A.
Applicants proposing regulated activities in the Municipality which do not fall under the exemption criteria shown in § 292-6 shall submit a stormwater management site plan consistent with this chapter and the applicable watershed stormwater management plan to the Municipality for review. The stormwater management criteria of this chapter shall apply to the total proposed development even if development is to take place in stages.
B.
No regulated activity within the Municipality shall commence until
the Municipality issues approval of a SWM plan, which demonstrates
compliance with the requirements of this chapter.
C.
The applicant is required to design the site to minimize surface
discharge of stormwater and the creation of impervious surfaces in
order to maintain, as much as possible, the natural hydrologic regime.
D.
The SWM site plan must be designed consistent with the sequencing provisions of § 292-17 to ensure maintenance of the natural hydrologic regime, to promote infiltration, and to protect groundwater and surface water quality and quantity. The SWM site plan designer must proceed sequentially in accordance with Article III of this chapter.
E.
Stormwater drainage systems shall be designed in order to preserve
natural flow conditions to the maximum extent practicable.
F.
Alteration of existing drainage discharge onto adjacent property
shall only be proposed in accordance with PADEP guidance document
"Chapter 102 Off-Site Discharges of Stormwater to Non-Surface Waters
— Frequently Asked Questions (FAQ)" dated January 2, 2019, or
latest guidance document from PADEP. Such discharge shall be subject
to any applicable discharge criteria specified in this chapter and
still must meet the requirements of Act 167.
G.
Areas of existing diffused drainage discharge, whether proposed to
be concentrated or maintained as diffused drainage areas, shall be
subject to any applicable discharge criteria in the general direction
of existing discharge, except as otherwise provided by this chapter.
If diffused drainage discharge is proposed to be concentrated and
discharged onto adjacent property, the applicant must document that
adequate downstream conveyance facilities exist to safely transport
the concentrated discharge or otherwise prove that no erosion, sedimentation,
flooding, or other impacts will result from the concentrated discharge.
H.
Where a development site is traversed by a stream, drainage easements
of adequate width as determined by the Municipal Engineer shall be
provided on either side of the stream and conform to the line of such
streams.
I.
Minimization of impervious surfaces and infiltration of runoff through
seepage beds, infiltration trenches, etc., is encouraged where soil
conditions permit in order to reduce the size or eliminate the need
for detention facilities or other structural BMPs.
J.
All stormwater runoff from new development or redevelopment shall
be pretreated for water quality prior to discharge to surface or groundwater.
Rooftop runoff may go directly to an infiltration BMP or be evapotranspirated.
K.
All regulated activities within the Municipality shall be designed,
implemented, operated, and maintained to meet the purposes of this
chapter, through these two elements:
L.
The BMPs shall be designed, implemented, and maintained to meet state
water quality requirements and any other more stringent requirements
as determined by the Municipality. 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.
N.
Operations and maintenance of permanent stormwater BMPs shall be addressed as required by Article VII.
O.
All BMPs used to meet the requirements of this chapter shall conform
to the state water quality requirements and any more stringent requirements
as set forth by the Municipality.
P.
Techniques described in Appendix E (Low Impact Development) of this
chapter[1] shall be considered because they reduce the costs of complying
with the requirements of this chapter and the state water quality
requirements.
[1]
Editor's Note: Appendix E is included as an attachment to this chapter.
Q.
In selecting the appropriate BMPs or combinations thereof, the applicant
shall consider the following:
(1)
Total contributing drainage area.
(2)
Permeability and infiltration rate of the site's soils.
(3)
Slope and depth to bedrock.
(4)
Seasonal high-water table.
(5)
Proximity to building foundations and wellheads.
(6)
Erodibility of soils.
(7)
Land availability and configuration of the topography.
(8)
Peak discharge and required volume control.
(9)
Stream bank erosion.
(10)
Efficiency of the BMPs to mitigate potential water quality problems.
(11)
The volume of runoff that will be effectively treated.
(12)
The nature of the pollutant being removed.
(13)
Maintenance requirements.
(14)
Creation/protection of aquatic and wildlife habitat.
(15)
Recreational value.
(16)
Enhancement of aesthetic and property values.
R.
The design of all stormwater management facilities shall incorporate
sound engineering principles and practices in a manner that does not
aggravate existing stormwater problems. The Municipality reserves
the right to disapprove any design that would result in construction
in or continuation of a stormwater problem area.
S.
The applicant may meet the stormwater management criteria through
off-site stormwater management measures as long as the proposed measures
are in the same subwatershed as shown in Appendix A.[2]
[2]
Editor's Note: Appendix A is included as an attachment to this chapter.
T.
Stormwater hotspots. Stormwater runoff from hotspots shall be pretreated
prior to surface or groundwater infiltration to prevent pollutant
runoff. Industrial sites referenced in 40 CFR 125 are examples of
hotspots.
(1)
Below 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 an appropriate
review authority.
(2)
The following land uses and activities are not normally considered
hotspots:
(3)
While streets and highways [average daily traffic volume (ADT) greater
than 30,000] are not considered stormwater hotspots, it is important
to ensure that highway stormwater management facilities are designed
to adequately protect receiving streams and/or groundwater.
(4)
The Environmental Protection Agency's (EPA) NPDES stormwater
program requires some industrial sites to prepare and implement a
stormwater pollution prevention plan.
U.
The following standards for protection of adjacent and downgradient
properties from off-site conveyance must be accomplished: For any
location where a new concentrated discharge of stormwater from any
frequency rainfall event, up to and including the 100-year storm and
the volume of runoff up to and including the two-year storm onto or
through adjacent property(ies) or downgradient property(ies), the
following are required:
(1)
A drainage easement (or other legal agreement/approval) must be obtained
for conveyance of discharges onto or through adjacent properties per
the PADEP guidance document "Chapter 102 Off-Site Discharges of Stormwater
to Non-Surface Wasters - Frequently Asked Questions (FAQ)" dated January
2, 2019, or latest guidance document from PADEP.
(2)
The conveyance must be designed to avoid erosion, flooding, or other
damage to the properties through which it is being conveyed.
The following permit requirements may apply to certain regulated
earth disturbance activities and must be met prior to commencement
of regulated earth disturbance activities, as applicable:
A.
All regulated earth disturbance activities subject to permit requirements
by PADEP under regulations at Title 25 Pennsylvania Code Chapter 102.
B.
Work within natural drainageways subject to permit by PADEP under Title 25 Pennsylvania Code Chapter 105.
C.
Any stormwater management facility 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 stormwater management facility that would be located on or discharging
to a state highway right-of-way or require access to or from a state
highway, shall be subject to approval by PennDOT.
A.
No regulated earth disturbance activities within the Municipality
shall commence until the Municipality receives an approval from the
PADEP in compliance with Title 25 Chapter 102 of the Pennsylvania
Code of an erosion and sediment control plan for construction activities
if applicable.
B.
PADEP has regulations regarding an erosion and sediment control under
Title 25 Pennsylvania Code Chapter 102.
C.
