Township of East Hempfield, PA
Lancaster County
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Table of Contents
Table of Contents
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 § 260-28 or the activity qualifies as a small project.
B. 
No regulated activities shall commence until the Township issues unconditional written approval of a SWM site plan.
C. 
The Township 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 Township 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 Erosion and Sediment Pollution Control Program Manual (E8S Manual), No. 363-2134-008 (March 2012), as amended and updated (NOTE: See § 260-53I.), 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 Township in accordance with § 260-13C. 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:
(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.
(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 Township 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.
(a) 
The stabilized infiltration rate is to be determined in the same location and within the same soil horizon as the bottom of the infiltration facility.
(b) 
The stabilized infiltration rate is to be determined as specified in the BMP Manual.
K. 
The calculation methodology to be used in the analysis of volume and peak rates of discharge shall be as required in § 260-17, 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) 
Stormwater 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 two 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 Township 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 Township in light of circumstances unique to the site, roof drains shall not be connected to streets, sanitary or storm sewers or to roadside ditches and instead shall discharge to infiltration areas or vegetative BMPs.
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 Subsection A below, or through continuous modeling approaches or other means as described in the BMP Manual. Small projects may use the method described in Subsection B 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 previous areas must be considered meadow in good condition.
(b) 
When the existing project 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 Township 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 § 260-27A 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 § 260-13.
(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 evapo-transpiration rate.
F. 
Applicable worksheets from of the BMP Manual shall be used when establishing volume controls.
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 affect of BMPs used to manage the peak rate and runoff volume. "Volume control" volumes as given in § 260-14 above may be used as part of this option.
B. 
Where the predevelopment hydrograph cannot be matched, the post-development rates of runoff from any regulated activity shall not exceed 50% of the peak rates of runoff for the new impervious surface area prior to development for the two-, ten-, twenty-five-, fifty-, and one-hundred-year storm events. [NOTE: A twenty-four-hour SCS type II storm or an IDF Curve Rational Method storm. See Table 2 in § 260-17 or (NOAA) Atlas 14 data for the specific project site.]
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 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, or other method acceptable to the Township 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 Township. 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 Township
EFH2
USDA NRCS
Applicable in rural and undeveloped areas subject to the program limits
Other methods
Varies
Other methodologies approved by the Township
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
Twenty-Four-Hour Storm Event
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 Subsection B above.) or PennDOT Publication 584 "PennDOT Drainage Manual," 2010 Edition, or latest, or the PDT-IDF chart included in Appendix B[1] 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.
[1]
Editor's Note: Said appendix is on file in the Township offices.
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 § 260-53K.)
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[2] 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.
[2]
Editor's Note: Said appendixes are on file in the Township offices.
(2) 
A sample worksheet for calculating Tc is provided in Appendix H.[3] 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,[4] or the Manning's kinematic solution shown in the sheet flow section of Worksheet No. 1 in Appendix H.[5]
[4]
Editor's Note: Said appendix is on file in the Township offices.
[5]
Editor's Note: Said appendix is on file in the Township offices.
(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.[6]
[6]
Editor's Note: Said appendix is on file in the Township offices.
(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 Township Engineer.
[3]
Editor's Note: Said appendix is on file in the Township offices.
(3) 
The developer may use stormwater credits for nonstructural BMPs in accordance with the BMP Manual. The allowable reduction will be determined by the Township 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 shall be created and recorded as part of any land development that encompasses a riparian corridor.
B. 
Except as otherwise required by Chapter 102, the riparian corridor easement shall be measured 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 Township 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 the 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.
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
3:1
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 Board of Supervisors. 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 Township 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 Township 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 § 260-13N 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 Township 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 Township.
I. 
SWM facilities which qualify as a dam per 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 per 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 Township 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 Township 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 Township Road Ordinance[1] or by the Township 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 claylike materials and generally uniform in gradation.
[1]
Editor's Note: See Ch. 222, Streets and Sidewalks.
(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 that 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
*SLHDPE, *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 to 1 inch
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
* Pipes crossing Township Collector Streets shall be RCP.
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 Township 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 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 Township Road Ordinance[1] or by the Township Engineer.
[1]
Editor's Note: See Ch. 222, Streets and Sidewalks.
(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 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 centerline and the street centerline.
(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,[2] or per the manufacturer's specifications.
[2]
Editor's Note: Said appendix is on file in the Township offices.
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 1/2 inches based on the twenty-five-year storm.
(4) 
Curbed streets.
(a) 
Inlets in streets shall be located along the curbline.
(b) 
Top units shall be PennDOT Type "C." The hood shall be aligned with the adjacent curb height.
(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 Township 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 Township 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 Township 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.
I. 
Level spreaders.
(1) 
Shall discharge at existing grade onto undisturbed vegetation.
(2) 
Discharge at a depth not exceeding 3.0 inches for a fifty-year, twenty-four-hour design storm.
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.