A.
For all regulated activities, unless specifically exempted in § 116-27:
(1)
Preparation and implementation of an approved SWM site plan is required.
(2)
No regulated activities shall commence until the Township issues
written approval of a SWM site plan, which demonstrates compliance
with the requirements of this chapter.
(4)
The SWM site plan as approved by the Township shall be on-site throughout
the duration of the regulated activities.
B.
For all regulated earth disturbance activities, erosion and sediment control 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 chapter and to meet all requirements under Title 25 of the Pennsylvania Code (including, but not limited to Chapter 102, Erosion and Sediment Control, and the Clean Streams Law[1]). Various BMPs and their design standards are listed in
the Erosion and Sediment Pollution Control Program Manual (E&S
Manual), No. 363-2134-008 (April 15, 2000), as amended and updated.
[1]
Editor's Note: See 35 P.S. § 691.1 et seq.
C.
For all regulated activities, stormwater BMPs shall be designed,
installed, implemented, operated, and maintained to meet the purposes
and requirements of this chapter and to meet all requirements under
Title 25 of the Pennsylvania Code and the Clean Streams Law,[2] conform to the state water quality requirements, meet
all requirements under the Storm Water Management Act[3] and any more stringent requirements as determined by the
Township.
D.
The Township, after consultation with PaDEP and the Conservation
District and with their written approval, may approve measures for
meeting the state water quality requirements other than those in this
chapter, provided that they meet the minimum requirements of, and
do not conflict with state law, including, but not limited to, the
Clean Streams Law.
E.
All regulated activities shall include, to the maximum extent practicable,
measures to:
(1)
Protect health, safety, and property.
(2)
Meet the water quality goals of this chapter by implementing measures
to:
(a)
Minimize disturbance to floodplains, wetlands, natural slopes,
existing native vegetation and woodlands;
(b)
Create, maintain, or extend riparian buffers and protect existing
forested buffers;
(c)
Provide trees and woodlands adjacent to impervious areas whenever
feasible;
(d)
Minimize the creation of impervious surfaces and the degradation
of waters of the commonwealth and promote groundwater recharge;
(e)
Protect natural systems and processes (drainageways, vegetation,
soils, and sensitive areas) and maintain, as much as possible, the
natural hydrologic regime;
(f)
Incorporate natural site elements (e.g., wetlands, stream corridors,
mature forests) as design elements;
(g)
Avoid erosive flow conditions in natural flow pathways;
(h)
Minimize soil disturbance and soil compaction;
(i)
Minimize thermal impacts to waters of the commonwealth; and
(j)
Disconnect impervious surfaces by directing runoff to pervious
areas, wherever possible, and decentralize and manage stormwater at
its source.
F.
Impervious areas.
G.
If diffused flow is proposed to be concentrated and discharged onto
adjacent property(ies), 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.
H.
Stormwater drainage systems shall be provided in order to permit
unimpeded flow along natural watercourses, except as modified by stormwater
management facilities or open channels consistent with this chapter.
I.
Where watercourses traverse a development site, drainage easements
(with a minimum width of 20 feet and including the one-hundred-year
water surface) shall be provided as a condition of plan approval,
conforming to the line of such watercourse(s).
(2)
The terms of the easement shall prohibit excavation, the placing
of fill, structures, shrubbery, trees, fences, structures, and/or
any alterations that may adversely affect the flow of stormwater within
any portion of the easement without the Township's prior written
approval.
(3)
Also, maintenance, including mowing of vegetation within the easement,
may be required, except as approved by the Township.
J.
When it can be shown that, due to topographic conditions, natural
drainageways on the site cannot adequately provide for drainage, open
channels may be constructed conforming substantially to the line and
grade of such natural drainageways.
K.
Design storm volumes and precipitation intensities to be used in
the analysis of discharge or runoff shall be obtained from either
of the following sources:
(1)
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
145 can be accessed at: http://hdsc.nws.noaa.gov/hdsc/pfds.
(2)
The Pennsylvania Department of Transportation Drainage Manual, Publication
584, Chapter 7, Appendix A.
L.
Any drainage facilities required by this chapter which are located
within state highway rights-of-way and/or which may discharge into
conveyance facilities within the state highway rights-of-way shall
be subject to the prior approval of the Pennsylvania Department of
Transportation (PennDOT).
