et seq.
[Ord. No. 1598, 3-9-2015]
Volume controls will mitigate increased runoff impacts, protect
stream channel morphology, maintain groundwater recharge, and contribute
to water quality improvements. Stormwater runoff volume control methods
are based on the net change in runoff volume for the two-year storm
event.
Volume controls shall be implemented using the Design Storm
Method in Subsection A or the Simplified Method in Subsection B below.
For regulated activities equal to or less than one (1) acre, this
Ordinance establishes no preference for either methodology; therefore,
the applicant may select either methodology on the basis of economic
considerations, the intrinsic limitations of the procedures associated
with each methodology, and other factors. All regulated activities
greater than one (1) acre must use the Design Storm Method.
A.
Design Storm Method (any regulated activity): This method requires
detailed modeling based on site conditions. For modeling assumptions
refer to Section 305A.
1.
Postdevelopment total runoff should not be increased from predevelopment
total runoff for all storms equal to or less than the 2-year 24-hour
duration precipitation.
2.
The following applies in order to estimate the increased volume of
runoff for the 2-year 24-hour duration precipitation event:
To calculate the runoff volume (cubic feet) for existing site
conditions (predevelopment) and for the proposed developed site conditions
(postdevelopment), it is recommended to use the Soil Cover Complex
Method as shown on the following page. Table B-3 in Ordinance Appendix
B[1] is available to guide a qualified professional and/or
an applicant to calculate the stormwater runoff volume. The calculated
volume shall be either reused, evapotranspired, or infiltrated through
structural or nonstructural means.
|
Soil Cover Complex Method:
| |
|
Step 1: Runoff (in) = Q = (P - 0.2S) 2/(P + 0.8S) where P =
2-year Rainfall (in)
| |
|
S = (1000 / CN) - 10, the potential maximum retention (including
initial abstraction, Ia)
| |
|
Step 2: Runoff Volume (Cubic Feet) = Q x Area x 1/12Q = Runoff
(in)
| |
|
Area = SWM Area (sq ft)
|
[1]
Editor's Note: Appendix B is included as an attachment to
this appendix.
B.
Simplified Method (regulated activities less than or equal to 1 acre):
1.
Stormwater facilities shall capture the runoff volume from at least
the first two inches (2") of runoff from all new impervious surfaces.
|
Volume (cubic feet) = (2" runoff/12 inches) * impervious surface
(sq ft)
|
2.
At least the first inch (1") of runoff volume from the new impervious
surfaces shall be permanently removed from the runoff flow —
i.e., it shall not be released into the surface waters of the commonwealth.
The calculated volume shall be either reused, evapotranspired or infiltrated
through structural or nonstructural means.
|
Volume (cubic feet) = (1" runoff/12 inches) * impervious surface
(sq ft)
|
3.
Infiltration facilities should be designed to accommodate the first
half inch (0.5") of the permanently removed runoff.
4.
No more than one inch (1") of runoff volume from impervious surfaces
shall be released from the site. The release time must be over 24
to 72 hours.
C.
Stormwater control measures. The applicant must demonstrate how the
required volume is controlled through stormwater best management practices
(BMPs) which shall provide the means necessary to capture, reuse,
evaporate, transpire or infiltrate the total runoff volume.
1.
If natural resources exist on the site and a SWM site plan submission
is required for the regulated activity, the applicant shall determine
and display the total acreage of protected area where no disturbance
is proposed on the plan. The acreage of the protected area should
be subtracted from the total site area and not included in the stormwater
management site area acreage used in determining the volume controls.
|
Stormwater Management Site Area = {Total Site Area (for both
pre and post development conditions) - Protected Area}
| |
|
Natural resource areas should be calculated based upon the municipality's
own natural resource protection ordinance. If no ordinance exists,
see Table B-2 in Ordinance Appendix B[2] for guidance to assess the total protected area. For additional reference see Chapter 5, Section 5.4.1, of the PA BMP Manual.
|
[2]
Editor's Note: Appendix B is included as an attachment to
this appendix.
3.
Volume controls provided through nonstructural BMPs should be subtracted
from the required volume to determine the necessary structural BMPs.
|
Required Volume Control (ft3) - Nonstructural
Volume Control (ft3) = Structural Volume
Requirement (ft3)
|
5.
Infiltration BMPs intended to receive runoff from developed areas
shall be selected based on the suitability of soils and site conditions
(see Table B-6 in Ordinance Appendix B for a list of infiltration
BMPs).[5] Infiltration BMPs shall be constructed on soils that have
the following characteristics:
a.
A minimum soil depth of twenty-four inches (24") between the bottom
of the infiltration BMPs and the top of bedrock or seasonally high
water table.
b.
An infiltration rate sufficient to accept the additional stormwater
load and dewater completely as determined by field tests. A minimum
of 0.2 inches/hour (in/hr) should be utilized and for acceptable rates
a safety factor of 50% should be applied for design purposes (e.g.,
for soil which measured 0.4 in/hr, the BMP design should use 0.2 in/hr
to insure safe infiltration rates after construction).
c.
All open-air infiltration facilities shall be designed to completely
infiltrate runoff volume within three (3) days (72 hours) from the
start of the design storm.