In addition, under Title 25 Pennsylvania Code Chapter 92, a PADEP
"NPDES Construction Activities" permit is required for regulated earth
disturbance activities.
D.
Evidence of any necessary permit(s) for regulated earth disturbance activities from the appropriate PADEP regional office or County Conservation District must be provided to the Municipality. The issuance of an NPDES construction permit [or permit coverage under the statewide general permit (PAG-2)] satisfies the requirements of § 292-27A.
E.
Soil erosion and sedimentation control.
(1)
Soil erosion and sedimentation control and stormwater management
plans shall be submitted for all subdivisions and/or land developments
within the Township. The Township or its designated representative
ensure and enforce compliance with the appropriate standards, including
without limitation the Township grading chapter,[1] which is incorporated herein by reference.
(2)
Measures used to control soil erosion and reduce sedimentation shall
as a minimum meet the latest revised standards, specifications and/or
regulations of:
(a)
The U.S. Department of Agriculture, Natural Resource Conservation
Service (NRCS), as adopted by the Delaware County Conservation District
in its Erosion and Sediment Control Handbook.
(b)
The Department in its Erosion and Sediment Pollution Control
Manual.
(c)
"Standards and Specifications for Soil Erosion and Sediment
Control in Developing Areas" by the U.S. Department of Agriculture,
Natural Resource Conservation Service (NRCS), College Park, Maryland.
(3)
Land disturbance activities for all subdivision and/or land developments
shall be conducted only in compliance with the following principles:
(a)
There shall be no increase in discharge of sediment or other
solid materials from the site as a result of stormwater runoff, and
any increase shall be deemed a violation of this chapter.
(b)
Erosion and sedimentation control devices, such as temporary
vegetation and mulch, temporary earthen berms, interceptor dikes,
ditches, diversion terraces, rock filter berms, crushed stone tire
scrubbers, silt basins, silt fences, and the like, appropriate to
the scale of operations, shall be installed concurrent with earthmoving
activities, and whenever any situation is created which would contribute
to increased soil erosion.
(c)
Earthmoving operations shall be minimized where possible and
practicable to preserve desirable natural features and the topography
of the site.
(d)
Stripping of vegetation, regrading or other development shall
be done in such a way that will minimize soil erosion.
(e)
To the maximum extent practicable, mature, healthy trees of
at least eight inches in caliper and other significant existing vegetation
shall be retained and protected. Such trees shall not be removed,
except as provided on the approved subdivision and/or land development
plan. The filling of soil more than five inches over the roots of
trees to be preserved is prohibited. The roots are presumed to extend
out from the tree as far as the branches of the tree extend outward.
(f)
Land disturbance shall be limited to the actual construction
site and an access strip. The amount of disturbed area and the duration
of exposure shall be kept to a minimum. Disturbed areas shall be stabilized
with vegetation, mulch, erosion control fabric, and the like as soon
as possible after earthmoving procedures.
(g)
Provisions shall be made to effectively accommodate the increased
runoff caused by changed soil and surface conditions during and after
development. Water runoff shall be minimized and retained on site
wherever possible to facilitate groundwater recharge.
(h)
Temporary vegetation and/or mulching shall be used to protect
critical areas during development. "Critical areas" shall be construed
to mean those portions of a site which are extremely vulnerable to
soil erosion.
(i)
The permanent final vegetation and structural soil erosion control
and drainage measures shall be installed as soon as practical in the
development in accordance with the approved plans.
(j)
Sediment in the runoff water shall be trapped until the disturbed
area is stabilized by the use of debris and sediment basins, silt
fences or other approved measures. Sediment deposits in basins, silt
fences, and the like shall be removed at periodic intervals during
the construction period.
(k)
Soils and rock or geologic formations with water supply potential
shall be protected from contamination by surface water or other source
or disruption caused by construction activity. Prior to any work,
protective procedures shall be developed and submitted to the Township
Engineer for review and approval.
(4)
The following practices shall be required for all subdivisions and/or
land developments, unless the Township determines that they are not
applicable.
(a)
Silt fences or other approved measures shall be utilized in
lieu of straw bale silt barriers and shall be securely anchored in
place.
(b)
Approved sediment filtering devices shall be placed at all inlets,
headwalls, basin outlets and similar drainage structures during the
construction period in order to prevent sediment from entering any
watercourse, storm drainage system, or other areas downstream.
(c)
Temporary on-lot berms may be required during construction.
The top width of the berms shall be a minimum of three feet in width,
with side slopes of 3:1 maximum.
(d)
Crushed stone tire scrubbers shall be placed at all entrances
to construction areas immediately at the time of commencement of construction.
Tire scrubbers shall be of sufficient width and length to prevent
the transportation of sediment off of the construction site.
(e)
Temporary and permanent seeding and mulch specifications shall
be noted on all plans. The specifications shall include lime and fertilizer
rates of application, as well as other provisions regarding procedures
and materials. The Township may require hydroseeding of all graded
areas associated with street construction and stormwater management
basins.
(f)
During roadway grading, interceptor dikes shall be installed
on all roadway subgrades with slopes in excess of 5% to prevent soil
erosion of the subgrade. The interceptor dikes shall divert stormwater
runoff through silt traps or silt fences.
(g)
The crushed stone base course for driveways, roadways and parking
areas shall be applied as soon as possible after grading procedures,
in order to prevent soil erosion of the subgrade.
(h)
Drainage swales and ditches, and all slopes greater than four
to one shall be protected against erosive velocities with soil erosion
control measures, such as erosion control fabric and other material
as approved by the Township.
(i)
Energy dissipaters and/or stilling basins shall be installed
at the outlet end of all storm drainage facilities.
(j)
Areas proposed for infiltration BMPs shall be protected from
sedimentation and compaction during the construction phase to maintain
their maximum infiltration capacity.
(5)
Maintenance of erosion and sedimentation control facilities.
(a)
Whenever sedimentation is caused by stripping vegetation, regarding
other development, it shall be the responsibility of the person causing
such sedimentation to remove the accumulated sediment from all adjoining
or downstream properties, surfaces, drainage systems and watercourses
and to repair any damage at his expense as quickly as possible.
(b)
All necessary soil erosion and sedimentation control measures installed under this chapter shall be adequately maintained by the landowner or developer after completion of the approved plan or until such measures are permanently stabilized as determined by the Township and thereafter as required by Article X of Chapter 305, Subdivision and Land Development (SALDO).
(6)
Additional erosion and sediment control design standards and criteria
are recommended to be applied where infiltration BMPs are proposed.
At a minimum, they shall include the following:
(a)
Areas proposed for infiltration BMPs shall be protected from
sedimentation and compaction during the construction phase to maintain
maximum infiltration capacity.
(b)
Infiltration BMPs shall not be constructed nor receive runoff
until the entire drainage area contributory to the infiltration BMPs
has achieved final stabilization.
F.
A copy of the erosion and sediment control plan and any required
permit, as required by Department regulations, shall be available
on the project site at all times.
