[Ord. 2007-6, 6/5/2007, § 401]
1. Applicants proposing regulated activities in the Township that do not fall under the exemption criteria shown in §
23-206 shall submit a drainage plan consistent with this Part and the respective Act 167 Stormwater Management Plan to the Township for review. The stormwater management criteria of this Part shall apply to the total proposed development even if development is to take place in stages.
2. 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.
3. The drainage plan must be designed consistent with the sequencing provisions of §
23-234 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 Part 2D.
4. 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 Part.
5. 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
Part.
6. 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 Part.
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.
7. 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 §
23-236, Subsection 1G, of this Part.
8. Any stormwater management facilities regulated by this Part 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.
9. Any proposed stormwater management facilities regulated by this Part
that would be located on state highway rights-of-way shall be subject
to approval by PennDOT.
10. 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.
11. All stormwater runoff shall be pre-treated for water quality prior
to discharge to surface or groundwater.
12. All regulated activities within the Township shall be designed, implemented,
operated, and maintained to meet the purposes of this Part, through
these two elements:
A. Erosion and sediment control during earth disturbance activities
(e.g., during construction).
B. Water quality protection measures after completion of earth disturbance
activities (i.e., after construction), including operations and maintenance.
13. No regulated earth disturbance activities within the Township shall
commence until the requirements of this Part are met.
14. Post-construction water quality protection shall be addressed as required by §
23-236.
15. Operations and maintenance of permanent stormwater BMPs shall be
addressed as required by Part 1G.
16. All BMPs used to meet the requirements of this Part shall conform
to the state water quality requirements and any more stringent requirements
as set forth by the Township.
17. Techniques described in Appendix 23-2-D (Low Impact Development)
of this Part shall be considered because they reduce the costs of
complying with the requirements of this Part and the state water quality
requirements.
18. In selecting the appropriate BMPs or combinations thereof, the applicant
shall consider the following:
B. Permeability and infiltration rate of the site's soils.
C. Slope and depth to bedrock.
D. Seasonal high water table.
E. Proximity to building foundations and wellheads.
G. Land availability and configuration of the topography.
H. Peak discharge and required volume control.
J. Effectiveness of the BMPs to mitigate potential water quality problems.
K. The volume of runoff that will be effectively treated.
L. The nature of the pollutant being removed.
M. Maintenance requirements.
N. Creation/protection of aquatic and wildlife habitat.
19. 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 23-2-A.
[Ord. 2007-6, 6/5/2007, § 402]
1. 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 Pa. Code, Chapter 102.
B. Work within natural drainageways subject to permit by DEP under 25
Pa. 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 Pa. 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 Pa. Code, Chapter
105.
[Ord. 2007-6, 6/5/2007, § 403]
1. No regulated earth disturbance activities within the Township shall
commence until the Township receives an approval from the Conservation
District of an erosion and sediment control plan for construction
activities.
2. 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 Pa. Code § 102.4(b).
3. In addition, under 25 Pa. Code, Chapter 92, a DEP permit for stormwater
discharges associated with construction activities is required for
land disturbances greater than one acre.
4. 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 Township.
5. 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.
6. Additional erosion and sediment control design standards and criteria
are recommended to be applied where infiltration BMPs are proposed.
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. Additional measures, such as placement of orange construction
fencing around proposed infiltration BMPs during construction to minimize
or eliminate traffic overtop of these areas, and temporary sealing
off of pipes and inlet connections to infiltration BMPs to prevent
sediment clogging should be given consideration.
B. Infiltration BMPs shall not be constructed nor receive runoff until
the entire drainage area contributory to the infiltration BMP has
achieved final stabilization.
[Ord. 2007-6, 6/5/2007, § 404]
1. The design of all regulated activities shall include the following
to minimize stormwater impacts:
A. The applicant shall find practicable alternatives to the surface
discharge of stormwater, such as those listed in Appendix 23-2-E,
Table 23-2-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.
B. 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 Township requirements.
C. 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.
2. 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:
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, hydrologic soil groups, wooded areas, and potential infiltration areas. Land development, any existing recharge areas, and other requirements outlined in the Township SALDO [Chapter
22] shall also be included.
B. Establish a stream buffer according to §
23-236, Subsection 1G.
C. Prepare a draft project layout avoiding sensitive areas identified
in Subsection 2A.
D. 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.
Evaluate nonstructural stormwater management alternatives:
(1)
Minimize earth disturbance.
(2)
Minimize impervious surfaces.
(3)
Break up large impervious surfaces.
