[Ord. 2014-02, 9/23/2014]
1.
Any person or entity intending to engage in a regulated activity in the municipality shall, prior to commencing the regulated activity, apply for and obtain a stormwater management permit and comply with the requirements of this chapter. An application for stormwater management permit and SWM site plan approval shall be submitted on a form prescribed by the municipality accompanied by the required fee. Prior to the commencement of any of the regulated activities set forth in § 23-105 of this chapter, a stormwater management permit and SWM site plan must be approved in writing by the Township. SWM site plans approved by the municipality shall be on site throughout the duration of the regulated activity.
2.
The stormwater management and runoff control criteria and standards
in this chapter shall apply to the total proposed regulated activity,
even if it is to take place in stages. The measurement of impervious
surfaces shall include all of the impervious surfaces in the total
proposed regulated activity even if the development is to take place
in stages.
3.
No regulated activity within the municipality shall commence until:
A.
The municipality issues the stormwater management permit and approval
of an SWM site plan, which demonstrates compliance with the requirements
of this chapter; and
B.
The applicant has received a letter of adequacy or approval for the
erosion and sediment control plan review by the municipality and the
Conservation District (if required) and has received all other local,
state and federal permit approvals required for the project involving
the regulated activity.
4.
Neither submission of an SWM site plan under the provisions herein
nor compliance with the provisions of this chapter shall relieve any
person from responsibility for damage to any person or property otherwise
imposed by law.
5.
The applicant shall design the site to minimize disturbances to land, site hydrology, and natural resources and to maintain the natural hydrologic regime, drainage patterns and flow conditions. The applicant shall apply the procedures set forth in § 23-304 for the overall site design and for selection, location and design of features and BMPs to be used to comply with the requirements of this chapter.
6.
To the maximum extent practicable, post-construction stormwater shall
be discharged within the drainage area of the same stream or water
body receiving the runoff prior to construction of the proposed regulated
activity.
7.
For regulated activities with one acre or more of proposed earth
disturbance, existing drainage peak rate discharges up to and including
the one-hundred-year storm onto or through adjacent property(ies)
or downgradient property(ies), including diffuse drainage discharge,
shall not be altered in any manner without written permission from,
and, where applicable as determined by the municipality, an easement
and agreement with, the affected landowner(s) for conveyance of discharges
onto or through their property(ies). Such discharge shall be subject
to any applicable discharge criteria specified in this chapter. Stormwater
runoff shall discharge to a suitable natural watercourse (except where
prohibited by a riparian buffer as defined in this chapter) or storm
sewer system.
A.
For regulated activities with one acre or less of proposed earth
disturbance, the applicant shall provide written notification to the
affected landowner(s) describing the proposed regulated activity and
proposed discharge(s), unless otherwise required by the municipality.
8.
Areas located outside of the site (i.e., areas outside of the regulated activity) that drain through a proposed site are not subject to water quality and volume control, infiltration, stream channel protection, or peak flow rate control requirements (as presented in §§ 23-305, 23-306, 23-307, and 23-308). Drainage facilities located on the site shall be designed to safely convey flows from outside of the site through the site.
9.
If site conditions preclude capture of runoff from limited portions
of the disturbed area for achieving water quality volume control standards,
stream channel protection standards, and the two-, five-, ten-, twenty-five-
and fifty-year storm event peak runoff rate reduction standards for
new development required by this chapter, the applicant shall propose
alternate methods to mitigate the bypass of the BMPs, subject to the
approval of the Municipal Engineer. In no case shall the resulting
peak rate be greater than the predevelopment peak rate for the equivalent
design storm.
10.
For all regulated activities, erosion and sediment control BMPs shall
be designed, implemented, operated, and maintained during the regulated
activities (i.e., during construction) as required to meet the purposes
and requirements of this chapter, to meet the erosion and sediment
control requirements of the municipality, if applicable, and to meet
all requirements under Title 25 of the Pa. Code and the Clean Streams
Law.
11.
For all regulated activities, permanent BMPs and conveyances shall
be designed, 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, the Clean Streams Law, and the
Storm Water Management Act.
12.
