A.
Preparation of a SWM site plan is required for all regulated activities, unless preparation and submission of the SWM site plan is specifically exempted according to § 315-51 or the activity qualifies as a small project.
B.
No regulated activities shall commence until the Borough issues unconditional
written approval of a SWM site plan.
C.
The Borough may, after consultation with DEP, approve measures for
meeting the state water quality requirements other than those in this
chapter, provided that they meet the minimum requirements of, and
do not conflict with, state law, including, but not limited to, the
Clean Streams Law.[1] The Borough shall maintain a record of consultations with
DEP pursuant to this subsection. Where an NPDES permit for stormwater
discharges associated with construction activities is required, issuance
of an NPDES permit shall constitute satisfaction of consultation with
DEP.
[1]
Editor's Note: See 35 P.S. § 691.1 et seq.
D.
For all regulated activities, erosion and sediment control and stormwater
management BMPs shall be designed, implemented, operated, and maintained
to meet the purposes and requirements of this chapter and to meet
all requirements under Title 25 of the Pennsylvania Code and the Clean
Streams Law. Various BMPs and their design standards are listed in
the E&S Manual and the BMP Manual.
E.
Developers have the option to propose a regional stormwater management plan or participate in a regional stormwater management plan developed by others. A regional stormwater management plan may include off-site volume and rate control, as appropriate and supported by a detailed design approved by the Borough in accordance with § 315-31. A regional stormwater management plan must meet all of the volume and rate control standards required by this chapter for the area defined by the regional stormwater management plan, but not necessarily for each individual development site. Appropriate agreements must be established to ensure the requirements of this chapter and the requirements of the regional stormwater management plan are met.
F.
Impervious areas.
(1)
The measurement of impervious area shall include all of the impervious
areas in the total proposed development even if development is to
take place in stages or phases.
(2)
For development taking place in stages or phases, the entire development
plan must be used in determining conformance with this chapter.
(3)
Any areas designed to initially be gravel or crushed stone shall
be assumed to be impervious.
G.
All regulated activities shall include such measures as necessary
to:
[Amended 9-27-2022 by Ord. No. 2022-04]
(1)
Protect health, safety, and property;
(2)
Meet the water quality goals of this chapter by implementing measures
to:
(a)
Protect and/or improve the function of floodplains, wetlands,
and wooded areas.
(b)
Protect and/or improve native plant communities including those
within the riparian corridor.
(c)
Protect and/or improve natural drainageways from erosion.
(d)
Minimize thermal impacts to waters of this commonwealth.
(e)
Disconnect impervious surfaces by directing runoff to pervious
areas, wherever possible.
(f)
Reduce or prevent to the maximum extent practicable the discharge
of a pollutant, polluted stormwater or an illicit discharge to the
MS4.
(3)
To the maximum extent practicable, the techniques for low-impact
development (LID) practices described in the BMP Manual shall be incorporated.
The proposed LID practices shall be noted on the stormwater management
site plan.
H.
The design of all SWM facilities over karst shall include an evaluation
of measures to minimize adverse effects.
I.
Infiltration BMPs shall be spread out, made as shallow as practicable,
and located to maximize use of natural on-site infiltration features
while still meeting the other requirements of this chapter. Infiltration
BMPs shall include pretreatment BMPs unless shown to be unnecessary.
J.
Infiltration BMPs intended to receive runoff from developed areas
shall be selected based on suitability of soils and development site
conditions and shall be constructed on soils that have the following
characteristics:
(1)
A minimum depth of 24 inches between the bottom of the facility and
the limiting zone, unless it is demonstrated to the satisfaction of
the Borough that the selected BMP has design criteria which allow
for a smaller separation.
(2)
A stabilized infiltration rate sufficient to accept the additional
stormwater load and drain completely as determined by field tests
conducted by the applicant's professional designer.
K.
The calculation methodology to be used in the analysis of volume and peak rates of discharge shall be as required in § 315-35, Calculation methodology.
L.
A planting plan is required for all vegetated stormwater BMPs.
(1)
Native or naturalized/noninvasive vegetation suitable to the soil
and hydrologic conditions of the development site shall be used unless
otherwise specified in the BMP Manual.
(2)
Invasive vegetation may not be included in any planting schedule.
(3)
The limit of existing, native vegetation to remain shall be delineated
on the plan along with proposed construction protection measures.
(4)
All planting shall be performed in conformance with good nursery
and landscape practice. Plant materials shall conform to the standards
recommended by the American Association of Nurseryman, Inc., in the
American Standard of Nursery Stock.
M.
Areas proposed for infiltration BMPs shall be protected from sedimentation
and compaction during the construction phase to maintain maximum infiltration
capacity. Staging of earthmoving activities and selection of construction
equipment should consider this protection.
N.
Infiltration BMPs shall not be constructed nor receive runoff from
disturbed areas until the entire contributory drainage area to the
infiltration BMP has achieved final stabilization.
O.
Stormwater easements.
(1)
A minimum ten-foot-wide access easement shall be provided for all
stormwater serving multiple properties and not located within a public
right-of-way. Easements shall provide for ingress and egress to a
public right-of-way.
(2)
Drainage easements shall be provided where the conveyance, treatment,
or storage of stormwater, either existing or proposed, is identified
on the SWM site plan. Drainage easements shall be provided to contain
and convey the one-hundred-year frequency flood.