(1)
Millcreek Township will not be responsible, nor shall it accept responsibility
for any stormwater management facilities located within or discharging
into conveyance facilities within the state highway right-of-way.
M.
Stormwater management facilities and appurtenances shall be designed
and provided so as to minimize erosion in watercourses and at all
points of discharge.
N.
All natural or man-made channels or swales must be able to convey
the increased runoff associated with a two-year return period within
their banks at velocities consistent with the protection of the channels
from erosion. Acceptable velocities shall be based upon criteria set
forth in the PaDEP Soil Erosion and Sedimentation Control Manual,
and Appendix B to this chapter [Permissible Velocities for Channels],
which is incorporated herein.[4]
[4]
Editor's Note: Appendix B is included as an attachment to this chapter.
O.
The developer shall ensure that all flows in excess of a twenty-five-year
design storm sewer flow up to and including the one-hundred-year storm
discharge from the developed site shall be adequately conveyed to
the detention basin via an adequately sized storm sewer system, a
designated swale and/or channel or a combination of such facilities
in such manner as will protect against public inconvenience and injury
to property or persons.
P.
All stormwater management facilities shall be constructed in accordance with the standards and specifications set forth in Chapter 29, Code Enforcement, Part 2, Public and Private Improvements, of the Code of the Township of Millcreek, as amended, or the regulations of an authority or other governmental agency having authority over the improvement.
Q.
A developer shall submit to the Township Engineer and to the engineer
retained to review stormwater management plans a listing of all materials
proposed to be used prior to commencement of construction.
R.
Minimization of impervious surfaces and infiltration of runoff through
seepage beds, infiltration trenches, etc. are encouraged, where soil
conditions and geology permit, to reduce the size or eliminate the
need for detention facilities.
S.
Infiltration BMPs should be dispersed throughout the site, made as
shallow as practicable and located to maximize use of natural on-site
infiltration features while still meeting the other requirements of
this chapter.
T.
The design of facilities over karst shall include an evaluation and
implementation of measures to minimize adverse effects.
U.
Roof drains should not be connected to streets, storm sewers or roadside
ditches and shall not be connected to sanitary sewers.
V.
When stormwater management facilities are proposed within 1,000 feet of a downstream municipality, the developer shall notify the downstream municipality and provide to it a copy of the SWM plan, if requested, for review and comment. See also §§ 116-28B(10)(g) and 116-31A.
W.
Applicants are encouraged to use low-impact development practices
to reduce the costs of complying with the requirements of this chapter
and the state water quality requirements.
X.
Sump pump and groundwater discharges shall be directly connected
to a storm sewer.
The stormwater performance standards contained in this section
are intended to implement the standards and criteria contained in
the Erie County Stormwater Management Plan adopted and approved in
accordance with the Act. If there is any discrepancy between the provisions
of this section and the standards and criteria of the plan, or if
the County's Stormwater Management Plan is subsequently amended,
then the standards/criteria of this section shall control if they
are more stringent than those in the plan and those of the plan or
amended watershed plan shall govern if they are more stringent than
those in this chapter.
A.
Storm frequencies/rainfall values. Stormwater management facilities
on all development sites shall control the peak stormwater discharge
for the two-, ten-, twenty-five-, fifty- and one-hundred-year storm
frequencies. The twenty-four-hour total rainfall for these storm frequencies
in the Township are:
|
Return Interval
(year)
|
24-hour Rainfall Total
(inches)
| |
|---|---|---|
|
1
|
2.13
| |
|
2
|
2.56
| |
|
10
|
3.71
| |
|
25
|
4.46
| |
|
50
|
5.09
| |
|
100
|
5.76
|
B.
Calculation methodology.
(1)
All calculations shall be consistent with the guidelines set forth
in the BMP Manual, as amended herein.
(2)
Runoff calculation. The NRCS twenty-four-hour, Type II Rainfall Distribution
shall be used for analyzing stormwater runoff for both pre- and postdevelopment
conditions.
(3)
Development sites. For the purpose of computing peak flow rates and
runoff hydrographs from development sites, calculations shall be performed
using one of the following: Soil-Cover-Complex (SCS) Method, Technical
Release TR-55 or 20, or the HECI or Penn State Runoff Model (PSRM)
or Stormwater Management Model (SWMM). Under special circumstances,
the Township may approve use of other computation methods.