The design of all BMPs and conveyances shall incorporate sound engineering
principles and practices in a manner that does not aggravate existing
stormwater problems as identified by the municipality. The municipality
reserves the right to disapprove any design that would result in construction
in an area affected by existing stormwater problem(s) or continuation
of an existing stormwater problem(s).
13.
Existing wetlands, either on the site or on an adjacent property,
shall not be used to meet the minimum design requirements for stormwater
management or stormwater runoff quality treatment. Stormwater discharges
to existing wetlands shall not degrade the quality or hydrologic integrity
of the wetland.
14.
Hotspot Runoff Controls. Specific structural or pollution-prevention
practices may be required, as determined to be necessary by the Municipal
Engineer, to pretreat runoff from hotspots prior to infiltration.
Following is a list of examples of hotspots:
A.
Vehicle salvage yards and recycling facilities;
B.
Vehicle fueling stations;
C.
Vehicle service and maintenance facilities;
D.
Vehicle and equipment cleaning facilities;
E.
Fleet storage areas (bus, truck, etc.);
F.
Industrial sites based on Standard Industrial Classification codes;
G.
Marinas (service and maintenance areas);
H.
Outdoor liquid container storage;
I.
Outdoor loading/unloading facilities;
J.
Public works storage areas;
K.
Facilities that generate or store hazardous materials;
L.
Commercial container nursery;
M.
Contaminated sites/brownfields;
N.
Other land uses and activities as designated by the municipality.
15.
Contaminated and Brownfield Sites. Where BMPs may contribute to the
migration of contaminants in groundwater, the water quality and runoff
volume, stream channel protection, and peak rate control standards
shall be met; however, at the Municipal Engineer's discretion,
the minimum infiltration requirement may be reduced or eliminated
commensurate with the contaminated area and the required water quality
and runoff control measures may be increased to mitigate the reduced
infiltration requirement for the contaminated area.
16.
Additional Water Quality Requirements. The municipality may require
additional stormwater control measures for stormwater discharges to
special management areas, including, but not limited to:
A.
Water bodies listed as "impaired" by PADEP.
B.
Any water body or watershed with an approved total maximum daily
load (TMDL).
C.
Areas of known existing flooding problems.
D.
Critical areas with sensitive resources (e.g., state-designated special-protection
waters, cold-water fisheries, carbonate geology or other groundwater
recharge areas that may be highly vulnerable to contamination, drainage
areas to water supply reservoirs, etc.).
17.
Applicants shall utilize the Pennsylvania Stormwater Best Management
Practices Manual (PA BMP Manual), as amended, or other sources acceptable
to the Municipal Engineer, for testing and design standards for BMPs;
and where there is a conflict with the provisions of this chapter,
the most restrictive applies.
18.
For areas underlain by karst or carbonate geology that may be susceptible
to the formation of sinkholes and other karst features, the location,
type, and design of infiltration BMPs shall be based on a site evaluation
conducted by a qualified licensed professional and based on the PA
BMP Manual or other design guidance acceptable to the Municipal Engineer.
19.
All regulated activities located within a special flood hazard area
designated by the Federal Emergency Management Agency (FEMA) shall
comply with Part 15, Flood Hazard District, of the Zoning Ordinance
and shall be designed to maintain the flood-carrying capacity of the
floodway such that the base flood elevations are not increased, either
upstream or downstream. The natural conveyance characteristics of
the site and the receiving floodplain shall be incorporated into the
stormwater management practices proposed for the site.
20.
Disturbance of existing ground cover during construction of the proposed
regulated activity is prohibited within 50 feet of the top-of-bank
of all perennial and intermittent waterways, water bodies (lakes,
ponds, etc.) and wetlands, except for activities otherwise approved
by state or local agencies (e.g., stream restoration projects, road
crossings, subsurface utility projects, etc.). At the Municipal Engineer's
discretion, and with Conservation District and PADEP approval where
necessary, the nondisturbance buffer may be reduced because of setback
or other site constraints, but it shall never be less than 10 feet.
[Ord. 2014-02, 9/23/2014]
1.
The following permit or other regulatory requirements may apply to
certain regulated activities and shall be met prior to (or as a condition
of) final approval by the municipality of the SWM site plan and prior
to commencement of any regulated activities, as applicable:
A.