E. Satisfy the groundwater recharge (infiltration) objective (§
23-235) and provide for stormwater pretreatment prior to infiltration.
F. Provide for water quality protection in accordance with §
23-236 water quality requirements.
G. Provide stream bank erosion protection in accordance with §
23-237 stream bank erosion requirements.
H. Determine into what management district the site falls (Appendix
23-2-A) and conduct an existing conditions runoff analysis.
I. 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.
J. Conduct a proposed conditions runoff analysis based on the final design that meets the management district requirements (§
23-238).
K. Manage any remaining runoff prior to discharge through detention,
bioretention, direct discharge, or other structural control.
[Ord. 2007-6, 6/5/2007, § 405]
1. 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 §
23-203 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 paragraph .A(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, preferably 36 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 pre-development 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. 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 a 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 such as double ring infiltrometer or hydraulic
conductivity tests (at the level of the proposed infiltration surface)
to determine the appropriate hydraulic conductivity rate. Percolation
tests are not recommended for design purposes.
(3)
Design the infiltration structure for the required recharge
(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 Township that
the soils are conducive to infiltrate on the lots identified.
C. Stormwater Hotspots.
(1)
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 wash off after
construction. The Environmental Protection Agency's (EPA) NPDES
stormwater program requires some industrial sites to prepare and implement
a stormwater pollution prevention plan. 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:
(a)
Residential streets and rural highways.
(c)
Institutional development.
(f)
Pervious areas, except golf courses and nurseries (which may
need an integrated pest management (IPM) plan).
(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 Township 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 Township.
I. The Township 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.
[Ord. 2007-6, 6/5/2007, § 406]
1. The applicant shall comply with the following water quality requirements
of this Part:
A. No regulated earth disturbance activities within the Township shall
commence until approval by the Township 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 Township.
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 pre-construction
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 pre-construction 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.
(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).
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 can not be physically accomplished, the water quality volume should be calculated using the net two-year volume approach described in §
23-235, Subsection
1A(2). In this case, the water quality volume may be captured and treated by methods other than infiltration BMPs.
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 Appendix
23-2-F.
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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.
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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 Township 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 Part, 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 Township. The issuance of an NPDES construction permit (or
permit coverage under the Statewide General Permit (PAG-2)) satisfies
the requirements of paragraph .A.
[Ord. 2007-6, 6/5/2007, § 407]
1. 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).
2. 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.
[Ord. 2007-6, 6/5/2007, § 408]
1. Within the Swamp Creek watershed, the criteria for peak runoff control
are designed to reduce the post-development peak flow to 50% of the
pre-development peak flow. Development sites must control proposed
conditions runoff rates to 50% of the existing conditions runoff rates
for the two-, five-, ten-, twenty-five-, fifty-year, and one-hundred-year
storm events.
2. 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 100% release rate provided that the overall site discharge
meets the weighted average release rate.
3. 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.
4. 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.
5. 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 Douglass Township:
A. Meet the full requirements specified by subsections .1 through .4.
B. Reduce the total impervious surface on the site by at least 20%,
based upon a comparison of existing impervious surface to proposed
impervious surface.
[Ord. 2007-6, 6/5/2007, § 409]
1. Stormwater runoff from all development sites with a drainage area
of greater than 200 acres shall be calculated using a generally accepted
calculation technique that is based on the NRCS Soil Cover Complex
Method. Table 23-239 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 23-239 are
also acceptable, such as WinTR55 for TR-55 and WinTR20 for TR-20.
The Township may allow the use of the Rational Method to estimate
peak discharges from drainage areas that contain less than 200 acres.
The Soil Cover Complex Method shall be used for drainage areas greater
than 200 acres.
Table 23-239
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 where limitations described
in TR-55.
|
HEC-1/HEC-HMS
|
U.S. 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 200 acres, or as approved by the Township
and/or Township Engineer.
|
Other Methods
|
Varies
|
Other computation methodologies approved by the Township and/or
Township Engineer.
|
2. All calculations consistent with this Part 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 23-2-E-1 in Appendix 23-2-E of this Part. 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.
3. The following criteria shall be used for runoff calculations:
A. For development sites not considered redevelopment, the ground cover
used in determining the existing conditions flow rates shall be as
follows:
(1)
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.
(2)
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 23-2-E-2 or
23-2-E-3 in Appendix 23-2-E of this Part.
(3)
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.
B. 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.
4. 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 in Figure 23-2-E-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.
5. Runoff curve numbers (CN) for both existing and proposed conditions
to be used in the Soil Cover Complex Method shall be obtained from
Table 23-2-E-2 in Appendix 23-2-E of this Part.
