A.
Applicants proposing regulated activities in the municipality that do not fall under the exemption criteria shown in § 101-6 shall submit a drainage plan consistent with this chapter and any respective Act 167 stormwater management plan(s) 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 § 101-19 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 § 101-21G 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 water 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.
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)
Streambank 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 Pa. Code Chapter 102.
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 encroaches on a state highway
right-of-way or requires access from a state highway shall be subject
to approval by PennDOT.
A.
No regulated earth disturbance activities within the municipality
shall commence until the municipality receives an approval from the
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 Pa. Code § 102.4(b).
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 overtop of these areas, and the 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 D, Table
D-4,[1] 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: Said appendix is on file in the Borough
offices.
(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.[2]
(3)
Prepare a draft project layout avoiding sensitive areas identified in § 101-19B(1).
(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, § 101-20, and provide for stormwater pretreatment prior to infiltration.
(8)
Provide streambank erosion protection in accordance with § 101-22, Streambank erosion requirements.
(9)
Conduct an existing conditions runoff analysis.
(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
the remaining site development has no surface or point discharge.
(11)
Conduct a proposed conditions runoff analysis based on the final
design.
(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 § 101-3 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 § 101-20A(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) between 24 hours to 72 hours.
(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. 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 subgrade 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 volume
(Rev) based on field-determined capacity at the level of the proposed
infiltration surface.
(4)
If on-lot infiltration structures are proposed by the applicant's
design professional, it must be demonstrated to the municipality that
the soils are conducive to infiltrate on the lots identified.
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.
(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 would be
a pollutant, such as municipal salt storage, 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 stored within this watershed in regards to its potential long-term
effects on the soils.
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.
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 (No. 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 the filtering
out of the chemical and physical pollutants from the stormwater runoff;
and
(3)
Streambank 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.
Storage and treatment facilities.
(1)
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 § 101-20A(2). In this case, the water quality volume may be captured and treated by methods other than infiltration BMPs.
(2)
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 E.[1]
[1]
Editor's Note: Said appendix is on file in the Borough
offices.
(3)
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 10 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 five 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 § 101-21A.
A.
In addition to the control of water quality volume (in order to minimize
the impact of stormwater runoff on downstream streambank 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 (post
two-year to the pre one-year). 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 of 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 orifice.
A.
Peak rate control. For all areas of the Borough, the design storm
criteria for peak runoff rate control are designed to reduce the post-development
peak flow to the predevelopment peak flow. Development sites must
control proposed conditions runoff rates to the existing conditions
runoff rates for the two-, five-, ten-, twenty-five-, fifty-, and
100-year storm events.
B.
The calculated peak discharges within the Borough 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.
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 West Conshohocken Borough:
A.
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 101-24 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 101-24 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 200 acres.
The Soil Cover Complex Method shall be used for drainage areas greater
than 200 acres.
|
Table 101-24 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 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 D-1 in Appendix D of this chapter.[1] 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: Said appendix is on file in the Borough
offices.
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.
I would not call this a wooded site (one two-inch tree every 10 feet).[2]
[2]
Editor's Note: So in original.
(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
number (CN) or Rational "C" value (i.e., woods) as listed in Table
D-2 or D-3 in Appendix D of this chapter.[3]
[3]
Editor's Note: Said appendix is on file in the Borough
offices.
(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 in Figure D-1.
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.
F.
Runoff coefficients (c) for both existing and proposed conditions
for use in the Rational Method shall be obtained from Table D-3 in
Appendix D 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 consistent with accepted published values.
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.
Stormwater facilities.
(1)
All wet basin designs shall incorporate biologic controls consistent
with the West Nile guidance.
(2)
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 100-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 100-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 100-year event.[1]
(3)
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.
(4)
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 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 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 identified floodplain areas must be able to convey runoff from a 100-year design storm. Any facility located within a PennDOT right-of-way must meet PennDOT minimum design standards and permit submission requirements.
(6)
Adequate erosion protection shall be provided along all open channels
and at all points of discharge.
(7)
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.
(8)
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 the following
(at a minimum):
(a)
The maximum water depth shall not exceed six feet for a detention
basin. A facility designed to have a permanent pool (i.e., retention
basin) shall not exceed a maximum permanent water depth of two feet
and shall be enclosed within a fence approved by the Borough Engineer.
(b)
The minimum top width of all dams/embankments/berms shall be
10 feet.
(c)
The interior side slopes shall not be greater than five horizontal
to one vertical. The top or toe of any slope shall be located a minimum
of five feet from a property line.
(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 100-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. Emergency spillways shall be constructed of concrete pavers,
gabions, or other similar materials approved by the Borough Engineer.