All regulated activities subject to permit or regulatory requirements
by PADEP under regulations at Title 25, Pennsylvania Code, Chapter
102, or erosion and sediment control requirements of the municipality.
B.
Work within natural drainageways subject to permit by PADEP under
Title 25, Pennsylvania Code, Chapter 105.
C.
Any BMP or conveyance that would be located in or adjacent to surface
waters of the commonwealth, including wetlands, subject to permit
by PADEP under Title 25, Pennsylvania Code, Chapter 105.
D.
Any BMP or conveyance that would be located on or discharge to a
state highway right-of-way, or require access to or from a state highway
and be subject to approval by PennDOT.
E.
Culverts, bridges, storm sewers, or any other facilities which must
pass or convey flows from the tributary area and any facility which
may constitute a dam subject to permit by PADEP under Title 25, Pennsylvania
Code, Chapter 105.
[Ord. 2014-02, 9/23/2014]
1.
No regulated activity within the municipality shall commence until:
A.
The municipality receives documentation that the applicant has received:
(1)
A "letter of adequacy" from the Conservation District or other
approval from PADEP in compliance with Title 25, Chapter 102, of the
Pennsylvania Code of an erosion and sediment control plan for construction
activities, if applicable;
(2)
A PADEP NPDES construction activities permit as required under
Title 25, Pennsylvania Code, Chapter 92a, if applicable;
(3)
Evidence of any other permit(s) or approvals required for the
regulated activities; and
B.
An erosion and sediment control plan has been approved by the municipality,
if required.
2.
A copy of the erosion and sediment control plan and any required
permit(s), as required by PADEP regulations, shall be available on
the site at all times.
[Ord. 2014-02, 9/23/2014]
1.
The applicant shall design the site to minimize the disturbances to land, site hydrology, and natural resources and to maintain the natural hydrologic regime, drainage patterns and flow conditions. For regulated activities with 10,000 or more square feet of proposed earth disturbance OR 2,000 or more square feet of proposed impervious surfaces, the applicant shall demonstrate in its SWM site plan (as required in § 23-402, Subsection 2C) that the design sequence, objectives and techniques described below were applied to the maximum extent practicable in the site design of the regulated activity while complying with all other requirements of this chapter. The site design shall:
B.
Second, provide a prioritized listing of these resources and features
to identify:
(1)
Those to be incorporated into the site design in a manner that
provides protection from any disturbance or impact from the proposed
regulated activity;
(2)
Those to be protected from further disturbance or impact but
for which the proposed regulated activity will provide improvement
to existing conditions;
(3)
Those that can be incorporated into and utilized as components
of the overall site design in a manner that protects or improves their
existing conditions while utilizing their hydrologic function within
the limits of their available capacity (e.g., for infiltration, evapotranspiration,
or reducing pollutant loads, runoff volume or peak discharge rates,
etc.) to reduce the need for or size of constructed BMPs; and
(4)
Those that may be considered for alteration, disturbance or
removal.
C.
Third, develop the site design to achieve the following:
(1)
Recognize and incorporate the priorities identified in § 23-304, Subsection 1B, as the basis for the proposed site layout, grading, construction, and permanent ground cover design;
(2)
Minimize earth disturbance (both surface and subsurface);
(3)
Maximize protection of or improvement to natural resources and
special management areas;
(4)
Minimize the disturbance of natural site hydrology, in particular
natural drainage features and patterns, discharge points and flow
characteristics, natural infiltration patterns and characteristics,
and natural channel and floodplain conveyance capacity;
(5)
Incorporate natural hydrologic features and functions identified in § 23-304, Subsection 1B, into the site design to protect and utilize those features and their hydrologic functions to reduce the need for or size of constructed BMPs;
(6)
Maximize infiltration and the use of natural site infiltration
features, patterns and conditions, and evapotranspiration features;
(7)
Apply selective grading design methods to provide final grading
patterns or preserve existing topography in order to evenly distribute
runoff and minimize concentrated flows;
(8)
Minimize the cumulative area to be covered by impervious surfaces
and:
(9)
Minimize the volume and peak discharge rates of stormwater generated;
(10)
Avoid or minimize stormwater runoff pollutant loads and receiving
stream channel erosion;
(13)
For flow volumes requiring conveyance from the source of generation
to a BMP for management, give preference to open channel conveyance
techniques that provide infiltration and water quality benefits, and
landscape-based management in common open space areas, where practicable;
and
(14)
Consider additional guidance for incorporating natural hydrology into the site and BMP designs, methods and techniques that support the objectives of §§ 23-304, Subsection 2B and C. Appendix B presents additional discussion of natural hydrology site design and sources of information for "conservation design," "low-impact design," and "sustainable design."[1]
[1]
Editor's Note: The appendixes are included as attachments
to this chapter.