The design of all regulated activities shall include the following
to minimize stormwater impacts to reduce the surface discharge of
stormwater, reduce the creation of unnecessary impervious surfaces,
prevent the degradation of waters of the Commonwealth, and maintain
as much as possible the natural hydrologic regime of the site.
A.
The applicant shall apply low-impact development (LID) methods such
as those listed in Appendix E,[1] provided that use of this method does not conflict with
other local codes.
[1]
Editor’s Note: Appendix E is included as an attachment to this chapter.
B.
The applicant shall demonstrate that the design process follows the
sequence noted below. The goal of the sequence is to minimize the
increases in stormwater runoff and impacts to water quality resulting
from the proposed regulated activity:
(1)
The following items in this subsection shall be addressed prior to
development of other stormwater management site plan design elements:
(a)
Prepare an existing resource and site analysis map (ERSAM) showing
environmentally sensitive areas including, but not limited to, steep
slopes, ponds, lakes, streams, wetlands, hydric soils, vernal pools,
stream buffers, and hydrologic soil groups. Land development, any
existing recharge areas, and other requirements outlined in the municipal
SALDO shall also be included.
(c)
Prepare a draft project layout avoiding sensitive areas identified in § 292-17B(1)(a).
(d)
Identify site-specific existing conditions drainage areas, discharge
points, recharge areas, and hydrologic soil groups A and B (areas
conducive to infiltration).
(f)
Determine into what management district the site falls (Appendix
A) and conduct an existing conditions runoff analysis.
(2)
The following items in this subsection may be addressed in any order provided that all items in § 292-17B(1) have been completed.
(a)
Satisfy the infiltration objective § 292-18 and provide for stormwater pretreatment prior to infiltration.
(c)
Provide stream bank erosion protection in accordance with § 292-20 stream bank erosion requirements.
(d)
Prepare final project design to maintain existing conditions
drainage areas and discharge points, to minimize earth disturbance
and impervious surfaces, and, to the maximum extent possible, to ensure
that the remaining site development has no surface or point discharge.
(e)
Conduct a proposed conditions runoff analysis based on the final design that meets the management district requirements (§ 292-21).
(f)
Manage any remaining runoff prior to discharge through detention,
bioretention, direct discharge, or other structural control.
A.
Providing for infiltration consistent with the natural hydrologic
regime is required.
(1)
Design of the infiltration facilities shall consider infiltration
to compensate for the reduction in the recharge that occurs when the
ground surface is disturbed, or impervious surface is created.
(2)
If it cannot be physically accomplished, then the design professional shall be responsible for demonstrating to the satisfaction of the municipality that this cannot be physically accomplished on the site (e.g., shallow depth to bedrock or limiting zone, open voids, steep slopes, etc. per the PA BMP Manual. A financial hardship as defined in § 292-13 is not acceptable to avoid implementing infiltration facilities. If infiltration can be physically accomplished, the volume of runoff to be infiltrated shall be determined from § 292-18B(2), dependent on demonstrated site conditions, and shall be the greatest volume that can be physically infiltrated or alternative methods consistent with the PA BMP Manual (as amended) or other PADEP guidance, such as the Managed Release Concept, may be used to manage this volume with approval from the Municipal Engineer. For example:
(a)
Any applicant (developer or redeveloper) shall first attempt to infiltrate the volume required in § 292-18B(2)(a).
(b)
If the § 292-18B(2)(a) requirement cannot be physically accomplished, then the applicant is required to attempt to infiltrate the volume required in § 292-18B(2)(b).
(c)
Finally, if the § 292-18B(2)(b) infiltration volume cannot be physically accomplished, the applicant must, at a minimum, infiltrate the volume required in § 292-18B(2)(c).
B.
Infiltration BMPs shall meet the following minimum requirements:
(1)
Infiltration BMPs intended to receive runoff from developed or redeveloped
areas shall be selected based on suitability of soils and site conditions
and shall be constructed on soils that have the following characteristics:
(a)
A minimum depth of 24 inches between the bottom of the BMP and
the top of the limiting zone.
(b)
An infiltration rate sufficient to accept the additional stormwater
volume and dewater completely as determined by field tests conducted
by the applicant's design professional.
(c)
The infiltration facility shall be capable of completely draining
the retention (infiltration) volume (Rev) within three days (72 hours)
from the end of the design storm.
(2)
The size of the infiltration facility and Rev shall be based upon
the following volume criteria:
(a)
Modified Control Guideline One (MCG-1) of the PA BMP Manual
- The retention (infiltration) volume (Rev) to be captured and infiltrated
shall be the net two-year twenty-four-hour volume. The net volume
is the difference between the post-development runoff volume and the
pre-development runoff volume. The post-development total runoff volume
for all storms equal to or less than the two-year twenty-four-hour
duration precipitation shall not be increased. For modeling purposes,
existing (pre-development) non-forested pervious areas must be considered
meadow in good condition or its equivalent, and 20% of existing impervious
area, when present, shall be considered meadow in good condition.
(b)
Infiltrating the entire Rev volume in § 292-18B(2)(a) (above) may not be feasible on every site due to site-specific limitations such as shallow depth to bedrock or the water table. If it cannot be physically accomplished, then the following criteria from Modified Control Guideline Two (MCG-2) of the PA BMP Manual must be satisfied:
[1]
At least the first one inch of runoff from new or replacement
impervious surfaces shall be infiltrated.
|
Rev = one (inch)* impervious area (square feet) ÷
12 (inches) = cubic feet (cf)
|
|
An asterisk (*) in equations denotes multiplication.
|
(c)
Only if infiltrating the entire Rev volume in § 292-18B(2)(b) (above) cannot be physically accomplished, then the following minimum criteria from Modified Control Guideline Two (MCG-2) of the PA BMP Manual must be satisfied:
[1]
Wherever possible, infiltration facilities should be designed to accommodate infiltration of the entire water quality volume (WQv) (§ 292-19B); however, in all cases at least the first 1/2 inch of the WQv shall be infiltrated. The minimum infiltration volume (Rev) required would, therefore, be computed as:
|
Rev = I * impervious area (square feet) ÷ 12 (inches)
= cubic feet (cf)
|
|
An asterisk (*) in equations denotes multiplication.
|
|
Where:
|
|
I = The maximum equivalent infiltration amount (inches) that
the site can physically accept or 0.50 inch, whichever is greater.
|
[2]
The retention volume values derived from the methods in § 292-18B(2)(a),(b) or (c) is the minimum volume the applicant must control through an infiltration BMP facility. If site conditions preclude capture of runoff from portions of the impervious area, the infiltration volume for the remaining area should be increased an equivalent amount to offset the loss.
C.
Soils. A detailed soils evaluation of the project site shall be required
to determine the suitability of infiltration facilities. The evaluation
shall be performed by a qualified design professional and at minimum
address soil permeability, depth to bedrock, and subgrade stability.
The general process for designing the infiltration BMP shall be:
(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 sub-grade stability; infiltration
may not be ruled out without conducting these tests.
(2)
Provide field tests as required in the PA BMP Manual.