6. Runoff coefficients (c) for both existing and proposed conditions
for use in the Rational Method shall be obtained from Table 23-2-E-3
in Appendix 23-2-E of this Part.
7. 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 consistent with accepted published values.
8. Outlet structures for stormwater management facilities shall be designed
to meet the performance standards of this Part using any generally
accepted hydraulic analysis technique or method.
9. The design of any stormwater detention facilities intended to meet
the performance standards of this Part 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 Township
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.
[Ord. 2007-6, 6/5/2007, § 410]
1. All wet basin designs shall incorporate biologic controls consistent
with the West Nile Guidance found in Appendix 23-2-G.
2. Any stormwater management facility (i.e., detention basin) required
or regulated by this Part 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 25 Pa. Code, Chapter 105, the facility shall be designed in accordance
with 25 Pa. Code, 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.
3. Any facilities that constitute water obstructions (e.g., culverts,
bridges, outfalls, or stream enclosures) and any work involving wetlands
governed by DEP 25 Pa. Code, 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.
4. Any other drainage conveyance facility that does not fall under 25
Pa. Code, 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 one 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.
5. Any drainage conveyance facility and/or channel not governed by 25
Pa. Code, 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
located within designated floodplain areas 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.
6. Storm sewers must be able to convey proposed conditions runoff from
a twenty-five-year design storm without surcharging inlets, where
appropriate.
7. Adequate erosion protection shall be provided along all open channels
and at all points of discharge.
8. The design of all stormwater management facilities shall incorporate
sound engineering principles and practices. The Township reserves
the right to disapprove any design that would result in construction
in or continuation of a stormwater problem area.
9. Any stormwater management facility (i.e., detention basin) required
or regulated by this Part designed to store runoff and requiring a
berm or earthen embankment shall be designed to provide the following
(at a minimum):
A. The maximum water depth shall not exceed six feet.
B. The minimum top width of all dams/embankments/berms shall be eight
feet.
C. The interior side slopes shall not be greater than three horizontal
to one vertical.
D. All basins shall be structurally sound and shall be constructed of
sound and durable materials. The completed structure and the foundation
of all basins shall be stable under all probable conditions of operation.
An emergency spillway shall be provided for the basin and shall be
capable of discharging the one-hundred-year peak rate of runoff that
enters the basin after development, in a manner that will not damage
the integrity of the facility and will not create a downstream hazard.
Where practical, the emergency spillway shall be constructed in undisturbed
ground. An easement for inspection and repair shall be provided when
the conveyance structure crosses property boundaries.
E. All basins not including groundwater recharge and/or water quality
storage shall include an outlet structure to permit draining the basin
to a completely dry position within 24 hours following the end of
the design rainfall. All basins that do include groundwater recharge
and/or water quality storage shall include an outlet structure to
permit draining the basin to the level of the groundwater recharge
and/or water quality storage within 24 hours following the end of
the design rainfall.
F. A cutoff trench and core of relatively impervious material shall
be provided within all basin embankments.
G. All structures passing through detention basin embankments shall
have properly spaced concrete cutoff collars and all piping must be
watertight. All structures passing through dam embankments shall have
seepage diaphragms and drains.
H. All discharge control devices with appurtenances shall be made of
reinforced concrete and stainless or hot dip galvanized steel. Bolts/fasteners
are to be stainless or galvanized steel.
I. Low flow channels shall be provided from each water carrying facility
to the outlet structure for all basins that do not include groundwater
recharge and/or water quality storage. Low flow channels shall be
1% minimum slope and shall be designed to enable ease of maintenance.
All basins that do include groundwater recharge and/or water quality
storage shall not be required to have a low flow channel.
J. Minimum slope within a basin that does not include groundwater recharge
and/or water quality storage shall be 2% positive grade to the low
flow channel.
K. Design storms for the computation of retention basins (where approved)
volumes shall be based upon a twenty-four-hour storm with one-hundred-year
return period (a storm with a one-percent chance of occurrence each
year).
L. The effect on downstream areas if the basin embankment fails shall
be considered in the design of all basins. Where possible, the basin
shall be designed to minimize the potential damage caused by such
failure of the embankment.
M. All structures (detention basins, cisterns, etc.), other than those
used for groundwater recharge volume and water quality volume, must
completely drain within 24 hours after the end of the design storm.
N. Soils used for the construction of basins shall have low erodibility
factors ("K" factors).