[Ord. 2014-02, 9/23/2014]
1.
To control post-construction stormwater impacts from regulated activities
and meet state water quality requirements, BMPs shall be provided
in the site design that replicate predevelopment stormwater infiltration
and runoff conditions, such that post-construction stormwater discharges
do not degrade the physical, chemical, or biological characteristics
of the receiving waters. The applicant shall comply with the following
water quality and runoff volume requirements for all regulated activities,
including all new development and redevelopment activities:
A.
The post-construction total runoff volume shall not exceed the predevelopment
total runoff volume for all storms equal to or less than the two-year,
twenty-four-hour duration precipitation (design storm). The water
quality and runoff volume to be managed shall consist of any runoff
volume generated by the proposed regulated activity over and above
the predevelopment total runoff volume and shall be captured and permanently
retained or infiltrated on the site. Permanent retention options may
include, but are not limited to, reuse, evaporation, transpiration,
and infiltration.
C.
The design of the facility outlet shall provide for protection from
clogging and unwanted sedimentation.
D.
BMPs that moderate the temperature of stormwater shall be used to
protect the temperature of receiving waters.
E.
Water quality improvement shall be achieved in conjunction with achieving the infiltration requirements of § 23-306. The infiltration volume required under § 23-306 may be included as a component of the water quality volume. If the calculated water quality and runoff volume is greater than the volume infiltrated, then the difference between the two volumes shall be managed for water quality and runoff volume control through other techniques or practices but shall not be discharged from the site.
F.
Runoff from the disturbed area shall be treated for water quality
prior to entering existing waterways or water bodies. If a stormwater
management practice does not provide water quality treatment, then
water quality BMPs shall be utilized to provide pretreatment prior
to the runoff entering the stormwater management practice.
H.
When the regulated activity contains or is divided by multiple drainage
areas, the water quality and runoff volume shall be separately addressed
for each drainage area.
I.
Weighted averaging of runoff coefficients shall not be used for manual
computations or input data for water quality and runoff volume calculations.
J.
Areas located outside of the site (i.e., areas outside of the regulated
activity) may be excluded from the calculation of the water quality
and runoff volume requirements.
[Ord. 2014-02, 9/23/2014]
1.
Providing for infiltration consistent with the natural hydrologic
regime is required to compensate for the reduction in the recharge
that occurs when the ground surface is disturbed or impervious surface
is created or expanded. The applicant shall achieve the following
infiltration requirements:
A.
Wherever possible, infiltration should be designed to accommodate the entire water quality and runoff volume required in § 23-305.
B.
For regulated activities involving new development, the volume of
a minimum of one inch of runoff from all proposed impervious surfaces
shall be infiltrated.
D.
If the requirements of § 23-306, Subsection 1B or C, cannot be physically accomplished, then the applicant shall be responsible for demonstrating with data or calculations, to the satisfaction of the Municipal Engineer, why this infiltration volume cannot be physically accomplished on the site (e.g., shallow depth to bedrock or limiting zone, open voids, steep slopes, etc.) and what alternative volume can be infiltrated; however, in all cases at least the first 0.5 inch of runoff volume shall be infiltrated.
E.
Only if a minimum of at least 0.5 inch infiltration requirement cannot be physically accomplished on the site shall a waiver from § 23-306 be considered by the municipality.
F.
If site conditions preclude capture of runoff from portions of the
impervious surfaces, the infiltration volume for the remaining area
shall be increased an equivalent amount to offset the loss.
G.