(3)
SWM facilities not located within a public right-of-way shall be
contained in and centered within a minimum twenty-foot-wide stormwater
management easement. All easements shall have adequate information
to be located in the field.
(4)
Stormwater management easements are required for all on-site areas
used to convey stormwater of 2 cfs or greater for a one-hundred-year
storm. Roof drains do not require stormwater management easements.
(5)
Unless a concentrated discharge of stormwater to an adjacent property
is within an existing watercourse, an easement burdening the adjacent
property shall be required.
(6)
Where a development site is traversed by watercourses other than
permanent streams, a drainage easement shall be provided conforming
substantially to the line of such watercourses. The terms of the easement
shall prohibit excavation, the placing of fill or structures, and
any alterations that may affect adversely the flow of stormwater within
any portion of the easement.
(7)
Nothing shall be placed, planted, set, or put within the area of
an easement that would adversely affect the function of the easement
or conflict with the easement agreement.
P.
The Borough may require additional stormwater control measures for
stormwater discharges to special management areas including but not
limited to:
(1)
Water bodies listed as "impaired" on Pennsylvania's Clean Water Act
303(d/305(b) integrated list.
(2)
Any water body or watershed with an approved total maximum daily
load (TMDL).
(3)
Critical areas with sensitive resources (e.g., state-designated special
protection waters, cold-water fisheries, carbonate or other groundwater
recharge areas highly vulnerable to contamination, drainage areas
to water supply reservoirs, source water protection zones, etc.)
Q.
Non-roof drains and sump pumps shall be tributary to infiltration
or vegetative BMPs. Use of catchment facilities for the purpose of
reuse is also permitted.
R.
Unless specifically approved by the Borough in light of circumstances
unique to the site, roof drains and sump pumps shall not be connected
to streets, sanitary or storm sewers, or to roadside ditches and shall
instead discharge to infiltration areas or vegetative BMPs.
[Amended 9-27-2022 by Ord. No. 2022-04]
S.
Incorporate
methods described in the BMP Manual. If methods other than green infrastructure
and low-impact development (LID) methods are proposed to achieve the
volume and rate controls required under this chapter, the SWM site
plan must include a detailed justification demonstrating that the
use of LID and green infrastructure is not practicable.
[Added 9-27-2022 by Ord. No. 2022-04]
Volume control BMPs are intended to maintain existing hydrologic conditions for small storm events by promoting groundwater recharge and/or evapotranspiration as described in this section. Runoff volume controls shall be implemented using the Design Storm Method described in § 315-32A below, or through continuous modeling approaches or other means as described in the BMP Manual. Small projects may use the method described in § 315-32B to design volume control BMPs.
A.
The Design Storm Method is applicable to any size of regulated activity.
This method requires detailed modeling based on site conditions.
(1)
Do not increase the post-development total runoff volume for all
storms equal to or less than the two-year twenty-four-hour storm event.
(2)
For modeling purposes:
(a)
Existing (predevelopment) non-forested pervious areas must be
considered meadow in good condition.
(b)
When the existing development site contains impervious area,
20% of existing impervious area to be disturbed shall be considered
meadow in good condition in the model for existing conditions.
(c)
The maximum loading ratio for volume control facilities in karst
areas shall be 3:1 impervious drainage area to infiltration area and
5:1 total drainage area to infiltration area. The maximum loading
ratio for volume control facilities in non-karst areas shall be 5:1
impervious drainage area to infiltration area and 8:1 total drainage
area to infiltration area. A higher ratio may be approved by the Borough
if justification is provided. Hydraulic depth may be used as an alternative
to an area-based loading ratio if the design hydraulic depth is shown
to be less than the depth that could result from the maximum area
loading ratio.
B.
Volume control for small projects.
(1)
At least the first one inch of runoff from new impervious surfaces
or an equivalent volume shall be permanently removed from the runoff
flow, i.e., it shall not be released into the surface waters of this
commonwealth. Removal options include reuse, evaporation, transpiration
and infiltration.
C.
A detailed geologic evaluation of the development site shall be performed in areas of carbonate geology to determine the design parameters of recharge facilities. A report shall be prepared in accordance with § 315-45A of this chapter.
(1)
If the developer can prove through analysis that the development
site is in an area underlain by carbonate geology, and such geologic
conditions may result in sinkhole formations, then the development
site is exempt from volume control requirements as described in this
chapter. However, the development site shall still be subject to NPDES
and E&S requirements.
D.
Storage facilities, including normally dry, open-top facilities,
shall completely drain the volume control storage over a period of
time not less than 24 hours and not more than 72 hours from the end
of the design storm. Any designed infiltration at such facilities
is exempt from the minimum twenty-four-hour standard, i.e., may infiltrate
in a shorter period of time, provided that none of this water will
be discharged into waters of this commonwealth.
E.
Any portion of the volume control storage that meets the following
criteria may also be used as rate control storage:
(1)
Volume control storage that depends on infiltration is designed according to the infiltration standards in § 315-31.
(2)
The volume control storage which will be used for rate control is
that storage which is available within 24 hours from the end of the
design storm based on the stabilized infiltration rate and/or the
evapotranspiration rate.
F.
Applicable worksheets from of the BMP Manual shall be used when establishing
volume controls.
G.
The green
infrastructure and LID practices provided in the BMP Manual shall
be used for all regulated activities wherever possible.