(3)
Design the infiltration structure for the required retention (Rev)
volume based on field determined capacity at the level of the proposed
infiltration surface.
(4)
If on-lot infiltration structures are proposed by the applicant's
design professional, it must be demonstrated to the Municipality that
the soils are conducive to infiltrate on the lots identified.
D.
Infiltration facilities should, to the greatest extent practicable,
be located to avoid introducing contaminants via groundwater, and
be in conformance with an approved source water protection assessment
or source water protection plan.
E.
Roadway drainage systems should provide an opportunity to capture
accidental spills. Road de-icing material storage facilities shall
be designed to avoid salt and chloride runoff from entering waterways
and infiltration facilities. The qualified design professional shall
evaluate the possibility of groundwater contamination from the proposed
infiltration facility and perform a hydrogeologic justification study
if necessary.
F.
The anti-degradation analysis found in Chapter 93 shall be applied
in HQ or EV streams.
G.
An impermeable liner will be required in detention basins where the
possibility of groundwater contamination exists. The Municipality
may require a detailed hydrogeologic investigation.
H.
The applicant should provide safeguards against groundwater contamination
for land uses that may cause groundwater contamination should there
be a mishap or spill.
I.
Subsurface disposal of stormwater.
(1)
Various methods of subsurface disposal may be employed. The effectiveness
and applicability of each should be evaluated at each location. Acceptable
methods include, but are not limited to, infiltration basins and/or
berms, seepage beds and/or trenches, and the like.
(2)
The design and construction of all subsurface facilities shall provide
proper procedures to prevent silt from clogging the aggregate backfill.
(3)
The following procedures and materials shall be required for all
subsurface facilities:
(a)
Excavation for the infiltration facility shall be performed
with equipment which will not compact the bottom of the seepage bed/trench,
or like facility.
(b)
The bottom of the bed and/or trench shall be scarified prior
to the placement of aggregate.
(c)
Only clean aggregate, free of fines, shall be allowed.
(d)
The top and sides of all seepage beds, trenches, or like facilities
shall be covered with drainage filtration fabric.
(e)
Perforated distribution pipes connected to centralized catch
basins and/or manholes with provisions for the collection of debris
shall be provided in all facilities. The perforated pipes shall distribute
stormwater throughout the entire seepage bed/trench, or like facility.
(4)
The landowner or developer shall be responsible for the proper installation,
operation, and maintenance of all subsurface stormwater management
facilities. If, in the opinion of the Township, the underground system
is not functioning properly, the landowner or developer shall be required
to make the necessary improvements/corrections to the system or provide
an alternate stormwater management facility which is functional.
The applicant shall comply with the following water quality
requirements of this article.
A.
To control post-construction stormwater impacts from regulated activities
and conform to state water quality requirements, BMPs which replicate
pre-development stormwater infiltration and runoff conditions must
be provided in the site design such that post-construction stormwater
discharges do not degrade the physical, chemical, or biological characteristics
of the receiving waters. The green infrastructure and low-impact development
(LID) practices provided in the PA BMP Manual, as well as the guidance
on green infrastructure and LID provided in Appendix E[1] shall be utilized for all regulated activities wherever
possible. This may be achieved by the following:
(1)
Infiltration: replication of pre-construction stormwater infiltration
conditions,
(2)
Treatment: use of water quality treatment BMPs to provide filtering
of chemical and physical pollutants from the stormwater runoff, and
(3)
Stream bank and stream bed protection: management of volume and rate
of post-construction stormwater discharges to prevent physical degradation
of receiving waters (e.g., from scouring).
[1]
Editor's Note: Appendix E is included as an attachment to this chapter.
B.
Developed areas shall provide adequate storage and treatment facilities necessary to capture and treat stormwater runoff. The infiltration volume computed under § 292-18 may be a component of the water quality volume if the applicant chooses to manage both components in a single facility. If the calculated water quality volume (WQv) is greater than the volume required to be infiltrated as described in § 292-18B(2), then the difference between the two volumes shall be treated for water quality by an acceptable stormwater management practice(s). The required water quality volume (WQv) is the storage capacity needed to capture and treat a portion of stormwater runoff from the developed areas of the site. To achieve this requirement, the following criterion is established:
(1)
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). If the Municipal
Engineer concurs that this criterion cannot be met, a minimum of 1/2
inches of runoff from all regulated impervious surfaces shall be managed.
For modeling purposes, existing (predevelopment) nonforested pervious
areas must be considered meadow in good condition or its equivalent,
and 20% of existing impervious area, when present, shall be considered
meadow in good condition.
(2)
This volume requirement can be managed by the permanent volume of
a wet basin or the detained volume from other BMPs. Where appropriate,
wet basins shall be utilized for water quality control and shall follow
the guidelines of the PA BMP Manual referenced in Appendix G.[2]
[2]
Editor's Note: Appendix G is included as an attachment to this chapter.
(3)
Release of water can begin at the start of the storm (i.e., the invert
of the water quality orifice is at the invert of the facility). The
design of the facility shall provide for protection from clogging
and unwanted sedimentation.
C.
The temperature of receiving waters shall be protected through the
use of BMPs that moderate temperature.
D.
If a perennial or intermittent stream passes through, or a water
body (i.e., lake, pond, wetland) is present on the site, the applicant
shall create a riparian buffer extending a minimum of the greater
of 50 feet or such other distance as required by federal and state
buffer policies and regulations, to either side of the top-of-bank
of the channel, lake, or wetland. The buffer area shall be planted
with native vegetation and maintained in a vegetated state (refer
to Appendix B, Pennsylvania Native Plant List, contained in the PA
BMP Manual).
(1)
The following provisions also apply to riparian buffers on lots in
existence at the time of adoption of this chapter:
(a)
If the applicable rear or side yard setback is less than 50
feet, the buffer width may be reduced to 25% of the setback or 35
feet, whichever is greater.
(b)
If a stream traverses a site in a manner that significantly
reduces the use of the site, the buffer may be reduced to 25% of the
setback or 35 feet, whichever is greater.
(c)
Notwithstanding the foregoing, if an existing buffer is legally
prescribed (e.g., deed covenant, easement, etc.) and it exceeds the
requirement of this chapter, the existing buffer shall be maintained.
(2)
Permitted uses within the buffer include the following, subject to
Township approval and provided that they comply with all federal,
state, and local regulations:
(a)
Recreational trails. See Riparian Buffer Trail Guidelines.[3]
[3]
Editor's Note: The document "Riparian Buffer Trail Guidelines" is included as an attachment to this chapter.
(b)
Utility rights-of-way.
(c)
Bridges.
(d)
Other uses subject to Township approval, provided they do not
interfere with, alter, or otherwise disturb the buffer.
E.
Evapotranspiration may be quantified and credited towards meeting
volume requirements according to the PADEP Post Construction Stormwater
Management (PCSM) Spreadsheet and Instructions (December 2020) or
the most recent guidance from PADEP.
F.
If an existing buffer is legally prescribed (i.e., deed, covenant,
easement, etc.) and it exceeds the requirements of this chapter, the
existing buffer shall be maintained.