When a project contains or is divided by multiple watersheds, the
infiltration volume shall be separately addressed for each watershed.
H.
Existing impervious surfaces located in areas outside of the site
(i.e., outside of the regulated activity) may be excluded from the
calculation of the required infiltration volume.
I.
A detailed soils evaluation of the site shall be conducted by a qualified
professional and at a minimum shall address soil permeability, depth
to bedrock, and subgrade stability. The general process for designing
the infiltration BMP shall be conducted by a qualified licensed professional
and shall be consistent with the PA BMP Manual (as amended) (or other
guidance acceptable to the Municipal Engineer) and in general shall:
(1)
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.
(2)
Provide field tests such as double-ring infiltrometer or other
hydraulic conductivity tests (at the elevation of the proposed infiltration
surface) to determine the appropriate hydraulic conductivity rate.
Standard septic/sewage percolation tests are not acceptable for design
purposes.
(3)
Design the infiltration facility for the required retention
(infiltration) volume based on field-determined infiltration capacity
(and apply safety factor as per applicable design guidelines) at the
elevation of the proposed infiltration surface.
(4)
On-lot infiltration features are encouraged; however, it shall
be demonstrated to the Municipal Engineer that the soils are conducive
to infiltration on the identified lots.
J.
Infiltration BMPs shall be selected based on suitability of soils
and site conditions and shall be constructed on soils that have the
following characteristics:
(1)
A minimum depth of 24 inches between the bottom of the BMP and the top of the limiting zone. Additional depth may be required in areas underlain by karst or carbonate geology (see § 23-306, Subsection 1N).
(2)
An infiltration rate sufficient to accept the additional stormwater
volume and drain completely as determined by field tests conducted
by the applicant.
(3)
The infiltration facility shall completely drain the retention
(infiltration) volume within three days (72 hours) from the end of
the design storm.
K.
All infiltration practices shall:
(1)
Be selected and designed to meet the criteria of § 23-304, Subsection 1C, that are applicable to infiltration;
(2)
Be set back at least 15 feet from all buildings and features
with subgrade elements (e.g., basements, foundation walls, etc.),
and property lines, unless otherwise approved by the Municipal Engineer;
(3)
For any infiltration practice that collects runoff from shared
or multiple features and that is located within a minimum of 25 feet
of a building or feature with subgrade elements (e.g., basements,
foundation walls, etc.), the bottom elevation shall be set below the
elevation of the subgrade element.
L.
Infiltration facilities shall, to the maximum extent practicable,
be located to avoid introducing contaminants to groundwater:
(1)
When a hotspot is located in the area draining to a proposed
infiltration facility, an evaluation of the potential of groundwater
contamination from the proposed infiltration facility shall be performed,
including a hydrogeologic investigation (if necessary) by a qualified
licensed professional to determine what, if any, pretreatment or additional
design considerations are needed to protect groundwater quality.
(2)
When located within a "wellhead protection area" of a public
water supply well, infiltration practices shall be in conformance
with the applicable approved source water protection assessment or
source water protection plan.
(3)
The applicant shall provide appropriate safeguards against groundwater
contamination for land uses that may cause groundwater contamination
should there be a mishap or spill.
M.
During site construction, all infiltration practice components shall
be protected from compaction due to heavy equipment operation or storage
of fill or construction material. Infiltration areas shall also be
protected from sedimentation. Areas that are accidentally compacted
or graded shall be remediated to restore soil composition and porosity.
Adequate documentation to this effect shall be submitted to the Municipal
Engineer for review. All areas designated for infiltration shall not
receive runoff until the contributory drainage area has achieved final
stabilization.
N.
Consideration of infiltration BMPs for areas underlain by karst or
carbonate geology is encouraged, but only where the design, supporting
calculations, results of soils or other site investigations or other
documentation is provided to the municipality demonstrating that the
potential or likelihood of subsidence or sinkholes is minimal. Evaluation
of site conditions and infiltration design shall rely on guidance
in the PA BMP Manual (as amended) or other guidance acceptable to
the Municipal Engineer.
O.