[Added 9-27-2022 by Ord. No. 2022-04]
Rate control for large storms, up to the one-hundred-year event,
is essential to protect against immediate downstream erosion and flooding.
A.
Match predevelopment hydrograph.
(1)
Applicants shall provide infiltration facilities or utilize other techniques which will allow the post-development one-hundred-year hydrograph to match the predevelopment one-hundred-year hydrograph, along all parts of the hydrograph, for the development site. To match the predevelopment hydrograph, the post-development peak rate must be less than or equal to the predevelopment peak rate, and the post-development runoff volume must be less than or equal to the predevelopment volume for the same storm event. A shift in hydrograph peak time of up to five minutes and a rate variation of up to 5% at a given time may be allowable to account for the timing effect of BMPs used to manage the peak rate and runoff volume. "Volume control" volumes as given in § 315-32 above may be used as part of this option.
B.
Where the predevelopment hydrograph cannot be matched, one of the
following shall apply:
(1)
Within the Little Conestoga Creek watershed, post-development rates of runoff from any regulated activity shall not exceed 50% of the peak rates of runoff prior to the development for all design storms [two-, five-, ten-, twenty-five-, fifty-, and one-hundred-year twenty-four-hour storms (NOTE: A twenty-four-hour SCS type II storm or an IDF Curve Rational Method storm. See Table 2 in § 315-35 or (NOAA) Atlas 14 data for the specific project site.)]. See the Release Rate Map in Appendix L[1] for additional information.
[1]
Editor's Note: Said appendix is included as an attachment to this chapter.
(2)
Within all remaining watersheds of Millersville Borough, the calculated post-development peak rate of stormwater runoff from any regulated activity shall not exceed the calculated predevelopment peak rate of runoff for all design storms [two-, five-, ten-, twenty-five-, fifty-, and one-hundred-year twenty-four-hour storms (NOTE: A twenty-four-hour SCS type II storm or an IDF Curve Rational Method storm. See Table 2 in § 315-35 or (NOAA) Atlas 14 data for the specific project site.)]. See the Release Rate Map in Appendix L[2] for additional information.
[2]
Editor's Note: Said appendix is included as an attachment to this chapter.
C.
All basins not including groundwater recharge and/or water quality
storage shall include an outlet structure to allow for draining the
basin to a completely dry position within 24 hours following the end
of the design rainfall. All basins that include groundwater recharge
and/or water quality storage shall include an outlet structure to
allow 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.
D.
A variety of BMPs should be employed and tailored to suit the development
site. The following is a partial listing of BMPs which can be utilized
in SWM systems for rate control where appropriate:
(1)
Decreased impervious surface coverage.
(2)
Routed flow over grass.
(3)
Grassed channels and vegetated strips.
(4)
Bio-retention areas (rain gardens).
(5)
Concrete lattice block or permeable surfaces.
(6)
Seepage pits, seepage trenches or other infiltration structures.
(7)
Rooftop detention.
(8)
Parking lot detention.
(9)
Cisterns and underground reservoirs.
(10)
Amended soils.
(11)
Retention basins.
(12)
Detention basins.
(13)
Other methods as may be found in the BMP Manual.
E.
Small projects are not required to provide for rate control.
A.
Runoff from impervious areas shall be drained to pervious areas within
the development site to the maximum extent practicable.
B.
Stormwater runoff from a development site to an adjacent property
shall flow directly into a natural drainageway, watercourse, or into
an existing storm sewer system, or onto adjacent properties in a manner
similar to the runoff characteristics of the predevelopment flow.
C.
Stormwater flows onto adjacent property shall not be created, increased,
decreased, relocated, or otherwise altered without written notification
of the adjacent property owner(s) by the developer. Such stormwater
flows shall be subject to the requirements of this chapter, including
the establishment of a drainage easement. Copies of all such notifications
shall be included in SWM site plan submissions.
D.
Existing on-site natural and man-made SWM facilities shall be used
to the maximum extent practicable.
E.
Stormwater runoff shall not be transferred from one sub-watershed
to another unless they are sub-watersheds of a common watershed that
join together within the perimeter of the development site and the
effect of the transfer does not alter the peak discharge onto adjacent
lands.
F.
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 elevation. 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.
G.
All stormwater conveyance facilities (excluding detention, retention,
and wetland basin outfall structures) shall be designed to convey
a twenty-five-year storm event. (NOTE: A twenty-four-hour SCS Type
II storm or an IDF Curve Rational Method storm.) All stormwater conveyance
facilities (excluding detention, retention, and wetland basin outfall
structures) conveying water originating from off site shall be designed
to convey a fifty-year storm event. (NOTE: A twenty-four-hour SCS
Type II storm or an IDF Curve Rational Method storm.) Safe conveyance
of the one-hundred-year runoff event (NOTE: A twenty-four-hour SCS
Type II storm or an IDF Curve Rational Method storm.) to appropriate
peak rate control BMPs and throughout the development site must be
demonstrated in the design.
H.
Erosion protection shall be provided along all open channels, and
at all points of discharge. Flow velocities from any storm sewer may
not result in erosion of the receiving channel.
A.
Any stormwater runoff calculations involving drainage areas greater
than 200 acres and time of concentration (Tc) greater than 60 minutes,
including on- and off-site areas, shall use generally accepted calculation
techniques based on the NRCS Soil-Cover Complex Method with the rainfall
depths provided in Table 2,[1] or other method acceptable to the Borough Engineer.
B.