A.
In addition to controlling the water quality volume (in order to
minimize the impact of stormwater runoff on downstream stream bank
erosion), the primary requirement to control stream bank erosion is
to design a BMP to detain the proposed conditions two-year, twenty-four-hour
design storm to the existing conditions one-year flow using the SCS
Type II distribution. Additionally, provisions shall be made (such
as adding a small orifice at the bottom of the outlet structure) to
release the proposed conditions one-year storm for a minimum of 24
hours from a point in time when the maximum volume of water from the
one-year storm is stored in a proposed BMP (i.e., 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 water quality
orifice is at the invert of the facility).
B.
The minimum orifice size in the outlet structure to the BMP shall
be three inches in diameter where possible, and a trash rack shall
be installed to prevent clogging. On sites with small drainage areas
contributing to this 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. 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. All facilities shall make use of measures to extend
the flow path and increase the travel time of flows in the facility.
A.
Each watershed has been divided into either stormwater management
districts or release rate districts as shown on the respective Management
District or Release Rate Maps in Appendix A.[1]
(1)
In addition to the watershed-specific requirements specified in Tables 308.1, 308.3, and 308.4 below, the erosion and sedimentation control (§ 292-16), the nonstructural project design (§ 292-17), the infiltration (§ 292-18), the water quality (§ 292-19), and the stream bank erosion (§ 292-20) requirements shall be implemented.
(2)
Standards for managing runoff from each subarea in a watershed for
the two-, five-, ten-, twenty-five-, fifty-, and 100-year design storms
are shown in Tables 308.1, 308.3, and 308.4. Development sites located
in each of the management/release rate districts must control proposed
conditions runoff rates to existing conditions runoff rates for the
design storms in accordance with the Table(s).
[1]
Editor's Note: Appendix A is included as an attachment to this chapter.
B.
General. Proposed conditions rates of runoff from any regulated activity
shall not exceed the peak release rates of runoff from existing conditions
for the design storms specified on the Stormwater Management District
Watershed Map (Appendix A)[2] and this section of the chapter.
[2]
Editor's Note: Appendix A is included as an attachment to this chapter.
C.
District boundaries. The boundaries of the stormwater management
districts are shown on an official map that is available for inspection
at the municipal and County Planning offices. A copy of the official
map at a reduced scale is included in Appendix A. The exact location
of the stormwater management district boundaries as they apply to
a given development site shall be determined by mapping the boundaries
using the two-foot topographic contours (or most accurate data required)
provided as part of the SWM site plan.
D.
Sites located in more than one district or watershed. For a proposed
development site located within two or more stormwater management
district subareas, the peak discharge rate from any subarea shall
meet the management district criteria for which the discharge is located.
The natural hydrology of each respective subarea shall be maintained,
and drainage shall not be redirected from one subarea to another.
Under circumstances where the applicant shows this cannot be accomplished,
a waiver is required by the Municipality.
|
Table 308.1
Peak Rate Control Standards In the Crum Creek Watershed
| |||
|---|---|---|---|
|
District
|
Proposed Condition Design Storm
|
(Reduce to)
|
Existing Condition Design Storm
|
|
A
|
2-year
|
1-year
| |
|
5-year
|
5-year
| ||
|
10-year
|
10-year
| ||
|
25-year
|
25-year
| ||
|
50-year
|
50-year
| ||
|
100-year
|
100-year
| ||
|
B
|
2-year
|
1-year
| |
|
5-year
|
2-year
| ||
|
10-year
|
5-year
| ||
|
25-year
|
10-year
| ||
|
50-year
|
25-year
| ||
|
100-year
|
100-year
| ||
|
Table 308.3
Ridley Creek Watersheds Peak Rate Control Standards in the Ridley
Creek Watersheds
| ||
|---|---|---|
|
Proposed Condition Design Storm
|
(Reduce to)
|
Existing Condition Design Storm
|
|
2-year
|
1-year
| |
|
5-year
|
5-year
| |
|
10-year
|
10-year
| |
|
25-year
|
25-year
| |
|
50-year
|
50-year
| |
|
100-year
|
100-year
| |
|
Table 308.3: For the Ridley Creek Watershed. The Delaware Direct
table should be used for all subareas listed as 100% release rate.
However, for subareas below 100%, the Municipal Engineer should make
a determination as to the correct requirement.
|
|
Table 308.3: For the Chester Creek Watershed. Watershed subareas
with a 100% should confirm that the peak rate control requirement
is at least as stringent as the Delaware Direct requirement.
|
|
Table 308.4
Control Criteria for Chester Creek Watershed Stormwater Management
Districts
| |
|---|---|
|
District
|
Control Criteria
|
|
100%
|
Post-development peak discharge for all design storms must be
no greater than pre-development peak discharges.
|
|
75%
|
Post-development peak discharge for all design storms must be
no greater than 75% of the pre-development peak discharges.
|
|
50%
|
Post-development peak discharge for all design storms must be
no greater than 50% of the pre-development peak discharges.
|
|
Table 308.4: Off-site Areas. Off-site areas that drain through
a proposed development site are not subject to release rate criteria
when determining allowable peak runoff rates. On-site drainage facilities
shall be designed to safely convey off-site flows through the development
site.
|
E.
Site areas. Where the site area to be impacted by a proposed development
activity differs significantly from the total site area, only the
proposed impact area utilizing stormwater management measures shall
be subject to the peak rate control standards noted above. Unimpacted
areas for which the discharge point has not changed are not subject
to the peak rate control standards.
F.
"Downstream hydraulic capacity analysis." Any downstream capacity
hydraulic analysis conducted in accordance with this chapter shall
use the following criteria for determining adequacy for accepting
increased peak flow rates:
(1)
Natural or man-made channels or swales must be able to convey the
increased runoff associated with a two-year storm event within their
banks at velocities consistent with protection of the channels from
erosion. Velocities shall be based upon criteria and methodologies
acceptable to the municipality.
(2)
Natural or man-made channels or swales must be able to convey increased
twenty-five-year storm event runoff without creating any increased
hazard to persons or property.
(3)
Culverts, bridges, storm sewers or any other hydraulic facilities which must pass or convey flows from the tributary area must be designed in accordance with PADEP Chapter 105 regulations (if applicable) and, at a minimum, pass the increased twenty-five-year storm event runoff.
(5)
Post construction peak rates shall not exceed the existing peak rates
for the respective subarea.
G.
Alternate criteria for redevelopment sites. For redevelopment sites,
one of the following minimum design parameters shall be accomplished,
whichever is most appropriate for the given site conditions as determined
by Edgmont Township;
H.
Stormwater management.
(1)
Where a development site is traversed by watercourses, riparian buffers
shall be provided conforming to the line of such watercourses. The
width of the buffers shall be determined as set forth in this chapter.
Excavating, placing of fill, building structures, or making any alterations
that may adversely affect the flow of stormwater within any portion
of the riparian buffer shall be prohibited unless the proposed work
is associated with a regulated wetlands mitigation program. The buffer
must be defined and preserved through a deed covenant.
