A.
Applicants proposing regulated activities in the municipality that do not fall under the exemption criteria shown in § 99-6 shall submit a drainage plan consistent with this chapter and the respective Act 167 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.
The applicant is required to find practicable alternatives to the
surface discharge of stormwater, the creation of impervious surfaces,
and the degradation of waters of the commonwealth and must maintain
as much as possible the natural hydrologic regime.
C.
The drainage plan must be designed consistent with the sequencing provisions of § 99-43 to ensure maintenance of the natural hydrologic regime, to promote groundwater recharge, and to protect groundwater and surface water quality and quantity. The drainage plan designer must proceed sequentially in accordance with Article IV of this chapter.
D.
Stormwater drainage systems shall be designed in order to permit
unimpeded flow along natural watercourses, except as modified by stormwater
management facilities or open channels consistent with this chapter.
E.
Existing points of concentrated drainage that discharge onto adjacent
property shall not be altered in any manner which could cause property
damage without permission of the affected property owner(s) and shall
be subject to any applicable discharge criteria specified in this
chapter.
F.
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.
G.
Where a development site is traversed by existing streams, drainage easements shall be provided conforming to the line of such streams. The terms of the easement shall conform to the stream buffer requirements contained in § 99-45G of this chapter.
H.
Any stormwater management facilities regulated by this chapter that
would be located in or adjacent to waters of the commonwealth or delineated
wetlands shall be subject to approval by DEP through the joint permit
application or the environmental assessment approval process, or where
deemed appropriate, by the DEP 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 DEP.
I.
Any proposed stormwater management facilities regulated by this chapter
that would be located on state highway rights-of-way shall be subject
to approval by PennDOT.
J.
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.
K.
All stormwater runoff shall be pretreated for water quality prior
to discharge to surface or groundwater.
L.
All regulated activities within the municipality shall be designed,
implemented, operated, and maintained to meet the purposes of this
chapter, through these two elements:
M.
No regulated earth disturbance activities within the municipality
shall commence until the requirements of this chapter are met.
O.
Operations and maintenance of permanent stormwater BMPs shall be addressed as required by Article VII.
P.
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.
Q.
Techniques described in Appendix D (Low Impact Development) of this
chapter shall be considered because they reduce the costs of complying
with the requirements of this chapter and the state water quality
requirements.
R.
In selecting the appropriate BMPs or combinations thereof, the applicant
shall consider the following:
(1)
Total contributing 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)
Effectiveness 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.
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 DEP under regulations at 25 Pennsylvania Code Chapter 102.
B.
Work within natural drainageways subject to permit by DEP under 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 DEP under 25 Pennsylvania Code Chapter 105.
D.
Any stormwater management facility that would be located on a state
highway right-of-way or require access from a state highway shall
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 DEP under 25 Pennsylvania
Code Chapter 105.
A.
No regulated earth-disturbance activities within the municipality
shall commence until the municipality receives an approval from the
Conservation District of an erosion and sediment control plan for
construction activities.
B.
DEP has regulations that require an erosion and sediment control
plan for any earth disturbance activity of 5,000 square feet or more,
under 25 Pennsylvania Code § 102.4(b).
C.
In addition, under 25 Pennsylvania Code Chapter 92, a DEP Permit
for Stormwater Discharges Associated with Construction Activities
is required for land disturbances greater than one acre.
D.
Evidence of any necessary permit(s) for regulated earth disturbance
activities from the appropriate DEP regional office or County Conservation
District must be provided to the municipality.
E.
A copy of the erosion and sediment control plan and any required
permit, as required by DEP regulations, shall be available on the
project site at all times.
F.
Additional erosion and sediment control design standards and criteria
are recommended to be applied where infiltration BMPs are proposed.
They shall include the following:
(1)
Areas proposed for infiltration BMPs shall be protected from sedimentation
and compaction during the construction phase to maintain maximum infiltration
capacity. Additional measures, such as placement of orange construction
fencing around proposed infiltration BMPs during construction to minimize
or eliminate traffic over top of these areas, and temporary sealing
off of pipes and inlet connections to infiltration BMPs to prevent
sediment clogging should be given consideration.