Stormwater runoff from all development sites shall be calculated
using either the modified Rational Method, a soil-cover-complex methodology,
or other method acceptable to the Borough. Table 1 summarizes acceptable
computation methods. It is assumed that all methods will be selected
by the design professional based on the individual limitations and
suitability of each method for a particular development site.
|
Table 1
Acceptable Computation Methodologies for Stormwater Management
Plans
| |||
|---|---|---|---|
|
Method
|
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
| |
|
Win TR-55 (or commercial computer package based on TR-55)
|
USDA NRCS
|
Applicable for land development plans within limitations described
in TR-55
| |
|
HEC-1/HEC-HMS
|
US Army Corps of Engineers
|
Applicable where use of full hydrologic computer model is desirable
or necessary
| |
|
Rational Method (or commercial computer package based on Rational
Method)
|
Emil Kuichling (1889)
|
For development sites less than 200 acres, Tc<60 min. or
as approved by the Borough
| |
|
EFH2
|
USDA NRCS
|
Applicable in rural and undeveloped areas subject to the program
limits
| |
|
Other methods
|
Varies
|
Other methodologies approved by the Borough
| |
C.
If the SCS Method is used, Antecedent Moisture Condition 1 is to
be used in areas of carbonate geology, and Antecedent Moisture Condition
2 is to be used in all other areas. A Type II distribution shall be
used in all areas.
|
Table 2
| ||
|---|---|---|
|
Storm Event
(years)
|
Rainfall
(inches)
| |
|
2
|
3.1
| |
|
5
|
4.1
| |
|
10
|
5.0
| |
|
25
|
5.5
| |
|
50
|
6.2
| |
|
100
|
7.0
| |
D.
If the Rational Method is used, the National Oceanic and Atmospheric Administration (NOAA) Atlas 14 data (See § 315-44B.) or PennDOT Publication 584, "PennDOT Drainage Manual," 2010 Edition, or latest, or the PDT-IDF chart included in Appendix B[2] shall be used to determine the rainfall intensity in inches
per hour based on the information for the five-through-sixty-minute-duration
storm events.
[2]
Editor's Note: Said appendix is included as an attachment to this chapter.
E.
Hydrographs may be obtained from NRCS methods such as TR-55, TR20, or from use of the "modified" or "unit hydrograph" Rational Methods. If "modified" or "unit hydrograph" Rational Methods are used, the ascending leg of the hydrograph shall have a length equal to three times the time of concentration (3xTc) and the descending leg shall have a length equal to seven times the time of concentration (7xTc) to approximate an SCS Type II hydrograph. (NOTE: See § 315-97K.)
F.
Runoff calculations shall include a hydrologic and hydraulic analysis
indicating volume and velocities of flow and the grades, sizes, and
capacities of water-carrying structures, sediment basins, retention
and detention structures and sufficient design information to construct
such facilities. Runoff calculations shall also indicate both predevelopment
and post-development rates for peak discharge of stormwater runoff
from all discharge points.
G.
For the purpose of calculating predevelopment peak discharges, all
runoff coefficients, both on site and off site, shall be based on
actual land use assuming summer or good land conditions. Post-development
runoff coefficients for off-site discharges used to design conveyance
facilities shall be based on actual land use assuming winter or poor
land conditions.
H.
Criteria and assumptions to be used in the determination of stormwater
runoff and design of management facilities are as follows:
(1)
Runoff coefficients shall be based on the information contained in
Appendix C and Appendix D[3] if the actual land use is listed in those appendixes.
If the actual land use is not listed in these appendixes, runoff coefficients
shall be chosen from other published documentation, and a copy of
said documentation shall be submitted with the SWM site plan.
[3]
Editor's Note: Said appendixes are included as attachments to this chapter.
(2)
A sample worksheet for calculating Tc is provided in Appendix H.[4] Times of concentration (Tc) shall be based on the following
design parameters:
(a)
Sheet flow. The maximum length for each reach of sheet or overland
flow before shallow concentrated or open channel flow develops is
150 feet. Flow lengths greater than 100 feet shall be justified based
on the actual conditions at each development site. Sheet flow may
be determined using the nomograph in Appendix F,[5] or the Manning's kinematic solution shown in the sheet
flow section of Worksheet No. 1 in Appendix H.[6]
[5]
Editor's Note: Said appendix is included as an attachment to this chapter.
[6]
Editor's Note: Said appendix is included as an attachment to this chapter.
(b)
Shallow concentrated flow. Travel time for shallow concentrated
flow shall be determined using Figure 3-1 from TR-55, Urban Hydrology
for Small Watersheds, as shown in Appendix G.[7]
[7]
Editor's Note: Said appendix is included as an attachment to this chapter.
(c)
Open channel flows. At points where sheet and shallow concentrated
flows concentrate in field depressions, swales, gutters, curbs, or
pipe collection systems, the travel times to downstream end of the
development site between these design points shall be based upon Manning's
Equation and/or acceptable engineering design standards as determined
by the Borough Engineer.
[4]
Editor's Note: Said appendix is included as an attachment to this chapter.
(3)
The developer may use stormwater credits for nonstructural BMPs in
accordance with the BMP Manual. The allowable reduction will be determined
by the Borough Engineer.
(4)
Peak rate control is not required for off-site runoff. Off-site runoff
may be bypassed around the site, provided all other discharge requirements
are met. If off-site runoff is routed through rate control facilities,
runoff coefficients for off-site discharges used to design those rate
control facilities shall be based on actual land use assuming winter
or poor land conditions.