Groundwater quality of the carbonate aquifer shall be protected from
infiltration of pollutants. At a minimum, stormwater runoff from hotspots
(i.e., sources of significant pollutant runoff) shall first be discharged
through a water quality BMP(s) to remove pollutants prior to infiltration.
Where soil characteristics are insufficient to provide removal of
pollutants from sources other than hotspots, stormwater runoff shall
first be discharged through a water quality BMP(s) to remove pollutants
prior to infiltration.
P.
Where sediment transport in the stormwater runoff is anticipated
to reach the infiltration system, appropriate permanent measures to
prevent or collect sediment shall be installed prior to discharge
to the infiltration system.
Q.
Where roof drains are designed to discharge to infiltration practices,
they shall have appropriate measures to prevent clogging by unwanted
debris (for example, silt, leaves and vegetation). Such measures shall
include but are not limited to leaf traps, gutter guards and cleanouts.
R.
All infiltration practices shall have appropriate positive overflow
controls.
S.
No sand, salt or other particulate matter may be applied to a porous
surface material for winter ice conditions.
T.
The following procedures and materials shall be required during the
construction of all subsurface facilities:
(1)
Excavation for the infiltration facility shall be performed
with equipment that will not compact the bottom of the seepage bed/trench
or like facility.
(2)
The bottom of the bed and/or trench shall be scarified prior
to the placement of aggregate.
(3)
Only clean aggregate with documented porosity, free of fines,
shall be allowed.
(4)
The tops, bottoms and sides of all seepage beds, trenches, or
like facilities shall be covered with drainage fabric. Fabric shall
be nonwoven fabric acceptable to the Municipal Engineer.
(5)
Stormwater shall be distributed throughout the entire seepage
bed/trench or like facility, and provisions for the collection of
debris shall be provided in all facilities.
[Ord. 2014-02, 9/23/2014]
1.
For regulated activities involving new development with one or more
acres of earth disturbance, the applicant shall comply with the following
stream channel protection requirements to minimize stream channel
erosion and associated water quality impacts to the receiving waters:
A.
The peak flow rate of the post-construction two-year, twenty-four-hour
design storm shall be reduced to the predevelopment peak flow rate
of the one-year, twenty-four-hour duration precipitation, using the
SCS Type II distribution or in accordance with methodology contained
in Table 309.1.
B.
To the maximum extent practicable, and unless otherwise approved
by the Municipal Engineer, the post-construction one-year, twenty-four-hour
storm flow shall be detained for a minimum of 24 hours and a maximum
not to exceed 72 hours from a point in time when the maximum volume
of water from the one-year, twenty-four-hour storm is stored in a
proposed BMP (i.e., when the maximum water surface elevation is achieved
in the facility). Release of water can begin at the start of the storm
(i.e., the invert of the orifice is at the invert of the proposed
BMP).
D.
The minimum orifice size in the outlet structure to the BMP shall
be three inches in diameter, unless otherwise approved by the Municipal
Engineer, and a trash rack shall be installed to prevent clogging.
For sites with small drainage areas contributing to the BMP that do
not provide enough runoff volume to allow a twenty-four-hour attenuation
with the three-inch orifice, the calculations shall be submitted showing
this condition.
E.
When the calculated orifice size is below three inches, gravel filters
(or other methods) are recommended to discharge low-flow rates, subject
to the Municipal Engineer's satisfaction. When filters are utilized,
maintenance provisions shall be provided to ensure filters meet the
design function.
F.
All proposed stormwater facilities shall make use of measures to
extend the flow path and increase the travel time of flows in the
facility.
G.
When a regulated activity contains or is divided by multiple drainage
areas, the peak flow rate control shall be separately addressed for
each drainage area.
[Ord. 2014-02, 9/23/2014]
1.
The applicant shall comply with the following peak flow rate control
requirements for all regulated activities, including those that involve
new development and redevelopment.
A.