(2)
Infiltration BMPs shall not be constructed nor receive runoff until
the entire drainage area contributory to the infiltration BMP has
achieved final stabilization.
A.
The design of all regulated activities shall include the following
to minimize stormwater impacts.
(1)
The applicant shall find practicable alternatives to the surface
discharge of stormwater, such as those listed in Appendix E,[1] Table E-4, the creation of impervious surfaces, and the
degradation of waters of the commonwealth and must maintain as much
as possible the natural hydrologic regime of the site.
[1]
Editor's Note: Appendix E is included at the end of this chapter.
(2)
An alternative is practicable if it is available and capable of implementation
after taking into consideration existing technology and logistics
in light of overall project purposes and other municipal requirements.
(3)
All practicable alternatives to the discharge of stormwater are presumed
to have less adverse impact on quantity and quality of waters of the
commonwealth unless otherwise demonstrated.
B.
The applicant shall demonstrate that the regulated activities were
designed in the following sequence. 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)
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, hydrologic soil groups, wooded areas, and potential
infiltration areas. Land development, any existing recharge areas,
and other requirements outlined in the Municipal SALDO shall also
be included.
(4)
Identify site-specific existing conditions drainage areas, discharge
points, recharge areas, and hydrologic soil groups A and B (areas
conducive to infiltration). Infiltration should still be considered
in well draining soils listed as hydrologic soil group C, but additional
soils testing should be performed to verify on-site conditions and
placement of these BMPs.
(6)
Satisfy the groundwater recharge (infiltration) objective (§ 99-44) and provide for stormwater pretreatment prior to infiltration.
(8)
Provide stream bank erosion protection in accordance with § 99-46 stream bank erosion requirements.
(9)
Determine into what management district the site falls (Chapter Appendix
A)[2] and conduct an existing conditions runoff analysis.
[2]
Editor's Note: Appendix A is included at the end of this chapter.
(10)
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.
(11)
Conduct a proposed conditions runoff analysis based on the final design that meets the management district requirements (§ 99-47).
(12)
Manage any remaining runoff prior to discharge through detention,
bioretention, direct discharge, or other structural control.
Maximizing the groundwater recharge capacity of the area being developed is required. Design of the infiltration facilities shall consider groundwater recharge to compensate for the reduction in the recharge that occurs when the ground surface is disturbed or impervious surface is created. It is recommended that roof runoff be directed to infiltration BMPs that may be designed to compensate for the runoff from parking areas. These measures are required to be consistent with § 99-3 and to take advantage of utilizing any existing recharge areas. Infiltration may not be feasible on every site due to site-specific limitations such as soil type. If it cannot be physically accomplished, then the design professional shall be responsible to show that this cannot be physically accomplished. Appropriate soils testing and/or geotechnical evaluation should be included as part of any documentation for infiltration BMPs. If it can be physically accomplished, then the volume of runoff to be infiltrated shall be determined from § 99-44A(2).
A.
Infiltration BMPs shall meet the following minimum requirements:
(1)
Infiltration BMPs intended to receive runoff from developed 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 (e.g., SHWT, groundwater, bedrock,
etc.).
(b)
An infiltration rate sufficient to accept the additional stormwater
load and dewater completely as determined by field tests conducted
by the applicant's design professional.
(c)
The infiltration facility shall be capable of completely infiltrating
the recharge (infiltration) volume (Rev) within
three days (72 hours) or less.
(d)
Pretreatment shall be provided prior to infiltration.
(2)
The size of the infiltration facility shall be based upon the Net
Two Year Volume Approach, where the recharge (infiltration) volume
(Rev) to be captured and infiltrated shall
be the volume difference between the predevelopment two-year, twenty-four-hour
storm event and post-development two-year, twenty-four -hour storm
event.
(3)
The recharge volume calculated using this section is the minimum
volume the applicant must control through an infiltration BMP facility.