I.
Times of concentration shall be calculated based on the methodology
recommended in the respective model used. Times of concentration for
channel and pipe flow shall be computed using Manning's Equation.
Supporting documentation and calculations must be submitted for review
and approval.
A.
In order to protect and improve water quality, a riparian corridor
easement is encouraged to be created and recorded as part of any subdivision
or land development that encompasses a riparian corridor.
B.
Except as otherwise required by Chapter 102, the riparian corridor
easement shall be measured to be 35 feet from the top of stream bank
(on each side).
C.
Minimum management requirements for riparian corridors.
(1)
Existing native vegetation shall be protected and maintained within
the riparian corridor easement.
(2)
Whenever practicable, invasive vegetation shall be actively removed
and the riparian corridor 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.
(3)
Any un-vegetated areas within the corridor shall be established with
permanent vegetation.
D.
The riparian corridor easement shall be enforceable by the Borough
and shall be recorded in the Lancaster 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 continued private
ownership and shall not be deemed a public right-of-way nor imply
public rights of access.
E.
Any permitted use within the riparian corridor easement shall be
conducted in a manner that will improve or maintain the stream stability,
and preserve and protect the ecological function of the floodplain.
F.
The following conditions shall apply when public and/or private recreation
trails are permitted within riparian corridors:
(1)
Trails shall be designed to have the least impact on native plant
species and other sensitive environmental features.
G.
Required
riparian buffers, upon being planted and permanently established with
the required vegetation, shall be professionally inspected, and certified
by a qualified professional that the established vegetation meets
the requirements of the approved planting plan, including required
native vegetation, permanent establishment, free from invasives, and
other such requirements.
[Added 9-27-2022 by Ord. No. 2022-04]
Aboveground storage facilities consist of all stormwater facilities
which store, infiltrate/evaporate/transpire, clean or otherwise affect
stormwater runoff, and the top of which is exposed to the natural
environment. Aboveground storage facilities are located above the
finished ground elevation. Aboveground storage facilities do not include
SWM facilities designed for conveyance or cisterns.
A.
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.
B.
Design criteria. Aboveground storage facilities shall comply with
the design criteria in the following table:
|
Aboveground Storage Facility Design Criteria
| ||||||
|---|---|---|---|---|---|---|
|
Facility Depth
| ||||||
|
Less than 2 feet
|
2 feet to 6 feet
|
Greater than 6 feet
| ||||
|
(a)
|
Embankment geometry
| |||||
|
[1]
|
Top width (minimum)
|
2 feet
|
5 feet
|
8 feet
| ||
|
[2]
|
Interior side slope (maximum)
|
2:1
|
5:1
|
5:1
| ||
|
[3]
|
Exterior side slope (maximum)
|
2:1
|
3:1
|
3:1
| ||
|
(b)
|
Embankment construction
| |||||
|
[1]
|
Key trench
|
Not required
|
Required
|
Required
| ||
|
[2]
|
Pipe collar
|
Not required
|
Required
|
Required
| ||
|
[3]
|
Compaction density
|
Not required
|
Required
|
Required
| ||
|
(c)
|
Internal construction
| |||||
|
[1]
|
Dewatering feature
|
N/A
|
Required
|
Required
| ||
|
[2]
|
Pretreatment elements
|
Not required*
|
Required
|
Required
| ||
|
(d)
|
Outlet structure
| |||||
|
[1]
|
Pipe size (minimum)
|
6 inches
|
12 inches
|
15 inches
| ||
|
[2]
|
Pipe material
|
SLHDPE, PVC, RCP
|
RCP
|
RCP
| ||
|
[3]
|
Anticlogging devices
|
Required
|
Required
|
Required
| ||
|
[4]
|
Antivortex design
|
Not required
|
Required
|
Required
| ||
|
[5]
|
Watertight joints in piping?
|
No
|
Yes
|
Yes
| ||
|
(e)
|
Spillway requirements
| |||||
|
[1]
|
Spillway freeboard (minimum)
|
Not required
|
6 inches
|
12 inches
| ||
|
[2]
|
Width (minimum)
|
Not required
|
10 feet
|
20 feet
| ||
|
[3]
|
Width (maximum)
|
Not required
|
50 feet
|
50 feet
| ||
|
[4]
|
Spillway channel design
|
Not required
|
Required
|
Required
| ||
|
[5]
|
Routing of 100 year storm
|
Permitted
|
Permitted
|
Permitted
| ||
|
*
|
Pretreatment required for infiltration BMPs unless shown to
be unnecessary
| |
|
N/A = Not applicable
| ||
|
SLHDPE = Smooth lined high-density polyethylene pipe
| ||
|
PVC = Polyvinyl chloride
| ||
|
RCP = Reinforced concrete pipe
|
C.
Facility depth.
(1)
For the purposes of the design criteria, the facility depth is defined
to be the depth between the bottom invert of the lowest orifice and
the invert of the spillway. If there is no spillway, the top of the
berm shall be used. For basins with no orifices or outlet structure
at the bottom of the basin, the bottom elevation of the basin shall
be used.
(2)
Facilities with a facility depth greater than six feet shall not
be permitted in residential areas.
(3)
Facilities with a facility depth greater than 15 feet require a dam
permit from DEP.