(2)
Any stormwater management facilities regulated by this chapter that
would be located in or adjacent to waters of the commonwealth or wetlands
shall be subject to approval by the Department through the joint permit
application process or, where deemed appropriate by the Department,
the general permit process. When there is a question as to whether
wetlands may be involved, it is the responsibility of the applicant
or his agent to show that the land in question cannot be classified
as wetlands; otherwise, approval to work in the area must be obtained
from the Department.
(3)
Minimization of impervious surfaces and infiltration of runoff through
seepage beds, infiltration structures, etc., are required, where soil
conditions permit, to reduce the size or eliminate the need for detention
facilities.
(4)
In order to promote overland flow and infiltration/percolation of
stormwater, roof drains must discharge into an accepted BMP providing
infiltration and filtering of the stormwater.
(5)
Detention/retention basins.
(a)
Detention basins shall be designed in accordance with the Soil
Cover Complex Method and the procedures developed by the U.S. Department
of Agriculture, Natural Resource Conservation Service (NRCS) as outlined
in its Technical Release No. 55, Urban Hydrology for Small Watersheds,
with specific attention given to antecedent moisture conditions, flood
routing, and peak discharge, and Hydrology National Engineering Handbook
Section 4. However, for development areas of less than five acres,
the Rational Method may be used to compute the sizing of basins.
(b)
Basins shall be designed to detain the quantity of water resulting from a 100-year, twenty-four-hour storm under full development conditions. Stormwater management calculations shall ensure that the predevelopment peak discharge from the development site is not exceeded after development for the two-, five-, ten-, twenty-five-, fifty-, and 100-year design storms (see § 292-21, Stormwater peak rate control, and § 292-22, Calculation methodology). Rainfall data shall be based on NOAA Atlas 14 partial duration series estimates (available online at http://hdsc.nws.noaa.gov/hdsc/pfds). The time of concentration method shall be utilized in the development of the runoff hydrographs and peak discharges. Storage discharge curves shall be provided for all basins. Average antecedent moisture conditions and Type II distribution storm shall be assumed.
(c)
Outlet control structures.
[1]
All outlet control structures shall be constructed of galvanized
steel, aluminum or concrete, properly anchored to prevent flotation,
and equipped with childproof, nonclogging, removable trash racks over
all design openings 12 inches or greater in diameter, except those
openings designed to carry perennial stream flows.
[2]
Temporary sedimentation controls shall be provided during construction
to prevent the flow of sediment through the basin outlet pipe. Such
measures may include temporary riser pipes, rock-filled gabions, and
the like.
(d)
Emergency spillways. Whenever possible, the emergency spillway
for basins shall be constructed on undisturbed ground. Emergency spillways
shall be constructed of reinforced concrete, concrete-mound slabs,
vegetated earth, concrete rubble, and the like. All emergency spillways
shall be constructed so that the basin berm is protected against soil
erosion. The minimum capacity of the emergency spillway shall be designed
to pass the difference between the 100-year post development and 100-year
predevelopment flows. Emergency spillways shall extend along the upstream
and downstream berm embankment slopes. The emergency spillway shall
not discharge stormwater over earthen fill and/or easily erodible
material without adequate protection against soil erosion.
(e)
Freeboard. The minimum freeboard shall be two feet. ("Freeboard"
is the difference between the design flow elevations in the emergency
spillway and the top of the settled basin embankment.)
(f)
Basin outlet pipes. Basin outlet pipes shall be equipped with
watertight joints.
(g)
Antiseep collars. Antiseep collars shall be installed around
the principal pipe barrel within the normal saturation zone of the
basin berms. The antiseep collars and their connections to the pipe
barrel shall be watertight. The antiseep collars shall extend to a
minimum of two feet beyond the outside of the principal pipe barrel.
The maximum spacing between collars shall be 14 times the minimum
projection of the collar measured perpendicular to the pipe. Design
calculations for antiseep collars must be submitted with the basin
calculations.
(h)
Basin outlets. Energy-dissipating devices (riprap, stilling
basin, concrete aprons, and the like) shall be placed at all basin
outlets. Rock level spreader berms shall be required where basins
do not discharge into an existing drainage swale, ditch or channel.
Concrete endwalls shall be placed at all basin outlets. All basin
outlet pipes 12 inches in diameter or greater shall be equipped with
childproof devices to deter entry by pedestrians or animals. Design
calculations for proposed energy dissipaters must be submitted with
basin calculations.
(i)
Slope of detention basin embankment. The maximum slope of earthen
basin embankments shall be four to one. The top or toe of any slope
shall be located a minimum of 15 feet from adjacent property lines
with the exception of the downstream property line where the toe of
the embankment shall be placed a sufficient distance to allow for
energy dissipating devices but in no case less than 40 feet unless
approved otherwise by the Township. Whenever possible, the side slopes
and basin shape shall blend with the natural topography. Straight
side slopes and rectangular basins shall be avoided whenever possible.
(j)
Width of berm. The minimum top width of detention basin berms
shall be 10 feet.
(k)
Construction specifications. The plans shall indicate the construction
specifications and compaction requirements for all detention/retention
basins.
(l)
Slope of basin bottom. In order to ensure proper drainage of
detention basins, a minimum grade of 2% shall be provided. A level
bottom may be permitted, provided that the basin bottom is landscaped
with appropriate wetland-type vegetation.
(m)
Cutoff trench. A cutoff trench shall be excavated along the
center line of the dam on earth-fill embankments. The minimum depth
shall be three feet. The minimum bottom width shall be 10 feet or
wide enough to permit operation of compaction equipment. The side
slopes shall be no steeper than 1:1. The trench shall be kept free
from standing water during the backfilling operations.
(n)
Cuts and fills. No excavation or fill shall be made with a cut or fill slope steeper than four horizontal to one vertical. A written statement shall be required from a civil engineer licensed by the commonwealth, having experience in soils engineering, certifying that he has inspected the site and that any proposed deviation from the slope specified above should not endanger any property or result in personal injury. Retaining walls will be required if a stable slope cannot be maintained. Any retaining wall design must be designed by an experienced structural engineer licensed by the commonwealth and approved by the Township. The toe of any cut or fill slope must be located a minimum of 15 feet from adjacent property lines with the exception stated in Subsection H(5)(i) above.
(o)
Landscaping.
[1]
A minimum of six inches of topsoil shall be placed on all areas
affected by the basin construction (bottom of basin, side slopes,
top of berm, and the like).
[2]
All earthen basins shall be hydroseeded with temporary and permanent
grasses or other approved ground covers within seven days after final
grading.
[3]
Fencing may be required around detention/retention basins where
the Board of Supervisors determines that circumstances warrant the
fencing. Whenever fencing is not required, shrub plantings shall be
installed around the facility.
[4]
The perimeter of all basins shall be built and maintained as
lawn.
(p)
Basins shall be installed prior to any earthmoving or land disturbances
which they will serve.