However, if a site has areas of soils where additional volume of recharge
can be achieved, the applicant is encouraged to infiltrate as much
of the stormwater runoff from the site as possible.
B.
Soils testing. If on-lot infiltration is proposed as part of a project,
the applicant's design professional must demonstrate to the satisfaction
of the municipality that the soils are conducive to infiltration at
the proposed location of infiltration facilities. Soils testing shall
be completed as follows:
(1)
Soils testing must be conducted by a qualified design professional
and at a minimum shall address depth to limiting zone, soil permeability,
and subgrade stability. Soils testing must be observed by a representative
of the municipality.
(2)
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.
(3)
Conduct field testing including test pits to determine soil horizons
and depth to limiting zone and permeability tests, at the elevation
of the proposed infiltration facility surface, to determine the appropriate
hydraulic conductivity rate. Double ring infiltrometer or hydraulic
conductivity tests should be used to determine soil permeability (percolation
tests are not recommended for design purposes). Site evaluation and
soils testing should be conducted in accordance with Appendix C of
the Pennsylvania Stormwater Best Management Practices Manual.
(4)
The proposed infiltration facilities shall be designed for the required
recharge (Rev) volume based on the field-determined
capacity at the surface elevation of the proposed infiltration facility.
C.
Stormwater hotspots. Below is a list of examples of designated hotspots.
If a site is designated as a hotspot, it has important implications
for how stormwater is managed. First and foremost, untreated stormwater
runoff from hotspots shall not be allowed to recharge into groundwater
where it may contaminate water supplies. Therefore, the Rev requirement shall not be applied to development sites
that fit into the hotspot category (the entire WQv must still be treated). Second, a greater level of stormwater treatment
shall be considered at hotspot sites to prevent pollutant washoff
after construction. The Environmental Protection Agency's (EPA) NPDES
stormwater program requires some industrial sites to prepare and implement
a stormwater pollution prevention plan.
(1)
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 codes.
(g)
Marinas (service and maintenance).
(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)
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 large highways (average daily traffic volume (ADT) greater
than 30,000 are not designated as stormwater hotspots, it is important
to ensure that highway stormwater management plans adequately protect
groundwater.
D.
Extreme caution shall be exercised where infiltration is proposed
in SWPAs as defined by the local municipality or water authority.
E.
Infiltration facilities shall be used in conjunction with other innovative
or traditional BMPs, stormwater control facilities, and nonstructural
stormwater management alternatives.
F.
Extreme caution shall be exercised where salt or chloride (municipal
salt storage) would be a pollutant since soils do little to filter
this pollutant, and it may contaminate the groundwater. The qualified
design professional shall evaluate the possibility of groundwater
contamination from the proposed infiltration facility and perform
a hydrogeologic justification study if necessary. Specific consideration
should be given to the particular type of salt or deicing material
to be used within this watershed in regards to its potential long-term
effects on the soils, especially in areas that contain clay soil.
G.
The infiltration requirement in HQ or EV waters shall be subject
to the Department's Chapter 93 Antidegradation Regulations.
H.
An impermeable liner will be required in detention basins where the
possibility of groundwater contamination exists. A detailed hydrogeologic
investigation may be required by the municipality.
I.
The municipality shall require the applicant to provide safeguards
against groundwater contamination for land uses that may cause groundwater
contamination should there be a mishap or spill.
J.
Infiltration design criteria.
(1)
All infiltration systems shall have appropriate positive overflow
controls to prevent storage within one foot of the finished surface
or grade.
(2)
All infiltration systems shall have a minimum setback of 15 feet
from principal structures, 10 feet from property lines, 100 feet from
wells, and 50 feet from septic system drain fields. Care should be
taken to prevent any seepage into subgrade structures.
(3)
Surface inflows shall be treated to prevent the direct discharge
of sediment and pollutants into the infiltration system; accumulated
sediment reduces stormwater storage capacity and ultimately clogs
the infiltration mechanism.
(4)
No sand or other particulate matter may be applied to a porous paving
surface for winter ice conditions.