Post-construction peak flow rates from any regulated activity shall
not exceed the predevelopment peak flow rates as shown for each of
the design storms specified in Table 308.1.
|
Table 308.1
| |||
|---|---|---|---|
|
Peak Rate Control Standards
| |||
|
(Peak Flow Rate of the Post-Construction Design Storm
Shall be Reduced to the Peak Flow Rate of the Corresponding Predevelopment
Design Storm Shown in the Table)
| |||
|
Post-Construction Design Storm Frequency
(24-Hour Duration)
|
Predevelopment Design Storm
| ||
|
New Development Regulated Activities
|
Redevelopment Regulated Activities
| ||
|
2-year
|
1-year
|
2-year
| |
|
5-year
|
2-year
|
5-year
| |
|
10-year
|
2-year
|
10-year
| |
|
25-year
|
10-year
|
25-year
| |
|
50-year
|
25-year
|
50-year
| |
|
100-year
|
100-year
|
100-year
| |
C.
For regulated activities involving only redevelopment, no peak flow rate controls are required when and only if the total proposed impervious surface area is at least 20% less than the total existing impervious surface area to be disturbed by the regulated activity. In all cases where this requirement is not met, the redevelopment regulated activity shall achieve the peak flow rate controls presented in Table 308.1, using the redevelopment ground cover assumptions presented in § 23-309, Subsection 4.
D.
Only the area of the proposed regulated activity shall be subject
to the peak flow rate control standards of this chapter. Undisturbed
areas for which the discharge point has not changed are not subject
to the peak flow rate control standards.
E.
Areas located outside of the site (i.e., areas outside of the regulated
activity) that drain through a proposed site are not subject to peak
flow rate control requirements. Drainage facilities located on the
site shall be designed to safely convey flows from outside of the
site through the site.
F.
When a regulated activity contains or is divided by multiple drainage
areas, the peak flow rate controls shall be separately addressed for
each drainage area.
G.
The effect of structural and nonstructural stormwater management
practices implemented as part of the overall site design may be taken
into consideration when calculating total storage volume and peak
flow rates.
[Ord. 2014-02, 9/23/2014]
1.
Stormwater runoff from all regulated activity sites with a drainage
area of greater than five acres shall be calculated using a generally
accepted calculation technique(s) that is based on the NRCS Soil Cover
Complex Method. Table 309.1 summarizes acceptable computation methods.
The method selected for use shall be based on the individual limitations
and suitability of each method for a particular site. The use of the
Rational Method to estimate peak discharges for drainage areas greater
than five acres shall be permitted only upon approval by the Municipal
Engineer.
|
Table 309.1
| |||
|---|---|---|---|
|
Acceptable Computation Methodologies for SWM Site Plan
| |||
|
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 where limitations described
in TR-55 are met
| |
|
HEC-1/HEC-HMS
|
US Army Corps of Engineers
|
Applicable where use of a full hydrologic computer model is
desirable or necessary
| |
|
Rational Method (or commercial computer package based on Rational
Method)
|
Emil Kuichling (1889)
|
For sites up to 5 acres, or as approved by the municipality
| |
|
Other methods
|
Varies
|
Other computation methodologies approved by the municipality
| |
2.
All calculations using the Soil Cover Complex Method shall use the
appropriate design rainfall depths for the various return period storms
consistent with this chapter. Rainfall depths used shall be obtained
from NOAA Atlas 14 values consistent with a partial duration series.
When stormwater calculations are performed for routing procedures
or infiltration, water quality and runoff volume functions, the duration
of rainfall shall be 24 hours.
3.
All calculations using the Rational Method shall use rainfall intensities
consistent with appropriate times of concentration (duration) and
storm events with rainfall intensities obtained from NOAA Atlas 14
partial duration series estimates, or the latest version of the PennDOT
Drainage Manual (PDM Publication 584). Times of concentration shall
be calculated based on the methodology recommended in the respective
model used. Times of concentration for channel and pipe flow shall
be computed using Manning's Equation.
4.
The applicant shall utilize the following ground cover assumptions
for all predevelopment water quality and runoff volume, infiltration
volume and peak flow rate calculations:
A.
For regulated activities involving new development, the following
ground cover assumptions shall be used:
B.
For regulated activities involving redevelopment, the following ground
cover assumptions shall be used:
(1)
For areas that are woods (as defined in Part 2 of this chapter), predevelopment calculations shall assume ground cover of "woods in good condition."
(2)
For areas that are not woods or not impervious surfaces, predevelopment
calculations shall assume ground cover of "meadow."