(q)
Whenever a basin is to be located in an area underlain by limestone
or other unstable rock formation, a geological evaluation of the proposed
location shall be conducted to determine susceptibility to sinkhole
formations. The design of all facilities over limestone formations
shall include measures to prevent groundwater contamination and, where
necessary, sinkhole formation.
(r)
Permanent pond. A five-foot-wide bench sloping at 4% shall be
provided for all detention/retention basins designed to contain a
permanent pond of water. When a permanent pond is proposed, a report
of a certified geotechnical specialist must be provided, certifying
that the water will not become stagnant. The basin side slopes below
the waterline must not exceed 4:1.
(6)
Maintenance of stormwater management facilities.
(a)
All stormwater management facilities, including detention and
retention basins designed and constructed for the purposes specified
in this chapter, shall be maintained in accordance with the plans
filed with and approved by the Township; the responsibility of the
property owner(s) upon whose property the facilities are located;
and in accordance with any deed restrictions or notes on the SWM site
plan. The landowner or developer may establish a homeowners'
association that, with the Township's approval, shall be the
responsible owner of all stormwater management facilities located
in the area of the development.
(b)
In order to ensure proper maintenance and function of stormwater
management facilities, the Township or its designee may perform inspections.
(c)
If at any time the Township, or its designee, discovers any
violation or condition not conforming with those designs and plans
filed with and approved by the Township in regards to the operation
of a stormwater management facility, the Township shall notify the
responsible owners of the violation and the manner in which it can
be corrected.
(d)
Under no conditions shall any person be allowed to remove, modify,
alter or change any previously approved stormwater management facility
unless an alternate facility is approved by the Township.
(e)
In the event the landowner, developer, or homeowner's association,
as the case may be, shall refuse or neglect to comply with the provisions
of this section as interpreted by the Township, the Township may direct
the work to correct any violation or noncompliance with the terms
of this chapter, and all other chapters and codes of the Township
and institute action for payment of costs incurred. In addition, the
Township may pursue any other remedy available under this chapter
or at law or in equity.
A.
Stormwater runoff from all development sites with a drainage area
of greater than five acres shall be calculated using a generally accepted
calculation technique that is based on the NRCS Soil Cover Complex
Method. Table 309.1 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 use of the Rational Method to estimate peak discharges for drainage
areas greater than five acres shall be permitted only upon approval
of 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.
|
|
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
and/or Municipal Engineer.
|
|
Other Methods
|
Varies
|
Other computation methodologies approved by the Municipality
and/or Municipal Engineer.
|
B.
All calculations consistent with this chapter using the Soil Cover
Complex Method shall use the appropriate design rainfall depths for
the various return period storms. Rainfall depths shall be according
to NOAA Atlas 14 values consistent with a partial duration series.
When stormwater calculations are performed for routing procedures
or water quality functions, the duration of rainfall shall be 24 hours.
C.
The following criteria shall be used for peak rate runoff calculations:
(1)
For development sites not considered redevelopment, the ground cover
used in determining the existing conditions flow rates shall be as
follows:
(a)
Wooded sites shall use a ground cover of "woods in good condition."
Portions of a site having more than one viable tree measuring a diameter
at breast height (DBH) of six inches or greater per 1,500 square feet
shall be considered wooded where such trees existed within three years
of application.
(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 (CN) number
or Rational "c" value (i.e., woods) as listed in Tables F-1 or F-2
in Appendix F.[1]
[1]
Editor's Note: Appendix F is included as an attachment to this chapter.
(2)
For redevelopment sites, existing non-wooded pervious areas shall
be considered meadow in good condition or its equivalent and 20% of
existing impervious area, when present, shall be considered meadow
when in good condition.
D.
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 a minimum of five minutes.
E.
Runoff curve numbers (CN) for both existing and proposed conditions
to be used in the Soil Cover Complex Method shall be obtained from
Table F-1 in Appendix F.
F.
Runoff coefficients (c) for both existing and proposed conditions
for use in the Rational Method shall be obtained from Table F-2 in
Appendix F.
G.
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). 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). Values for Manning's roughness coefficient (n) shall
be consistent with Table F-3 in Appendix F.
H.
Outlet structures for stormwater management facilities shall be designed
to meet the performance standards of this chapter using any generally
accepted hydraulic analysis technique or method.
I.
The design of any stormwater detention facilities intended to meet
the performance standards of this chapter shall be verified by routing
the design storm hydrograph through these facilities using an acceptable
method. 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.
A.
Storm sewer system.
(1)
Design flow rate.
(a)
The storm sewer system shall be designed to carry a twenty-five-year
peak flow rate. Storm sewer systems which convey runoff to a stormwater
management facility must be designed to convey the 100-year peak flow
rate. The peak flow rate into each inlet shall be indicated on the
SWM site plan. The design flow rate shall be determined by the rational
formula, Q = CIA.
|
Where:
| ||
|
Q
|
=
|
Peak runoff rate, cubic feet per second (CFS)
|
|
C
|
=
|
Runoff coefficient equal to the ratio of the peak runoff rate
to the average rate of rainfall over a time period equal to the time
of concentration.
|
|
I
|
=
|
Average rainfall intensity in inches per hour for a time equal
to the time of concentration.
|
|
A
|
=
|
Drainage area in acres.
|
(b)
Appropriate values for the runoff coefficient and rainfall intensity
shall be taken from the following source:
|
PA DOT
|
|
Design Manual, Part 2, Highway Design
|
|
August 1981 (or the latest revisions thereto)
|
(2)
Storm sewer system design.
(a)
The storm sewer system shall be designed to the more restrictive
of the following: to collect stormwater at any point where three cubic
feet to five cubic feet per second is accumulated during the design
storm, and/or inlets/manholes shall not be spaced more than 300 feet
apart on pipe sizes up to 24 inches in diameter and not more than
400 feet apart on greater sizes.
(b)
Inlets, manholes, grates, covers, frames and the like shall
conform to the PA DOT roadway construction standards and Form 408
specifications and all amendments, revisions or updates thereto. All
inlets and manholes shall be precast concrete.
(3)
Bridge/culvert/channel design.
(a)
Bridges and culverts shall have ample waterway to carry expected
flows, based on minimum storm frequency of 25 years or as required
by the Department. Bridge and/or culvert design shall be in accordance
with the PA DOT and/or the Department requirements. All culverts shall
be provided with concrete end walls or concrete end sections unless
approved otherwise by the Township.
(b)
All drainage channels shall be designed to carry a flow rate
equal to a 100-year, twenty-four-hour storm.
(c)
All drainage channels shall be designed to prevent the erosion
of the stream bed and stream bank areas. The flow velocity in all
vegetated drainage channels shall not exceed the maximum permissible
velocity to prevent soil erosion. Suitable bank stabilization shall
be provided where required to prevent soil erosion of the drainage
channels. Where storm sewers discharge into existing drainage channels
at an angle greater than 30° from parallel with the downstream
channel flow, the far side bank shall be stabilized by the use of
riprap, masonry and/or concrete walls. The stabilization shall be
designed to prevent soil erosion and frost heave under and behind
the stabilizing media.