(5)
During site construction, all recharge system components shall be
protected from compaction due to heavy equipment operation or storage
of fill or construction material. Recharge areas shall be protected
from sedimentation. All areas designated for recharge shall not receive
runoff until the contributory drainage area has achieved final stabilization.
(6)
The following procedures and materials shall be required during the
construction of 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. Fabric shall meet
the specifications of PennDOT Publication 408, Section 735 Construction
Class 1.
(e)
Perforated distribution pipes connected to centralized catch
basins and/or manholes with provision 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.
(7)
All infiltration facilities which service more than one lot and are
considered a common facility shall have an easement provided to the
Township for future access if necessary.
(8)
No more than 50% of the required infiltration volume may be provided
in detention basin bottoms. The remaining 50% of infiltration volumes
shall be provided at or near the proposed impervious coverage.
The applicant shall comply with the following water quality
requirements of this article.
A.
No regulated earth disturbance activities within the municipality
shall commence until approval by the municipality of a plan that demonstrates
compliance with post-construction state water quality requirements.
B.
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.
C.
To control post-construction stormwater impacts from regulated earth
disturbance activities, state water quality requirements can be met
by BMPs, including site design, which provide for replication of preconstruction
stormwater infiltration and runoff conditions so that post-construction
stormwater discharges do not degrade the physical, chemical, or biological
characteristics of the receiving waters. As described in the DEP Comprehensive
Stormwater Management Policy (#392-0300-002, September 28, 2002),
this may be achieved by the following:
(1)
Infiltration: replication of preconstruction stormwater infiltration
conditions;
(2)
Treatment: use of water quality treatment BMPs to ensure filtering
out of the chemical and physical pollutants from the stormwater runoff;
and
(3)
Stream bank and streambed protection: management of volume and rate
of post-construction stormwater discharges to prevent physical degradation
of receiving waters (e.g., from scouring).
D.
Developed areas shall provide adequate storage and treatment facilities necessary to capture and treat stormwater runoff. If site conditions allow for infiltration, the water quality volume and the recharge volume are the same volume and may be managed in a single facility. If infiltration cannot be physically accomplished, the water quality volume should be calculated using the net two-year volume approach described in § 99-44A(2). In this case, the water quality volume may be captured and treated by methods other than infiltration BMPs.
(1)
This volume requirement can be accomplished 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 BMP manuals referenced in Chapter Appendix F.[1]
[1]
Editor's Note: Appendix F is included at the end of this chapter.
(2)
The water quality volume shall take a minimum of 24 hours to be discharged
from a BMP facility. Release of the water quality volume 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.
E.
For areas within defined special protection subwatersheds that include
EV and HQ waters, the temperature and quality of water and streams
shall be maintained through the use of temperature-sensitive BMPs
and stormwater conveyance systems.
F.
To accomplish the above, the applicant shall submit original and
innovative designs to the Municipal Engineer for review and approval.
Such designs may achieve the water quality objectives through a combination
of different BMPs.
G.
If a perennial or intermittent stream passes through the site, the
applicant shall create a stream buffer extending a minimum of 50 feet
to either side of the top-of-bank of the channel. The buffer area
shall be maintained with and encouraged to use appropriate native
vegetation. (Refer to Appendix B of the Pennsylvania Stormwater Best
Management Practices Manual, latest version, for plant lists.) If
the applicable rear- or side-yard setback is less than 50 feet or
a stream traverses the site, the buffer width may be reduced to 25%
of the setback and/or to a minimum of 10 feet. 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. This does not include lakes or wetlands.
H.
Evidence of any necessary permit(s) for regulated earth disturbance activities from the appropriate DEP regional office 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 § 99-45A.
A.
In addition to the control of water quality volume (in order to minimize
the impact of stormwater runoff on downstream stream bank erosion),
the primary requirement 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) so that the proposed conditions one-year
storm takes a minimum of 24 hours to drain from the facility from
a point where the maximum volume of water from the one-year storm
is captured (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. Orifice
sizes less than three inches can be utilized, provided that the design
will prevent clogging of the intake.
A.