(4)
The maximum depth of water for aboveground storage facilities without
restricted access shall not exceed six feet unless approved by modification
or waiver by the Borough Council or its designee. Access to basins
with a maximum depth of water greater than six feet shall be restricted
by fencing that will discourage access.
D.
Embankment construction.
(1)
Impervious core/key trench. An impervious core/key trench, when required,
shall consist of a cutoff trench (below existing grade) and a core
trench (above existing grade). A key trench may not be required wherever
it can be shown that another design feature, such as the use of an
impermeable liner, accomplishes the same purpose.
(a)
Materials used for the core shall conform to the Unified Soil
Classification GC, SC, CH, or CL and must have at least 30% passing
the No. 200 sieve.
(b)
The dimensions of the core shall provide a minimum trench depth
of two feet below existing grade, minimum width of four feet and side
slope of 1H:1V or flatter.
(c)
The core should extend up both abutments to the ten-year water
surface elevation or six inches below the emergency spillway elevation,
whichever is lower.
(d)
The core shall extend four feet below any pipe penetrations
through the impervious core. The core shall be installed along or
parallel to the center line of the embankment.
(e)
The area under the embankment shall be cleared, grubbed and
stripped of topsoil to a depth of two feet prior to any placement
and compaction of earthfill.
(2)
Compaction.
(a)
Compaction requirements shall be the same as those for the embankment
to assure maximum density and minimum permeability.
(b)
The core shall be constructed concurrently with the outer shell
of the embankment. Core and key trench shall be constructed to a minimum
of 95% standard Proctor density.
(c)
The trench shall be dewatered during backfilling and compaction
operations.
(3)
Pipe collars. All pipe collars, when required, shall be designed
in accordance with Chapter 7 of the E&S Manual. The material shall
consist of concrete or otherwise nondegradable material around the
outfall barrel and shall be watertight.
(4)
Embankment fill material. The embankment fill material shall be taken
from an appropriate borrow area which shall be free of roots, stumps,
wood, rubbish, stones greater than six inches, frozen or other objectionable
materials.
(5)
Embankment compaction. When required, embankments shall be compacted
by sheepsfoot or pad roller. The loose lift thickness shall be nine
inches or less, depending on roller size, and the maximum particle
size is six inches or less (2/3 of the lift thickness). Five passes
of the compaction equipment over the entire surface of each lift is
required. Embankment compaction to visible nonmovement is also required.
E.
Internal construction.
(1)
Bottom slope. The minimum bottom slope of facilities not designed
for infiltration shall be 1%. A flatter slope may be used if an equivalent
dewatering mechanism is provided.
(2)
Dewatering features. When required, dewatering shall be provided
through the use of underdrain, surface device, or alternate approved
by the Borough Engineer. If the facility is to be used for infiltration,
the dewatering device should be capable of being disconnected and
only be made operational if the basin is not dewatering within the
required time frame.
(3)
Pretreatment elements. When required, pretreatment elements shall
consist of forebays, or alternate approved by the Borough Engineer,
to keep silt to a smaller portion of the facility for ease of maintenance.
(4)
Infiltration basins. Within basins designed for infiltration, existing native vegetation shall be preserved, if possible. For existing unvegetated areas or for infiltration basins that require excavation, a planting plan shall be prepared in accordance with § 315-31L and the BMP Manual which is designed to promote infiltration.
F.
Outlet configuration.
(1)
For facilities with a depth of two feet or greater, a Type D-W endwall
or riser box outlet structure shall be provided.
(2)
For facilities with a depth less than two feet, the designer must
specify a suitable outlet structure.
(3)
All discharge control devices with appurtenances shall be made of
reinforced concrete and stainless steel. Bolts/fasteners shall be
stainless steel.
G.
Spillway.
(1)
Material. The spillway shall be designed to provide a nonerosive,
stable condition when the project is completed.
(2)
Nonemergency use. Use of the spillway to convey flows greater than
the fifty-year design storm is permitted.
(3)
Emergency use. The spillway shall be designed to convey the one-hundred-year
peak rate of runoff which enters the basin after development in a
manner which will not damage the integrity of the facility and will
not create a downstream hazard.
(4)
When required, freeboard shall be measured from the top of the water
surface elevation for emergency use.
H.
Breach analysis. The Borough may require a breach analysis based
on site-specific conditions and concern of threat for downstream property.
When required, the breach analysis shall be conducted in accordance
with the NRCS methodology, the United States Army Corps of Engineers
methodology (HEC-1) or other methodologies as approved by the Borough.
I.
SWM facilities which qualify as a dam under DEP regulations or facilities
deemed a potential threat to the life, safety or welfare of the general
public shall be subject to the following requirements:
(1)
Facilities which qualify as a dam under DEP regulation shall obtain
the required permit through DEP and design the facility in accordance
with DEP standards.
(2)
Additional requirements and analysis may be required by the Borough
to prove that the proposed facility has been designed to limit the
potential risk to the life, safety or welfare of the general public.
Subsurface storage facilities consist of all stormwater facilities
which store, infiltrate/evaporate/transpire, clean or otherwise affect
stormwater runoff and the top of which is not exposed to the natural
environment. Subsurface facilities are located below the finished
ground elevation. Subsurface facilities do not include SWM facilities
designed for conveyance.
A.