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 between 24 hours and 72 hours following
the end of the design rainfall, unless the Borough Engineer finds
that the downstream conditions may warrant other design criteria for
stormwater release. 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 between 24 hours and 72 hours following
the end of the design rainfall.
(f)
A cutoff trench of relatively impervious material shall be provided
beneath all embankments requiring fill material. The keyway shall
be a minimum of eight feet wide and a minimum of three feet deep and
have 1:1 side slopes.
(g)
All structures passing through detention basin embankments shall
have properly spaced antiseep collars installed around the pipe barrel
within the normal saturation zone of the detention basin berms. The
antiseep collars and their connections to the pipe barrels shall be
watertight. The antiseep collars shall extend a minimum of two feet
in all directions 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 antiseep collars shall be installed on each outlet pipe.
(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)
The 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.
(j)
Design storms for the computation of retention basin (where
approved) volumes shall be based upon a twenty-four-hour storm with
100-year return period (a storm with a 1% chance of occurrence each
year).
(k)
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.
(l)
All structures (detention basins, cisterns, etc.), other than
those used for groundwater recharge volume and water quality volume,
must completely drain between 24 hours and 72 hours after the end
of the design storm, unless the Borough Engineer finds that downstream
conditions may warrant other design criteria for stormwater release.
(m)
Soils used for the construction of basins shall have low erodibility
factors ("K" factors).
(n)
Unless permitted by the Floodplain Administrator, stormwater
management facilities shall not be located within floodplains or within
the identified floodplain areas or alluvial soils.
(o)
Detention basins shall be designed to facilitate regular maintenance,
mowing and periodic silt removal and reseeding. Shallow broad basins
are preferred to steep-sided basins.
(p)
All detention and retention basin embankments shall be placed
in eight-inch maximum lifts to a minimum 95% dry density. Prior to
proceeding to the next lift, compaction shall be checked by the Borough
Engineer or an approved soils engineer, who shall provide the Borough
Engineer with a written report. Compaction tests shall be performed
using the Modified Proctor Method in accordance with ASTM D-1577.
Compaction tests shall be run on the leading and trailing edge as
well as the top of the berm.
(9)
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 100-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 floodproofing design information shall be
submitted for approval.
(10)
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.
(11)
All groundwater recharge facilities shall be designed to empty
between 24 hours and 72 hours. 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
(predevelopment versus post-development) shall be designed to empty
between 24 hours and 72 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 (predevelopment
versus post-development).
B.
Stormwater conveyance systems.
(1)
General.
(a)
Storm sewers, culverts, bridges and related installations shall
be provided:
[1]
To permit unimpeded flow of natural watercourses and in such
a manner as to protect the natural character of the watercourses and
to provide regulated discharge;
[2]
To insure adequate drainage of all low points along the line
of streets; and
[3]
To intercept stormwater runoff along streets at intervals reasonably
related to the extent and grade of the area drained and to prevent
substantial flow of water across intersections.
(b)
All storm sewer system components shall conform to current PennDOT
standards.
(c)
Drainage structures which drain watershed areas in excess of
1/2 square mile (320 acres), or which have a span of eight feet or
more, shall be designed for a maximum expected runoff as calculated
using the Soil Conservation Service Technical Release 55, "Urban Hydrology
for Small Watersheds (less than 2,000 acres)."
(d)
The design storm for the above structures shall be a 100-year
storm. A water obstruction permit shall be obtained from the Pennsylvania
Department of Environmental Protection for the waterway opening before
final design is undertaken.
(e)
The cartway over the culvert or bridge shall be as wide as the
ultimate width of the roadway approaches. Additional width may be
required to provide sidewalk on one or both sides of the cartway.
(2)
Storm sewer design and construction requirements.
(a)
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. 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 100-year-frequency flood
throughout the development site. Easements shall begin at the furthest
upstream property line of the proposed development site in a watershed.
(b)
All storm sewer pipes and inlets intended to drain to detention
facilities shall be designed to accommodate the 100-year storm if
the bypass or overflow runoff will not reach the basin by overland
flow. In cases where the bypass or overflow runoff will flow over
land, a stable swale shall be constructed to accommodate the excess
runoff.
(c)
Storm sewer pipes, other than those used as roof drains, detention
basin underdrains, and street subbase underdrains shall have a minimum
diameter of 18 inches and be made of reinforced concrete pipe, smooth
lined corrugated polyethylene pipe, or approved equivalent. Where
installation conditions merit, structural calculations that address
the actual design requirements will be required.
(d)
Storm sewer pipes and culverts shall be installed on sufficient
slopes to provide a minimum velocity of three feet per second when
flowing full.