Peak runoff control shall be designed to reduce the post-development
peak flow to 50% of the predevelopment peak flow. Development sites
must control proposed conditions runoff rates to 50% of the existing
conditions runoff rates for the two-year, five-year, ten-year, twenty-five-year,
fifty-year, and one-hundred-year storm events.
B.
The calculated peak discharges shall apply regardless of whether
the grading plan changes the drainage area by subarea. An exception
to the above may be granted if discharges from multiple subareas recombine
in proximity to the site. In this case, peak discharge in any direction
may be a one-hundred-percent release rate provided that the overall
site discharge meets the weighted-average release rate.
C.
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. However, on-site drainage facilities shall be designed
to safely convey off-site flows through the development site.
D.
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. In other
words, unimpacted areas bypassing the stormwater management facilities
would not be subject to the peak rate control standards.
E.
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 Washington Township:
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 99-48.1 summarizes acceptable computation methods, and
the method selected by the design professional shall be based on the
individual limitations and suitability of each method for a particular
site. Note that successors to the methods listed in Table 99-48.1
are also acceptable, such as WinTR55 for TR-55 and WinTR20 for TR-20.
The municipality may allow the use of the Rational Method to estimate
peak discharges from drainage areas that contain less than five acres.
|
Table 99-48.1
| |||
|---|---|---|---|
|
Acceptable Computation Methodologies for Stormwater Management
Plans
| |||
|
Method
|
Developed By
|
Applicability
| |
|
TR-20 (or commercial computer package based on TR-20)
|
USDA NRCS
|
Applicable where use of full hydrology computer model is desirable
or necessary
| |
|
TR-55 (or commercial computer package based on TR-55)
|
USDA NRCS
|
Applicable for land development plans within limitations described
in TR-55
| |
|
HEC-1/HEC-HMS
|
United States Army Corps of Engineers
|
Applicable where use of a full hydrologic computer is desirable
or necessary
| |
|
PSRM
|
Penn State University
|
Applicable where use of a hydrologic model is desirable or necessary;
simpler than TR-20 or HEC-1
| |
|
Rational Method (or commercial computer package based on Rational
Method)
|
Emil Kuichling (1889)
|
For sites less than five 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 according to the region in which
they are located as presented in Table E-1 in Appendix E[1] of this chapter. If a hydrologic computer model such as
PSRM or HEC-1/HEC-HMS is used for stormwater runoff calculations,
then the duration of rainfall shall be 24 hours.
[1]
Editor's Note: Appendix E is included at the end of this chapter.
C.
The following criteria shall be used for 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."
A site shall be considered to be a wooded site where a biological
community dominated by trees and other woody plants exists that covers
an area of 10,000 square feet or more, and contains at least 100 trees
with at least 50% of those trees having a dbh of two inches or greater.
(Duerksen, Christopher J., with Suzanne Richman, Tree Conservation
Ordinances, Planning Advisory Service Report Number 446, American
Planning Association, Chicago, Illinois, and Scenic America, Washington,
D.C., August, 1993.)
(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 E-2 or E-3
in Appendix E[2] of this chapter.
[2]
Editor's Note: Appendix E is included at the end of this chapter.
(c)
Off-site land use conditions used to determine storm flows for
designing storm facilities shall be based on existing land uses assuming
winter or poor land cover conditions.
(2)
For development considered redevelopment sites, the ground cover
used in determining the existing conditions flow rates for the developed
portion of the site shall be based upon actual land cover conditions.
D.
All calculations using the Rational Method shall use rainfall intensities
consistent with appropriate times-of-concentration for overland flow
and return periods presented in the appropriate curves from the PennDOT
Storm-Duration-Frequency Chart (Region 4 is included as Figure E-3A.[3] The user should refer to the Atlas 14, Volume 2, Storm-Duration-Frequency
Chart). Times-of-concentration for overland flow shall be calculated
using the methodology presented in Chapter 3 of Urban Hydrology for
Small Watersheds, NRCS, TR-55 (as amended or replaced from time to
time by NRCS). Times-of-concentration for channel and pipe flow shall
be computed using Manning's equation.