10. Minimum floor elevations for all structures that would be affected
by a basin, other temporary impoundments, or open conveyance systems
where ponding may occur shall be two feet above the one-hundred-year
water surface. If basement or underground facilities are proposed,
detailed calculations addressing the effects of stormwater ponding
on the structure and waterproofing and/or flood-proofing design information
shall be submitted for approval.
11. All storm sewer pipes, culverts and bridges (excluding detention
and retention basin outfall structures), gutters and swales conveying
water originating only from within the boundaries of the development
site shall be designed for a twenty-five-year storm event as a minimum.
All storm sewer pipes, culverts and bridges (excluding detention and
retention basin outfall structures) conveying water originating from
off-site shall be designed for a fifty-year storm event. Drainage
easements shall be provided to contain and convey the one-hundred-year
frequency flood throughout the development site. Easements shall begin
at the furthest upstream property line of the proposed development
site in a watershed.
12. A concentrated discharge of stormwater to an adjacent property shall
be within an existing natural drainageway or watercourse or otherwise
an easement shall be required.
13. Storm sewer pipes other than those used as roof drains, detention
basin underdrains, and street subbase underdrains, shall have a minimum
diameter of 15 inches and be made of reinforced concrete pipe, corrugated
galvanized metal pipe, smooth lined corrugated polyethylene pipe,
or approved equivalent. Where installation conditions merit, structural
calculations that address the actual design requirements will be required.
14. Storm sewer pipes and culverts shall be installed on sufficient slopes
to provide a minimum velocity of three feet per second when flowing
full.
15. All storm sewer pipe and culverts shall be laid to a minimum depth
of one foot from finished subgrade to the crown of pipe in paved areas
and one foot from finished grade to the crown of pipe in grassed areas.
16. Curves in pipes or box culverts without an inlet or manhole are prohibited.
Tee joints, elbows and wyes are also prohibited.
17. Manholes, inlets, headwalls and endwalls proposed for dedication
or located along streets or subject to vehicular traffic, shall conform
to the requirements of the PennDOT, Bureau of Design, Standards for
Roadway Construction, in effect at the time the design is submitted,
or as otherwise modified by the Township.
18. Headwalls and endwalls shall be used where stormwater runoff enters
or leaves the storm sewer horizontally from a natural or man-made
channel. PennDOT Type "DW" headwalls and endwalls shall be utilized.
19. Stormwater roof drains, sump pumps, and pipes, shall not directly
discharge water into a street right-of-way or discharge into a sanitary
sewer or storm sewer.
20. All existing and natural watercourses, channels, drainage systems,
wetlands and areas of surface water concentration shall be maintained
in their existing condition unless an alteration is approved by the
Township and any other necessary approving body.
21. Flow velocities from any storm sewer may not result in erosion of
the receiving channel.
22. Energy dissipators shall be placed at the outlets of all storm sewer
pipes, culverts, and bridges where flow velocities exceed maximum
permitted channel velocities as specified below:
A. Three feet per second where only sparse vegetation can be established
and maintained because of shade or soil condition.
B. Four feet per second where normal growing conditions exist and vegetation
is to be established by seeding.
C. Five feet per second where a dense, vigorous sod can be quickly established
or where water can be temporarily diverted during establishment of
vegetation. Netting and mulch or the equivalent methods for establishing
vegetation shall be used.
D. Six feet per second where there exists a well-established sod of
good quality.
23. The following conditions shall be met for all swales:
A. Capacities and velocities shall be computed using the Manning equation.
The design parameters shall be as follows:
(1)
All vegetated swales shall have a minimum slope of 1% unless
approved by the Township Engineer.
(2)
The "n" factors to be used for swales or gutters shall be based
upon accepted engineering design practices as approved by the Township.
B. All swales shall be designed to concentrate low flows to minimize
siltation and meandering.
24. Manning "n" values used for design of pipes and culverts shall be
in accordance with accepted published values.
25. Storm facilities not located within a public right-of-way shall be
contained in and centered within an easement. Easements shall follow
property boundaries where possible.
26. Adequate erosion protection shall be provided along all open channels,
and at all points of discharge.
27. All groundwater recharge facilities shall be designed to empty within
three days (72 hours) or less subsequent to any storm event. All water
quality facilities shall be designed so that water is released slowly
for a minimum of 24 hours subsequent to any storm event. All infiltration,
detention or retention facilities the volume of which will be used
for stormwater management (pre-development vs. post-development) shall
be designed to empty within 24 hours subsequent to any storm event.
Volumes that will not be available within 24 hours subsequent to any
storm event shall not be used for stormwater management (pre-development
vs. post-development).