(3)
For areas that are impervious surfaces, predevelopment calculations
shall assume at least 20% of the existing impervious surface area
to be disturbed as "meadow" ground cover.
C.
The applicant shall determine which stormwater standards apply to
the proposed regulated activity as follows:
(1)
Stormwater standards for new development shall apply to all
proposed regulated activities that involve only new development activities
as defined in this chapter.
(2)
Stormwater standards for redevelopment shall apply to all proposed
regulated activities that involve only redevelopment activities as
defined in this chapter.
(3)
At the discretion of the Municipal Engineer, regulated activities
that involve a combination of both new development and redevelopment
activities, as defined in this chapter, may either:
(a)
Apply the stormwater standards (redevelopment or new development)
that are associated with the activity that involves the greatest amount
of land area; or
(b)
Apply the redevelopment and new development stormwater standards
to the corresponding redevelopment and new development portions of
the proposed regulated activity.
7.
Weighted averaging of runoff coefficients shall not be used for manual
computations or input data for water quality and runoff volume calculations.
8.
Hydraulic computations to determine the capacity of pipes, culverts,
and storm sewers shall be consistent with methods and computations
contained in the Federal Highway Administration Hydraulic Design Series
Number 5 (Publication No. FHWA-NHI-01-020 HDS No. 5, as amended).
Hydraulic computations to determine the capacity of open channels
shall be consistent with methods and computations contained in the
Federal Highway Administration Hydraulic Engineering Circular Number
15 (Publication No. FHWA-NHI-05-114 HEC 15, as amended). Values for
Manning's roughness coefficient (n) shall be consistent with
Table C-3 in Appendix C of this chapter.[3]
[3]
Editor's Note: The appendixes are included as attachments
to this chapter.
[Ord. 2014-02, 9/23/2014]
1.
Any BMP intended to hold standing water for four days or longer shall
be designed to incorporate biologic controls consistent with the West
Nile Guidance found in Appendix D, PADEP Document 363-0300-001, "Design
Criteria - Wetlands Replacement/Monitoring" (as amended) (or contact
the Pennsylvania State Cooperative Wetland Center or the Penn State
Cooperative Extension Office for design information.)
2.
Any stormwater basin required or regulated by this chapter designed
to store runoff and requiring a berm or earthen embankment shall be
designed to provide an emergency spillway to safely convey flow up
to and including the one-hundred-year proposed conditions. The height
of embankment shall provide a minimum 1.0 foot of freeboard above
the maximum pool elevation computed when the facility functions for
the one-hundred-year proposed conditions' inflow. Should any
BMP require a dam safety permit under Pa. Code Chapter 105 regulations,
the facility shall be designed in accordance with and meet the regulations
of Pa. Code Chapter 105 concerning dam safety. Pa. Code Chapter 105
may require the safe conveyance of storms larger than one-hundred-year
event.
3.
Any drainage conveyance facility and/or channel not governed by Pa.
Code Chapter 105 regulations shall be designed to convey, without
damage to the drainage facility or roadway, runoff from the twenty-five-year
storm event. Larger storm events (fifty-year and one hundred-year
storms) shall also be safely conveyed in the direction of natural
flow without creating additional damage to any drainage facilities,
nearby structures, or roadways.
4.
Conveyance facilities to or exiting from stormwater management facilities
(i.e., detention basins) shall be designed to convey the design flow
to or from the facility.
5.
Roadway crossings or structures located within designated floodplain
areas shall be able to convey runoff from a one-hundred-year design
storm consistent with Federal Emergency Management Agency National
Flood Insurance Program — Floodplain Management Requirements.
6.
Any facility located within a PennDOT right-of-way shall comply with
PennDOT minimum design standards and permit submission and approval
requirements.
7.
Adequate erosion protection and energy dissipation shall be provided
along all open channels and at all points of discharge. Design methods
shall be consistent with the Federal Highway Administration Hydraulic
Engineering Circular Number 11 (Publication No. FHWA-IP-89-016, as
amended) and the PADEP Erosion and Sediment Pollution Control Program
Manual (Publication No. 363-2134-008, as amended), or other design
guidance acceptable to the Municipal Engineer.