[3]
Editor's Note: Said figure is included in Appendix E at the
end of this chapter.
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 E-2 in Appendix E of this chapter.
F.
Runoff coefficients (c) for both existing and proposed conditions
for use in the Rational Method shall be obtained from Table E-3 in
Appendix E of this chapter.
G.
Where uniform flow is anticipated, the Manning equation shall be
used for hydraulic computations and to determine the capacity of open
channels, pipes, and storm sewers. Values for Manning's roughness
coefficient (n) shall be in accordance with Table E-5 in Appendix
E.[4]
[4]
Editor's Note: Appendix E is included at the end of this chapter.
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 the storage-indication
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.
All wet basin designs shall incorporate biologic controls consistent
with the West Nile guidance found in Appendix G.[1]
[1]
Editor's Note: Appendix G is included at the end of this chapter.
B.
Any stormwater management facility (i.e., detention 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 one-hundred-year proposed
conditions. The height of embankment must provide a minimum one foot
of freeboard above the maximum pool elevation computed when the facility
functions for the one-hundred-year proposed conditions inflow. Should
any stormwater management facility require a dam safety permit under
DEP 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 be required to pass storms larger than the
one-hundred-year event.
C.
Any facilities that constitute water obstructions (e.g., culverts,
bridges, outfalls, or stream enclosures) and any work involving wetlands
governed by DEP Chapter 105 regulations (as amended or replaced from
time to time by DEP) shall be designed in accordance with Chapter
105 and will require a permit from DEP.
D.
Any other drainage conveyance facility that does not fall under Chapter
105 regulations must be able to convey, without damage to the drainage
structure or roadway, runoff from the twenty-five-year design storm
with a minimum 1.0 foot of freeboard measured below the lowest point
along the top of the roadway. Any facility that constitutes a dam
as defined in DEP Chapter 105 regulations may require a permit under
dam safety regulations. Any facility located within a PennDOT right-of-way
must meet PennDOT minimum design standards and permit submission requirements.
E.
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 design storm.
Conveyance facilities to or exiting from stormwater management facilities
(i.e., detention basins) shall be designed to convey the design flow
to or from that structure. Roadway crossings must be able to convey
runoff from a one-hundred-year design storm. Any facility located
within a PennDOT right-of-way must meet PennDOT minimum design standards
and permit submission requirements.
F.
Storm sewers must be able to convey proposed conditions runoff from
a twenty-five-year design storm without surcharging inlets, where
appropriate.
G.
Adequate erosion protection shall be provided along all open channels
and at all points of discharge.
H.
The design of all stormwater management facilities shall incorporate
sound engineering principles and practices. The municipality reserves
the right to disapprove any design that would result in construction
in or continuation of a stormwater problem area.
A.
The following criteria shall be used for the design of detention
and wet basins. Any reference to detention basin shall also include
wet basins.
(1)
When detention basins are provided, they shall be designed to utilize
the natural contours of the land wherever possible. When such design
is impracticable, the construction of the basin shall utilize slopes
as flat as possible to blend the structure into the terrain. All basins
shall have maximum side slope of three horizontal to one vertical
(3:1).
(2)
A routed hydrograph and calculations shall be furnished for each
storm through the detention basin.
(3)
Emergency overflow facilities (i.e., emergency spillway) shall be
provided for all detention facilities which shall pass a one-hundred-year
storm under orifice block conditions.
B.
In the design of detention basins, the following items listed below
shall be submitted to the Township for review. Any reference to detention
basin shall also include retention basin.
(1)
Design computations for the sizing of the outlet device.
(2)
A stage-storage curve for said detention/retention basin.
(3)
Flood routing and/or storage requirement calculations.
(4)
A plan showing the berm embankment and outlet structure. The plan
shall indicate top of berm elevations, width of the top of the berm,
side slopes, emergency spillway elevation, and elevations of the outlet
structure, including riser, dimensions and spacing of anti-seep collars.
(5)
A cross section through the outlet structure, emergency spillway
and berm embankment.