(4)
The Rational Method shall not be used to calculate runoff volume.
(5)
Peak flow rates.
(a)
The NRCS Rainfall-Runoff Method is recommended for design of
stormwater management facilities and where stormwater runoff must
be taken into consideration. As to the model applicability: there
are no size limitations as to NRCS TR-20 or HEC-HMS and SWMM; other
models shall be as preapproved by the Township. Other methods upon
approval by the Township may be applied.
(b)
The NRCS antecedent runoff condition II (ARC II, previously
AMC II) must be used for all simulations. Use of continuous simulation
models that vary the ARC is not permitted for stormwater management
purposes.
(c)
Rainfall intensities for design should be obtained from those sources authorized in § 116-12K above.
(d)
The Rational Method may be used for design of conveyance facilities
only. Extreme caution should be used by the design professional if
the watershed has more than one main drainage channel. If the watershed
is divided so that hydrologic properties are significantly different
in one versus the other, if the time of concentration exceeds 60 minutes,
or if stormwater runoff volume is an important factor. The combination
of Rational Method hydrographs based on timing is prohibited.
(e)
For comparison of peak flow rates, flows shall be rounded to
a tenth of a cubic foot per second (cfs).
(6)
Routing of hydrographs through detention/retention facilities for
the purpose of designing those facilities shall be accomplished using
the Modified-Plus Method or other recognized reservoir routing method
subject to Township's approval.
(7)
HEC-HMS Model Calibrated and/or SWMM modeling is acceptable.
(8)
The Township may require that computed existing runoff rates be reconciled
with field observations, conditions and site history. If the designer
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 recommendation
of the Township.
(9)
Runoff coefficients.
(a)
Rational Method (as authorized): use Table D-1 (Appendix D).[1]
[1]
Editor's Note: Appendix D is included as an attachment to this chapter.
(b)
NRCS Rainfall-Runoff Method: use Table D-2 (Appendix D). Curve
numbers (CN) should be rounded to tenths for use in hydrologic models
as they are a design tool with statistical variability. For large
sites, CNs should realistically be rounded to the nearest whole number.
(10)
Time of concentration.
(a)
The time of concentration is to represent the average condition
that best reflects the hydrologic response of the area. The following
time of concentration (Tc) computational methodologies
shall be used unless another method is preapproved by the Township:
[1]
Predevelopment: NRCS's Lag Equation:
|
Time of Concentration = Tc = [(Tlag"/0.6)* 60] (minutes)
|
 |
|
Where:
| ||||
|
Tlag
|
=
|
Lag time (hours)
| ||
|
L
|
=
|
Hydraulic length of watershed (feet)
| ||
|
Y
|
=
|
Average overland slope of watershed (percent)
| ||
|
S
|
=
|
Maximum retention in watershed as defined by:
S = [(1,000/CN) - 10]
| ||
|
CN
|
=
|
NRCS curve number for watershed
| ||
[2]
Postdevelopment commercial, industrial or other areas with large
impervious areas (> 20% impervious area): NRCS Segmental Method. The
length of sheet flow shall be limited to 100 feet. Tc for channel and pipe flow shall be computed using Manning's
equation.
(b)
Additionally, the following provisions shall apply to calculations
for time of concentration:
[1]
The postdevelopment Tc shall never be
greater than the predevelopment Tc for any
watershed or subwatershed. This includes when the designer has specifically
used swales to reduce flow velocities. In the event that the designer
believes that the postdevelopment Tc is greater,
it will still be set by default equal to the predevelopment Tc for modeling purposes.
[2]
The minimum Tc for any watershed shall
be five minutes.
[3]
The designer must provide computations for all predevelopment
Tc paths. A five-minute Tc cannot be assumed for predevelopment.
[4]
Undetained fringe areas (areas that are not tributary to a stormwater
facility but where a reasonable effort has been made to convey runoff
from all new impervious coverage to best management practices) may
be assumed to represent the predevelopment conditions for purposes
of Tc calculation.
C.
Predevelopment conditions. Predevelopment conditions shall be assumed
to be those which existed on any site at the time prior to the commencement
of any development activities. Where a site has been developed previously
and is proposed to be redeveloped, the predevelopment coefficient
for the developed portion of the site shall be based upon an assumed
50% native land cover (i.e., grass, trees, etc.) and 50% developed
land cover (e.g., buildings, pavement, etc.).