(d)
Any vegetated drainage channel requiring mowing of the vegetation
shall have a maximum slope of four horizontal to one vertical on those
areas to be mowed.
(e)
The design of all channels shall, as a minimum, conform to the
design procedures outlined in:
[1]
United States Department of Transportation.
[2]
Federal Highway Administration.
[3]
Roadside Drainage Channels.
[4]
Hydraulic Design Series No. 4.
[5]
Hydraulic Design Series No. 3.
[6]
Standards and Specifications for Soil.
[7]
Erosion and Sediment Control in Developing Areas.
[8]
United States Department of Agriculture.
[9]
Soil Conservation Service.
[10]
College Park, Maryland.
(4)
Overflow system. An overflow system shall be provided to carry flow
to the detention basin when the capacity of the storm drain pipe system
is exceeded. The overflow system shall be sufficient capacity to carry
the difference between the 100-year and the twenty-five-year peak
flow rates.
(5)
Inlet capacity.
(a)
All inlets must be designed to accommodate the twenty-five-year
peak flow rate except at low points where they shall accommodate the
twenty-five-year peak flow rate. The capacity of Type C, M, or S inlets
shall be determined from the following source:
(b)
The capacity of each inlet shall be indicated on the SWM site
plan. All SWM site plans shall indicate that inlet grates be installed
in such a manner that the roadway stormwater will be directed into
the inlet and away from the roadway. At curbed street/driveway intersections,
inlets shall be placed on the tangent section and not in the curved
portion of the curbing.
(6)
Straight pipe sections. All storm sewers shall be designed to follow
straight courses. No angular deflections of storm sewer pipe sections
in excess of 5° shall be permitted. No vertical curves shall be
permitted in any storm sewer system.
(7)
Minimum grade and size. All storm drain pipes shall be designed to
maintain a minimum grade of 1%. All storm sewer pipes shall have a
minimum inside diameter of 15 inches.
(8)
Pipe capacity. The capacity of all pipe culverts shall, as a minimum,
provide the required carrying capacity as determined by the following
sources:
(a)
United States Department of Transportation Federal Highway Administration
Hydraulic Engineering Circular No. 5, Hydraulic Charts for the Selection
of Highway Culverts.
(b)
United States Department of Transportation Federal Highway Administration
Hydraulic Design Series No. 3, Design Charts for Open Channel Flow.
(c)
United States Department of Transportation Bureau of Public
Roads Hydraulic Engineering Circular No. 10, Capacity Charts for the
Hydraulic Design of Highway Culverts.
(9)
Pipe arches. Where headroom is restricted, equivalent pipe arches
may be used in lieu of circular pipes.
(10)
Pipe material and gauge thickness. All storm sewers shall be
either reinforced cement concrete, corrugated aluminum or corrugated
galvanized steel pipe. Storm sewers shall be of the proper class and
thickness to support the above fill material. Pipe class and gauge
or thickness shall be noted on the plans.
(11)
Allowable headwater depth. At all inlets or manholes, the maximum
allowable headwater depth shall be one foot below the top of the inlet
grate or the manhole cover.
(12)
Horizontal pipe deflections. A manhole or inlet shall be provided
at all horizontal deflections in the storm pipe system exceeding 5°.
(13)
Minimum and maximum cover. In lawn areas, a minimum cover satisfactory
to the Township shall be maintained over all storm drain pipes. Under
streets, the top of storm drain pipes shall be at least 0.5 foot below
subgrade elevation. The maximum cover over storm drain pipes shall
be 10 feet unless approved otherwise by the Township.
(14)
Storm sewer system outlets. Storm sewer system outlet pipes
shall extend to proposed stormwater management facilities, natural,
and the like. A concrete end wall shall be required on all storm sewer
system outlet pipes. All storm/sewer outlets 12 inches in diameter
or greater shall be equipped with a galvanized, childproof bar rack,
bolted to the end wall.
(15)
Drainage easements.
(a)
All storm sewer easements through undedicated land shall be
a minimum of 20 feet.
(b)
Where a site is traversed by a watercourse, a drainage easement
or right-of-way conforming substantially with the line of such watercourse
and of such width as will be adequate to preserve natural drainage
and provide sufficient width for maintenance shall be created, as
determined by the Township.
(16)
Diversion of surface water runoff. All storm sewers and/or drainage
swales shall be designed to carry such runoff into a detention basin
or similar facility utilized to control the rate of runoff, unless
approved otherwise by the Township.
B.
Maintenance of storm sewer systems and watercourses.
(2)
It is the responsibility of any landowner or developer doing any
act on or across a watercourse or swale or upon the floodplain or
right-of-way thereof to maintain as nearly as possible in its present
state the stream, watercourse, swale, floodplain or right-of-way for
the duration of the construction activity and to return it to its
original or equal condition after such activity is completed.
(3)
Maintenance of drainage facilities or watercourses originating on
private property is the responsibility of the owner to the owner's
point of open discharge at the property line or at a watercourse within
the property.
(4)
No landowner or developer shall block, impede the flow of, alter,
construct any structure, or deposit any material, or commit any act
which will affect normal or flood flow in any watercourse without
having obtained prior approval from the Township and the Department.
A.
All wet basin designs shall incorporate biologic controls consistent
with the West Nile Guidance found in Appendix H,[1] PADEP document 363-0300-001 "Design Criteria - Wetlands
Replacement/Monitoring," or contact the Pennsylvania State Cooperative
Wetland Center (www.wetlands.psu.edu/) or the Penn State Cooperative
Extension Office (www.extension.psu.edu/extmap.html).
[1]
Editor's Note: Appendix H is included as an attachment to this chapter.
B.
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 handle flow up to and including the 100-year proposed conditions. The height of embankment must provide a minimum of one foot of freeboard above the maximum pool elevation computed when the facility functions for the 100-year proposed conditions inflow. Should any stormwater management facility require a dam safety permit under PADEP Chapter 105, the facility shall be designed in accordance with Chapter 105 and meet the regulations of Chapter 105 concerning dam safety. Chapter 105 may require the passing of storms larger than 100-year event.
C.
Any drainage conveyance facility and/or channel not governed by Chapter 105 regulations must be able to convey, without damage to the drainage structure or roadway, runoff from the twenty-five-year storm event. The larger the events fifty-year and 100-year) must also be safely conveyed in the direction of natural flow without creating additional damage to any drainage structures, nearby structures, or roadways.
D.
Conveyance facilities transporting flow to or exiting from stormwater
management facilities (i.e., detention basins) shall be designed to
convey the 100-year frequency storm.
E.
Roadway crossings or structures located within designated floodplain
areas must be able to convey runoff from a 100-year design storm consistent
with Federal Emergency Management Agency National Flood Insurance
Program - Floodplain Management Requirements.
F.
Any facility located within a PennDOT right-of-way must meet PennDOT
minimum design standards and permit submission requirements.
G.
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) and
the PADEP Erosion and Sediment Pollution Control Program Manual (Publication
No. 363-2134-008).