(6)
A detailed plan of the trash rack and anti-vortex device (if required).
(7)
The maximum side slope of earthen detention embankments shall be
three horizontal to one vertical (3:1). The top and/or toe (whichever
is closer to a property line) of any slope shall be located a minimum
of 25 feet from any property line. Whenever possible, the side slopes
and basin shape shall conform to the natural topography.
(8)
The minimum top width of the detention basin berm shall be 10 feet.
A cutoff trench (key-way) of impervious material shall be provided
under all embankments that require fill material. The cutoff trench
shall be a minimum of eight feet wide, two feet deep and minimum of
two feet over the pipe, and have maximum side slopes of one horizontal
to one vertical (1:1).
(9)
In order to ensure proper drainage on the floor of the basin, a minimum
grade of 2% shall be maintained for areas of sheet flow. For channel
flow, a minimum grade of 1% shall be maintained. Under certain circumstances,
such as continuous seasonal flow, the Township may require a low flow
channel to be constructed. Wet basins may be designed with a flat
bottom upon approval by the Township Engineer.
(10)
All detention basin embankments shall be placed in a maximum
of eight-inch lifts to a minimum of 95% of maximum dry density as
established by ASTM D-1557. Prior to proceeding to the next lift,
the compaction shall be checked by the Township Engineer or the Soils
Engineer. The developer's contractor shall obtain the services of
a qualified laboratory technician to conduct compaction tests on the
leading and the trailing edge of the berm along with the top of the
berm. All tests shall be furnished to the Township for review.
(11)
Whenever possible, the emergency spillway for detention basins
shall be constructed on undisturbed ground. Emergency spillways shall
be constructed of reinforced concrete checker blocks or other permanent
material approved by the Township Engineer. All emergency spillways
shall be constructed so that the detention basin berm is protected
against erosion. The minimum capacity of all emergency spillways shall
be the peak flow rate from the one-hundred-year design storm after
development. The construction material of the emergency spillway shall
extend along the upstream and downstream berm embankment slopes. Construction
material on the upstream slope of the emergency spillway shall extend
to a minimum of two feet below the spillway crest elevation. The downstream
slope of the spillway shall as a minimum extend to the toe of the
berm embankment. The emergency spillway shall not discharge over earthen
fill and/or easily erodible material.
(12)
A minimum freeboard of one foot shall be provided between the
one-hundred-year design stormwater elevation and the invert of the
emergency spillway, and between the design flow through the emergency
spillway and the top of the berm.
(13)
Anti-seep collars shall be installed around the pipe barrel
within the normal saturation zone of the detention basin berms. The
anti-seep collars and their connections to the pipe barrel shall be
watertight. The anti-seep collars shall extend 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. A minimum of two anti-seep collars
shall be installed on each pipe outlet.
(14)
All outlet pipes through the basin berm shall be reinforced
concrete pipe having O-ring joints. All outlet structures shall be
concrete.
(15)
Energy dissipating devices shall be placed at all basin outlets.
(16)
Easements of all basins shall be provided.
(17)
Permanent detention basins outlet structures shall be designed
to incorporate multiple stage outlet release devices.
(18)
A cross section through the basin from the proposed pipe termination
into the basin to the termination of the outlet pipe shall be provided.
(19)
At the discretion of the Township, the perimeter of the detention/retention
pond shall be enclosed with a standard chain link security fence or
approved equal with a minimum height of four feet with locking gates.
Each basin shall be provided with two gates, one wide enough for maintenance
vehicles and a second gate for pedestrian access. The security fence
and gate shall be subject to the approval of the Township.
(20)
Permanent plantings for wet ponds shall be designed by a wetland
biologist to have a mixture of plants that thrive in wet areas.
A.
Drainage easements shall be provided to accommodate all storm drainage
requirements and shall be a minimum of 20 feet in width. Easements
shall be provided for all watercourses and storm drainage piping that
are not located within street rights-of-way.
B.
Storm sewers, as required, shall be placed in the cartway of curbed
streets and parallel to the roadway shoulders of streets without curbs.