(1)
NRCS runoff curve numbers selected for use in the calculations shall
accurately reflect existing conditions subject to the approval of
the Township.
(2)
Hydrologic conditions for all areas with pervious cover (i.e., fields,
woods, lawn areas, pastures, cropland, etc.) shall be assumed to be
in "good" condition, and the lowest recommended NRCS runoff curve
number (CN) shall be applied for all pervious land uses within the
respective range for each land use and hydrologic soil group.
D.
Release rate.
(1)
Definition. The release rate percentage defines the percentage of
the predevelopment peak rate of runoff that can be discharged from
an outfall on the site after development under the one-, two-, ten-,
twenty-five- and fifty-year storm conditions. Under all circumstances,
a one-hundred-percent release rate percentage shall be applied to
the one-hundred-year frequency storm, whether or not lower reduced
release rate percentage are specified for the one-, two-, ten-, twenty-five
and fifty-year return frequency storms. The assigned release rate
percentage for each subarea applies uniformly to all land development
or alterations within the subarea. A listing of the release rate percentage
by subarea appears in Appendix A of this chapter;[2] the subareas are delineated on the Release Rate Percentage
Map attached hereto and made a part of Appendix A. This Appendix was
established under the County of Erie's 1996 Act 167 Plan and
is being retained to better protect the public health and safety.
[2]
Editor's Note: Appendix A is included as an attachment to this chapter.
(2)
Procedure for use.
(a)
Identify the specific subarea in which the development site
is located from the watershed map and obtain the subarea release rate
percentage from the information in Appendix A.
(b)
Compute the pre- and postdevelopment runoff hydrographs for
each stormwater outfall on the development site using an acceptable
calculation method for the one-, two-, ten-, twenty-five-, fifty-
and one-hundred-year storms. Apply no on-site detention for stormwater
management, but include any techniques to minimize impervious surfaces
and/or increase the time of concentration for stormwater runoff flowing
from the development site.
(c)
If the postdevelopment peak runoff rate is greater than the
predevelopment peak runoff rate, or if such runoff exceeds release
rates established in this chapter, then stormwater detention shall
be required. The capacity of the detention facility shall be calculated
by multiplying the predevelopment rate of runoff from the one-, two-,
ten-, twenty-five-, and fifty-year frequency storm by the subarea
release rate percentage and the predevelopment runoff rate from the
one-hundred-year storm event by 100% to determine the maximum allowable
releases from any detention basin.
E.
Exception areas. Exceptions to the peak discharge standards will
be considered only in the following instances:
(1)
In the Lake Erie Area Watershed: only in instances where the discharge
from the development site occurs directly to Lake Erie, an adequately
sized storm or combined sewer which discharges directly into Lake
Erie, or through a properly sized and designed regional stormwater
detention facility.
(2)
The analyses of storm or combined sewers and/or regional detention
facilities as are necessary to demonstrate their adequacy for the
proposed discharges shall be completed by the applicant using methods
and procedures as directed by the Township.
A.
No exemption shall be allowed as to an applicant's obligation
to implement such measures as are necessary to:
B.
The applicant must utilize the following BMPs to the maximum extent
practicable to receive consideration for exemptions:
(1)
Design around and limit disturbance of floodplains, wetlands, natural
slopes over 15%, existing native vegetation and other sensitive and
special value features.
(2)
Maintain riparian and forested buffers.
(3)
Limit grading and maintain nonerosive flow conditions in natural
flow paths.
(4)
Maintain existing tree canopies near impervious areas.
(5)
Minimize soil disturbance and reclaim disturbed areas with topsoil
and vegetation.
(6)
Direct runoff to pervious areas.
C.
The applicant's proposed development/additional impervious area
may not adversely affect:
D.
An applicant proposing regulated activities, after demonstrating compliance with § 116-14A, B and C, may be exempted from various requirements of this chapter if documentation can be provided that a downstream man-made water body (i.e., reservoir, lake, man-made wetlands or other BMP) has been designed or modified to address the potential stormwater flooding and volume impacts of the proposed development.
E.