Subsurface storage facilities shall comply with the design criteria
in the following table:
|
Subsurface Storage Facility Design Criteria
| |||||
|---|---|---|---|---|---|
|
Facility Type
| |||||
|
Infiltration and Storage
|
Storage without Infiltration
| ||||
|
(a)
|
Facility geometry
| ||||
|
[1]
|
Depth from surface (maximum)
|
2 feet less than limiting zone
|
N/A
| ||
|
[2]
|
Loading ratio (maximum)
|
Per BMP Manual*
|
N/A
| ||
|
(b)
|
Distribution system requirements
| ||||
|
[1]
|
Pipe size (minimum)
|
4 inches
|
4 inches
| ||
|
[2]
|
Pretreatment
|
Required
|
Required
| ||
|
[3]
|
Loading/balancing
|
Required
|
Not required
| ||
|
[4]
|
Observation/access ports
|
Required
|
Required
| ||
|
*
|
Unless otherwise determined by professional geologic evaluation
|
B.
Distribution system requirements.
(1)
Pretreatment requirements. The facility shall be designed to provide
a method to eliminate solids, sediment, and other debris from entering
the subsurface facility.
(2)
Loading/balancing. The facility shall be designed to provide a means
of evenly balancing the flow across the surface of the facility to
be used for infiltration.
(3)
Observation/access ports.
(a)
For facilities with the bottom less than five feet below the
average grade of the ground surface, a cleanout shall be an acceptable
observation port.
(b)
For facilities with the bottom five feet or more below the average
grade of the ground surface, a manhole or other means acceptable to
the Borough shall be provided for access to and monitoring of the
facility.
(c)
The number of access points shall be sufficient to flush or
otherwise clean out the system.
C.
Materials.
(1)
Pipe material. Distribution system piping may be PVC, SLHDPE, or
RCP.
(2)
Stone for infiltration beds. The stone used for infiltration beds
shall be clean washed, uniformly graded coarse aggregate. The void
ratio for design shall be assumed to be 40%.
(3)
Backfill material. Material consistency and placement depths for
backfill shall be (at a minimum) per all applicable pipe manufacturer's
recommendations, further providing it should be free of large (not
exceeding six inches in any dimension) objectionable or detritus material.
Select non-aggregate material should be indigenous to the surrounding
soil material for non-vehicular areas. Backfill within vehicular areas
shall comply with this section unless otherwise specified in the Borough
Street or Subdivision and Land Development Ordinance or by the Borough
Engineer. Furthermore, if the design concept includes the migration
of runoff through the backfill to reach the infiltration facility,
the material shall be well drained, free of excess clay or clay-like
materials and generally uniform in gradation.
(4)
Lining material. Nonwoven geotextiles shall be placed on the sides
and top of subsurface infiltration facilities. No geotextiles shall
be placed on the bottom of subsurface infiltration facilities.
D.
Cover.
(1)
When located under pavement, the top of the subsurface facility shall
be a minimum of three inches below the bottom of pavement subbase.
(2)
Where located under vegetative cover, the top of the subsurface facility
shall be a minimum of 12 inches below the surface elevation or as
required to establish vegetation.
E.
Subsurface facilities shall be designed to safely convey and/or bypass
flows from storms exceeding the design storm.
F.
Infiltration facilities shall be designed with measures to protect
infiltration facilities from compaction and sedimentation during and
after construction.
Conveyance facilities consist of all stormwater facilities which
carry flow, which may be located either above or below the finished
grade. Conveyance facilities do not include SWM facilities which store,
infiltrate/evaporate/transpire, or clean stormwater runoff.
A.
Design criteria. Conveyance facilities shall comply with the design
criteria in the following table:
|
Conveyance Facility Design Criteria
| ||||||
|---|---|---|---|---|---|---|
|
Location
|
Within Public Street Right-of-Way
|
Outside Public Street Right-of-Way
| ||||
|
Loading
|
All
|
Vehicular Loading
|
Non-Vehicular Loading
| |||
|
(a)
|
Pipe design
| |||||
|
[1]
|
Material
|
RCP
|
PVC, SLHDPE, RCP
|
PVC, SLHDPE, RCP
| ||
|
[2]
|
Slope (minimum)
|
0.5%
|
0.5%
|
0.5%
| ||
|
[3]
|
Cover
|
1 foot to stone subgrade
|
1 foot to stone subgrade
|
1 foot to surface
| ||
|
[4]
|
Diameter (minimum)
|
15 inches
|
12 inches
|
8 inches
| ||
|
[5]
|
Street crossing angle
|
75° to 90°
|
N/A
|
N/A
| ||
|
[6]
|
Access/ maintenance port frequency (maximum)
|
400 feet
|
400 feet
|
600 feet
| ||
|
(b)
|
Inlet design
| |||||
|
[1]
|
Material
|
Concrete
|
Concrete
|
N/A
| ||
|
[2]
|
Grate depression
|
1/2 inch or less
|
2 inches
|
1 inch minimum
| ||
|
(c)
|
Manhole design
| |||||
|
[1]
|
Material
|
Concrete
|
Concrete
|
Concrete
| ||
|
(d)
|
Swale design
| |||||
|
[1]
|
Freeboard (minimum)
|
6 inches
|
N/A
|
6 inches
| ||
|
[2]
|
Velocity (maximum)
|
Stability check
|
N/A
|
Stability check
| ||
|
[3]
|
Slope (minimum)
|
1%
|
N/A
|
1%
| ||
|
[4]
|
Side slopes (residential area)
|
4:1 max
|
N/A
|
4:1 max
| ||
|
[5]
|
Side slopes (nonresidential area)
|
4:1 max
|
N/A
|
3:1 max
| ||
|
[6]
|
Bottom width to flow depth ratio
|
12:1
|
N/A
|
12:1
| ||
|
(e)
|
Outlet design
| |||||
|
[1]
|
End treatment
|
Headwall/endwall
|
N/A
|
Headwall/endwall or flared end section
| ||
|
[2]
|
Energy dissipater
|
Required
|
N/A
|
Required
| ||
|
N/A = Not applicable or no criteria specified
| |
|
SLHDPE = Smooth lined high-density polyethylene pipe
| |
|
PVC = Polyvinyl chloride
| |
|
RCP = Reinforced concrete pipe
|
B.