[5]
Editor's Note: Appendix B is included as an attachment to
this appendix.
6.
Soils. A soils evaluation of the project site shall be required to
determine the suitability of infiltration facilities. All regulated
activities are required to perform a detailed soils evaluation by
a qualified design professional which at a minimum address soil permeability,
depth to bedrock, and subgrade stability. The general process for
designing the infiltration BMP shall be:
a.
Analyze hydrologic soil groups as well as natural and man-made features
within the site to determine general areas of suitability for infiltration
practices. In areas where development on fill material is under consideration,
conduct geotechnical investigations of subgrade stability; infiltration
may not be ruled out without conducting these tests.
b.
Provide field tests such as double ring infiltrometer or hydraulic
conductivity tests (at the level of the proposed infiltration surface)
to determine the appropriate hydraulic conductivity rate. Percolation
tests are not recommended for design purposes.
c.
Design the infiltration structure based on field-determined capacity
at the level of the proposed infiltration surface and based on the
safety factor of 50%.
d.
If on-lot infiltration structures are proposed, it must be demonstrated
to the municipality that the soils are conducive to infiltrate on
the lots identified.
e.
An impermeable liner will be required in detention basins where the
possibility of groundwater contamination exists. A detailed hydrogeologic
investigation may be required by the municipality.
[Ord. No. 1598, 3-9-2015]
A.
The following criteria shall be used for runoff calculations:
1.
For development sites not considered redevelopment, the ground cover
used to determine the existing conditions runoff volume and flow rate
shall be as follows:
a.
Wooded sites shall use a ground cover of "woods in good condition."
A site is classified as wooded if a continuous canopy of trees exists
over a 1/4 acre.
b.
The undeveloped portion of the site including agriculture, bare earth,
and fallow ground shall be considered as "meadow in good condition,"
unless the natural ground cover generates a lower curve number (CN)
or Rational "c" value (i.e., woods) as listed in Tables B-4 or B-7
in Appendix B of this Ordinance.[1]
[1]
Editor's Note: Appendix B is included as an attachment to
this appendix.
2.
For development and redevelopment sites, the ground cover used to
determine the existing conditions runoff volume and flow rate for
the developed portion of the site shall be based upon actual land
cover conditions. If the developed site contains impervious surfaces,
20 percent of the impervious surface area shall be considered meadow
in the model for existing conditions.
B.
Stormwater runoff peak discharges from all development sites with
a drainage area equal to or greater than 200 acres shall be calculated
using a generally accepted calculation technique that is based on
the NRCS Soil Cover Complex Method. Table 305.1 summarizes acceptable
computation methods. The method selected by the design professional
shall be based on the individual limitations and suitability of each
method for a particular site. The municipality may allow the use of
the Rational Method (Q=CIA) to estimate peak discharges from drainage
areas that contain less than 200 acres.
|
Q = Peak flow rate, cubic feet per second (CFS)
| |
|
C = Runoff coefficient, dependent on land use/cover
| |
|
I = Design rainfall intensity, inches per hour
| |
|
A = Drainage Area, acres.
|
C.
All calculations consistent with this ordinance 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 rain gage, seen in Table B-1 in Ordinance Appendix B.[2] The SCS Type II rainfall curve from NOAA is found on Figure
B-1 in Ordinance Appendix B. This data may also be directly retrieved
from the NOAA Atlas 14 website: hdsc.nws.noaa.gov/hdsc/pfds/orb/pa_pfds.html.
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.
|
Table 305.1
| |||
|---|---|---|---|
|
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
|
US 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 Kuichling (1889)
|
For sites less than 200 acres, or as approved by the municipality
and/or Municipal Engineer.
| |
|
Other methods
|
Varies
|
Other computation methodologies approved by the municipality
and/or Municipal Engineer.
| |
[2]
Editor's Note: Appendix B is included as an attachment to
this appendix.
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. 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.
F.
Runoff coefficients (C) for both existing and proposed conditions
for use in the Rational Method shall be consistent with Table B-4
in Ordinance Appendix B.
G.
Runoff from proposed sites graded to the subsoil will not have the
same runoff conditions as the site under existing conditions because
of soil compaction, even after top-soiling or seeding. The proposed
condition "CN" or "C" shall increase by 5% to better reflect proposed
soil conditions.
H.
The Manning Equation is preferred for one-dimensional, gradually
varied, open-channel flow. In other cases, appropriate, applicable
methods should be applied, however, early coordination with the municipality
is necessary.
I.
Outlet structures for stormwater management facilities shall be designed
to meet the performance standards of this Ordinance using the generally
accepted hydraulic analysis technique or method of the municipality.
J.
The design of any stormwater detention facilities intended to meet
the performance standards of this Ordinance shall be verified by routing
the design storm hydrograph through these facilities using the Storage-Indication
Method. For drainage areas greater than 200 acres in size, the design
storm hydrograph shall be computed using a calculation method that
produces a full hydrograph. The municipality may approve the use of
any generally accepted full hydrograph approximation technique that
shall use a total runoff volume that is consistent with the volume
from a method that produces a full hydrograph.