(e)
All storm sewer pipe and culverts shall be laid to a minimum
depth of two feet from finished subgrade to the crown of pipe in paved
areas and two feet from finished grade to the crown of pipe in grassed
areas.
(f)
Curves in pipes or box culverts without an inlet or manhole
are prohibited. Tee joints, elbows and wyes are also prohibited.
(g)
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 municipality.
(h)
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.
(i)
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.
(j)
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
municipality and any other necessary approving body.
(k)
Flow velocities from any storm sewer may not result in erosion
of the receiving channel.
(l)
Minimum pipe size is 18 inches.
(m)
Minimum pipe slope shall be 0.005 foot/foot.
(n)
Minimum drop across junctions shall be two inches. At changes
in pipe diameter, pipe crowns shall be matched at junctions (manhole,
inlet or junction box).
(o)
Maximum distance between junctions shall be 300 feet.
(p)
Runoff to proposed storm sewers and inlets shall be calculated
using the Rational Method.
(q)
The time of concentration to inlets for grate capacity calculations
shall be assumed five minutes.
(r)
All storm sewer systems shall be analyzed for both inlet and
outlet control (including tailwater effects) by using the equations
and nomographs as shown in the FHA's Hydraulic Design Services
No. 5. In lieu of this, computer programs that calculate the actual
hydraulic grade line for the storm sewer system can be used, provided
all losses (friction, bend, junction, etc.) are taken into account.
Documentation for the program must be submitted for approval.
(s)
Inlet capacities shall be calculated using PennDOT or manufacturer's
nomographs. Documentation for manufacturer's nomograph must be
provided to the Borough Engineer.
(t)
Manning "n" values used for design of pipes and culverts shall
be in accordance with accepted published values.
(u)
All storm sewer crossings of streets shall be perpendicular
to the street center line, to the maximum extent possible.
(v)
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.
(w)
Adequate erosion protection shall be provided along all open
channels and at all points of discharge.
(3)
Shoulders in cut areas (without swales).
(a)
Water flowing in the shoulder shall not encroach more than 2/3
the shoulder width during a twenty-five-year-frequency storm of five-minute
duration.
(b)
The maximum velocity as determined by Manning's Equation
shall not exceed the allowable velocities for the specific type of
shoulder material.
(c)
Inlets shall be provided to control the shoulder encroachment
and water velocity.
(4)
Swales adjacent to shoulders.
(a)
Swales in cut areas shall be designed to prevent the passage
of water on the cartway during a twenty-five-year-frequency storm
of five-minute duration.
(b)
The maximum velocity as determined by Manning's Equation
shall not exceed the allowable velocities for the specific type of
shoulder material.
(6)
Inlets: general.
(a)
At street intersections, inlets shall be placed in the tangent
portion, rather than the curved portion, of the curbing.
(b)
If the capacity of the shoulder, swale, curb section, or depressed
median section exceeds the assumed inlet capacities, the inlet capacities
shall govern the spacing of inlets.
(c)
If the capacity of the shoulder, swale, curb section, or depressed
median section is less than the inlet capacities, then the shoulder,
swale, curb section or depressed section capacity shall govern the
spacing of inlets.
(7)
Energy dissipaters. Energy dissipaters 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.
(8)
Swales. 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:
(b)
All swales shall be designed to concentrate low flows to minimize
siltation and meandering.
A.
In order to protect and improve water quality, a riparian buffer
easement shall be created and recorded as part of any subdivision
or land development that encompasses a riparian buffer.
B.
Except as required by Chapter 102, the riparian buffer easement shall be measured to be the greater of the limit of the 100-year floodplain or a minimum of 35 feet from the top of the streambank (on each side).
C.
Minimum management requirements for riparian buffers.
(1)
Existing native vegetation shall be protected and maintained
within the riparian buffer easement.
(2)
Whenever practicable invasive vegetation shall be actively removed
and the riparian buffer easement shall be planted with native trees,
shrubs and other vegetation to create a diverse native plant community
appropriate to the intended ecological context of the site.
D.
The riparian buffer easement shall be enforceable by the municipality
and shall be recorded in the appropriate County Recorder of Deeds
office, so that it shall run with the land and shall limit the use
of the property located therein. The easement shall allow for the
continued private ownership and shall count toward the minimum lot
area a required by zoning, unless otherwise specified in the Municipal
Zoning Ordinance.[1]
E.
Any permitted use within the riparian buffer easement shall be conducted
in a manner that will maintain the extent of the existing 100-year
floodplain, improve or maintain the stream stability, and preserve
and protect the ecological function of the floodplain.