Conveyance pipes, culverts, manholes, inlets and endwalls within
the public street right-of-way or proposed for dedication shall conform
to the requirements of PennDOT Standards for Roadway Construction,
Publication No. 72M as directed by the Borough Engineer.
C.
Conveyance pipes, culverts, manholes, inlets and endwalls outside
the public street right-of-way which are subject to vehicular loading
shall be designed for the HS-25 loading condition.
D.
All material and workmanship for conveyance facilities shall conform
to current PennDOT Publication 408 specifications.
E.
Conveyance pipes.
(1)
Backfill requirements. Backfill material. Material consistency and
placement depths for backfill shall be (at a minimum) per all applicable
pipe manufacturer's recommendations, further providing it should be
free of large (not exceeding six inches in any dimension) objectionable
or detritus material. Select non-aggregate material should be indigenous
to the surrounding soil material for non-vehicular areas. Backfill
within vehicular areas shall comply with this section unless otherwise
specified in the Borough Street or Subdivision and Land Development
Ordinance or by the Borough Engineer.
(2)
Inlets or manholes shall be placed at all points of changes in the
horizontal or vertical directions of conveyance pipes. Curved pipe
sections are prohibited.
(3)
Access/maintenance ports. An access/maintenance port is required
and may either be an inlet or manhole.
(4)
Watertight joints shall be provided where pipe sections are joined,
except for perforated pipe installed as pavement base drain.
(5)
The street-crossing angle shall be measured between the pipe center
line and the street center line.
(6)
Elliptical pipe of an equivalent cross-sectional area may be substituted
in lieu of circular pipe where cover or utility conflict conditions
exist.
(7)
The roughness coefficient (Manning "n" values) used for conveyance
pipe capacity calculations should be determined in accordance with
Appendix E,[1] or per the manufacturer's specifications.
[1]
Editor's Note: Said appendix is included as an attachment to this chapter.
F.
Inlets.
(1)
All pipes must enter inlets completely through one of the sides.
No corner entry of pipes is permitted.
(2)
Within the public street right-of-way, the gutter spread based on
the twenty-five-year storm shall be no greater than 1/2 of the travel
lane and have a maximum depth of three inches at the curbline. A parking
lane shall not be considered as part of the travel lane. In the absence
of pavement markings separating a travel lane from the parking lane,
the parking lane shall be assumed to be seven feet wide if parking
is permitted on the street.
(3)
Flow depth within intersections. Within intersections of streets,
the maximum depth of flow shall be 1.5 inches based on the twenty-five-year
storm.
(5)
All inlets placed in paved areas shall have heavy-duty bicycle-safe
grating consistent with PennDOT Publication 72M, latest edition. A
note to this effect shall be added to the SWM site plan or inlet details
therein.
(6)
Inlets, junction boxes, or manholes greater than five feet in depth
shall be equipped with ladder rungs and shall be detailed on the SWM
site plan.
G.
Swales.
(1)
A swale shall be considered as any man-made ditch designed to convey
stormwater directly to another stormwater management facility or surface
waters.
(2)
Inlets within swales shall have PennDOT Type "M" top units or equivalent
approved by the Borough Engineer.
(3)
Swale capacities and velocities shall be computed using the Manning
Equation using the following design parameters:
(a)
The first condition shall consider swale stability based upon
a low degree of retardance ("n" = 0.03);
(b)
The second condition shall consider swale capacity based upon
a higher degree of retardance ("n" = 0.05); and
(c)
All vegetated swales shall have a minimum slope of 1% unless
otherwise approved by the Borough Engineer.
(d)
The "n" factors to be used for paved or riprap swales or gutters
shall be based upon accepted engineering design practices, as approved
by the Borough Engineer.
(4)
All swales shall be designed to maximize infiltration and concentrate
low flows to minimize siltation and meandering, unless geotechnical
conditions do not permit infiltration.
H.
Culverts. In addition to the material requirements in this section,
culverts designed to convey waters of this commonwealth may be constructed
with either a corrugated metal arch or a precast concrete culvert.
J.
Energy dissipaters. Energy dissipaters shall be designed in accordance
with the requirements in the E&S Manual.
K.
End treatments.
(1)
Where the connecting pipe has a diameter 18 inches or greater, headwalls
and endwalls shall be provided with a protective barrier device to
prevent entry of the storm sewer pipe by unauthorized persons. Such
protection devices shall be designed to be removable for cleaning.
(2)
Headwalls and endwalls shall be constructed of concrete.
(3)
Flared end sections shall be of the same material as the connecting
pipe and be designed for the size of the connecting pipe.