The purpose of this section is to ensure consistency of stormwater management planning between this chapter and NPDES permitting (when required) and to ensure that the applicant has a single and clear set of stormwater management standards to which the applicant is subject. The Township may accept alternative stormwater management controls under this section, provided that the Township, in consultation with the PaDEP, determines that meeting the volume control requirements (see § 116-15) is not possible or places an undue hardship on the applicant.
A.
The low-impact development practices provided in the BMP Manual shall
be utilized for all regulated activities to the maximum extent possible.
B.
Stormwater runoff volume controls shall be implemented using the
Design Storm Method or the Simplified Method, as defined below. For
Regulated Activity areas equal to or less than one acre that do not
require hydrologic routing to design the stormwater facilities, this
chapter establishes no preference for either method; therefore, an
applicant may select either method. The Simplified Method is authorized
only where regulated activities involve no more than one acre and
where the project does not require detailed design of stormwater management
facilities.
C.
The Design Storm Method (CG-1 in the BMP Manual) is applicable to
any sized regulated activity. This method requires detailed modeling
based on site conditions.
D.
When allowed, the Simplified Method (CG-2 in the BMP Manual) is independent
of site conditions and should be used if the Design Storm Method is
not followed. 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 the commonwealth). Removal
options include reuse, evaporation, transpiration and infiltration.
(3)
Wherever possible, infiltration facilities should 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 should be infiltrated.
(4)
Actual field infiltration tests at the location of the proposed elevation
of the stormwater BMPs are required. Infiltration tests shall be conducted
in accordance with the BMP Manual. A minimum forty-eight-hour notification
of the Township shall be provided to allow witnessing of the testing.
E.
In cases where it is not possible or desirable to use infiltration-based best management practices to partially fulfill the requirements in either § 116-15C or D, the following procedure shall be provided to allow witnessing of the testing:
(1)
At a minimum, the following documentation shall be provided to justify
the decision to reduce the use of infiltration BMPs:
(a)
Description of and justification for field infiltration/permeability
testing with respect to the type of test and test locations.
(b)
An interpretive narrative describing existing site soils and
their structure as these relate to the interaction between soils and
water. In addition to providing soil and soil profile descriptions,
this narrative shall identify depth to seasonal high water tables
and depth to bedrock, and provide a description of all substance elements
(fragipans and other restrictive layers, geology, etc.) that influence
the direction and rate of subsurface water movement.
(c)
A qualitative assessment of the site's contribution to
annual aquifer recharge shall be made, along with identification of
any restrictions or limitations associated with the use of engineered
infiltration facilities.
(d)
The provided documentation must be signed and sealed by a professional
engineer or a professional geologist.
(e)
A professional opinion, stated under signature and seal of a
professional engineer or a professional geologist, stating that the
proposed groundwater recharge is consistent with natural recharge,
is reasonable and is consistent with this chapter.
(2)
The following water quality pollutant load reductions will be required
for all disturbed areas within the proposed development:
|
Pollutant Load
|
Units
|
Required Reduction
| |
|---|---|---|---|
|
Total Suspended Solids (TSS)
|
Pounds
|
85%
| |
|
Total Phosphorus (TP)
|
Pounds
|
85%
| |
|
Total Nitrate (NO3)
|
Pounds
|
50%
|
(3)
The performance criteria for water quality best management practices
shall be determined from the Pennsylvania Stormwater Best Management
Practices Manual ("BMP Manual"), most current version.
F.
The applicable worksheets from the BMP Manual must be used in calculations
to establish volume control.
On all lands within Millcreek Township, the postdevelopment
peak discharge rates shall be in accordance with the release rate
map for the Lake Erie Watershed as set forth in Appendix A of this
chapter.[1] See also § 116-13D.
[1]
Editor's Note: Appendix A is included as an attachment to this chapter.
In developments not subject to this chapter under § 116-3, a grading plan must be submitted to and approved by the Township Engineer prior to issuance of any building or local street access permit.
No person shall modify or alter any approved and/or implemented stormwater management or grading plan without the prior approval of the Board of Supervisors. This prohibition shall extend to the developer and to all successors and assigns, including grantees, owners, lessees, occupants and their contractors and/or agents. Any violation of this section shall be subject to the remedies and penalties set forth in Article XIII, Prohibited Acts, Enforcement Procedures and Remedies, of this chapter.