When located in undedicated land with approval by the Board of Supervisors,
storm sewers shall be placed within an easement not less than 20 feet
wide.
C.
Storm drainage pipes. The minimum diameter of all storm drainage
pipes shall be 15 inches or an equivalent thereto. All storm drainage
piping shall be laid in a straight line. Storm drainage piping shall
not be permitted under buildings or structures. The minimum grade
of piping shall be 0.5%. All pipe shall be reinforced concrete with
O-rings joints, or smooth bore high density polyethylene pipe. All
structures exposed to the surface shall be reinforced concrete.
D.
When proposed, manholes and inlets (catch basins) shall not be spaced
more than 400 feet apart for pipes of less than or equal to twenty-four-inch
diameter and 500 feet apart for pipes of greater than twenty-four-inch
diameter. Additional manholes or inlets shall be placed at all changes
in alignment, grade or pipe size, and at all points of convergence
of two or more influent storm sewer lines. Inlets may be substituted
for manholes where they will serve a useful purpose. In addition,
the following standards shall apply:
(1)
Manholes and inlets must conform to the standards established by
the Pennsylvania Department of Transportation and must be supplied
by a PennDOT Bulletin 15 approved supplier. Such requirement shall
be listed on the plan.
(2)
At street intersections, inlets shall be placed to prevent the flow
of water across intersections.
(3)
Inlets shall be spaced to limit the gutter spread to within the parking
lane, not more than eight feet during the ten-year storm. Capacity
of the inlets shall not exceed four cubic feet per second (cfs) for
four-foot inlets and five cfs for six-foot inlets in nonponding areas.
(4)
Inlets with a depth greater than five feet must be provided with
ladder rungs and noted on the plan as such.
(5)
When there is a change in pipe size in the inlet, the elevation of
the top of pipes shall be the same or the smaller pipe higher. A minimum
drop of two inches shall be provided at the inlet pipe invert elevation
and the outlet pipe elevation.
(6)
Inlets shall contain a marker which discourages the discharge of
anything other than stormwater into the inlet.
(7)
Inlets in paved areas shall be equipped with bicycle safe grates.
Inlets in nonpaved areas shall be equipped with standard grates.
(8)
Manhole covers shall have the word "STORM" cast on the top of the
cover.
E.
Properly designed, graded and lined drainage swales may be permitted
in lieu of storm sewers in commercial, industrial and residential
areas where approved by the Township. Swale lining must meet the County
Soil Conservation design standards. All drainage channels shall have
a maximum side slope grade of three horizontal to one vertical (3:1).
All drainage swales shall be provided with a minimum six-inch freeboard,
measured from the top of the design storm flow to the top of the swale.
F.
Curb requirements shall vary according to street hierarchy and intensity
of the proposed development. Curbing may also be required for any
one or more of the following reasons:
G.
The maximum encroachment of water on the roadway pavement shall not
exceed four inches in depth at the curbline during a ten-year frequency
storm. Inlets shall be provided to control the encroachment of water
on the pavement.
H.
Pipe underdrains and/or pavement base drains shall be provided in
areas delineated as having a "seasonal high water table" and in areas
deemed necessary by the Township Engineer during the construction
phase of the project. The installation of the underdrain system shall
be approved by the Township Engineer and paid for by the developer.
Pipe underdrains and pavement base drains shall be constructed in
accordance with PennDOT Pub. 408, Section 610, as amended.
I.
Pipe end sections and/or head walls shall be utilized at all terminated
pipe segments.
J.
All drainage structures located within a state highway right-of-way
shall be reviewed and approved by the Pennsylvania Department of Transportation
(PennDOT). A letter from PennDOT indicating such approval shall be
submitted to the Township prior to municipal approval.
K.
Energy dissipaters shall be provided at all pipe end treatments.
L.
At the discretion of the Township, in situations in which the design
standards and requirements do not apply to the site conditions, the
Township Engineer shall suggest or provide additional and/or alternative
design methods to meet the objectives of this chapter.
