Township of Warwick, PA Stormwater Management Standards

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.

For development taking place in stages or phases, the entire development plan must be used in determining conformance with this chapter.

Any areas designed to initially be gravel or crushed stone shall be assumed to be impervious.

All regulated activities shall include such measures as necessary to:

Protect health, safety, and property;

Meet the water quality goals of this chapter by implementing measures to:

Protect and/or improve the function of floodplains, wetlands, and wooded areas.

Protect and/or improve native plant communities including those within the riparian corridor.

Protect and/or improve natural drainageways from erosion.

Minimize thermal impacts to waters of this commonwealth.

(e)

Disconnect impervious surfaces by directing runoff to pervious areas, wherever possible.

The design of all SWM facilities over karst shall include an evaluation of measures to minimize adverse effects.

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.

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:

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.

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.

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.

The stabilized infiltration rate is to be determined as specified in the BMP Manual.

The calculation methodology to be used in the analysis of volume and peak rates of discharge shall be as required in § 270-35.

A planting plan is required for all vegetated stormwater BMPs.

Native or naturalized/non-invasive vegetation suitable to the soil and hydrologic conditions of the development site shall be used unless otherwise specified in the BMP Manual.

Invasive vegetation may not be included in any planting schedule.

The limit of existing, native vegetation to remain shall be delineated on the plan along with proposed construction protection measures.

Prior to construction, a tree protection zone shall be delineated at the dripline of the tree canopy. All trees scheduled to remain during construction shall be marked; however, where groups of trees exist, only the trees on the outside edge need to be marked. A forty-eight-inch-high snow fence or forty-eight-inch-high construction fence mounted on steel posts located eight feet on center shall be placed along the tree protection boundary, or another method of identification approved by the Township prior to construction. No construction, storage of material, temporary parking, pollution of soil, or regrading shall occur within the tree protection zone.

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.

Planting designs are encouraged to share planting space for optimal root growth whenever possible.

No staking or wiring of trees shall be allowed without a maintenance note for the stake and/or wire removal within one year of planting.

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.

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.

A minimum ten-foot-wide access easement shall be provided for all stormwater facilities with tributary areas equal or greater than 1,000 square feet and not located within a public right-of-way. Easements shall provide for ingress and egress to a public right-of-way.

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.

The Township may require additional stormwater control measures for stormwater discharges to special management areas including but not limited to:

Water bodies listed as "impaired" on Pennsylvania's Clean Water Act 303-d/305(b) Integrated List.

Any water body or watershed with an approved total maximum daily load (TMDL).

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.).

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.

Nonstructural BMPs shall be utilized for all regulated activities unless proven to be impractical.

§ 270-32 Volume controls.

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 § 270-32B to design volume control BMPs.

The design storm method is applicable to any size of regulated activity. This method requires detailed modeling based on site conditions.

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.

For modeling purposes:

Existing (predevelopment) nonforested pervious areas must be considered meadow in good condition.

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.

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.

Volume control for small projects.

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.

Wherever possible, infiltration facilities should be designed to accommodate infiltration of the entire permanently removed runoff; however, in all cases at least the first 1/2 inch of the permanently removed runoff should be infiltrated.

A detailed geologic evaluation of the development site shall be performed in areas of carbonate geology to determine the design parameters of recharge facilities. The evaluation shall be performed by a registered professional geologist, and shall at a minimum, address soil permeability, depth to bedrock, susceptibility to sinkhole formation, and subgrade stability. A report shall be prepared in accordance with § 270-45A of this chapter.

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.

Any portion of the volume control storage that meets all of the following criteria may also be used as rate control storage:

Volume control storage that depends on infiltration is designed according to the infiltration standards in § 270-31.

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.

Volume control storage facilities designed to infiltrate shall avoid, to the greatest extent practicable, the least permeable hydrologic soil group(s) at the development site.

§ 270-33 Rate controls.

Rate control for large storms, up to the one-hundred-year event, is essential to protect against immediate downstream erosion and flooding.

Match pre-development hydrograph. Applicants shall provide infiltration facilities or utilize other techniques which will allow the post-development one-hundred-year hydrograph to match the pre-development one-hundred-year hydrograph, along all parts of the hydrograph, for the development site. To match the pre-development hydrograph, the post development peak rate must be less than or equal to the pre-development peak rate, and the post development runoff volume must be less than or equal to the pre-development 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 § 270-32 may be used as part of this option.

Where the pre-development hydrograph cannot be matched, one of the following shall apply:

For areas not covered by a release rate map from the Conestoga River Act 167 Plan: Post development discharge rates shall not exceed the predevelopment discharge rates for the two-, ten-, twenty-five-, fifty-, and one-hundred-year twenty-four-hour storm events.* If it is shown that the peak rates of discharge indicated by the post development analysis are less than or equal to the peak rates of discharge indicated by the predevelopment analysis for two-, ten-, twenty-five-, fifty-, and one-hundred-year twenty-four-hour storms,* then the requirements of this section have been met. Otherwise, the applicant shall provide additional controls as necessary to satisfy the peak rate of discharge requirement.

	NOTE:
	*	A twenty-four-hour SCS type II storm or an IDF Curve Rational Method storm. See Table III-1 in Section 305.

For areas covered by a release rate map in the Conestoga River Act 167 Plan: For the two-, ten-, twenty-five-, fifty-, and one-hundred-year storm events,* the post development peak discharge rates will follow the applicable approved release rate maps.

	NOTE:
	*	A twenty-four-hour SCS type II storm or an IDF Curve Rational Method storm. See Table III-1 in § 270-35.

Normally dry, open top, storage facilities shall completely drain the rate control storage over a period of time less than or equal to 24 hours from the peak one-hundred-year water surface design elevation.

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:

Decreased impervious surface coverage.

Routed flow over grass.

Grassed channels and vegetated strips.

Bio-retention areas (rain gardens).

Concrete lattice block or permeable surfaces.

Seepage pits, seepage trenches or other infiltration structures.

Rooftop detention.

(8)

Parking lot detention.

(9)

Cisterns and underground reservoirs.

(10)

Retention basins.

(11)

Detention basins.

(12)

Other methods as may be found in the BMP Manual, as approved by the Township.

Small projects are not required to provide for rate control.

§ 270-34 Stormwater management performance standards.

Runoff from impervious areas shall be drained to pervious areas within the development site, unless the site has 85% or more impervious cover and is a redevelopment,[1] in which case the portion of the site that discharges to pervious areas shall be maximized.

Editor's Note: See § 270-97J.

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.

Maximum use shall be made of the existing on-site natural and man-made stormwater management facilities. The applicant must provide proof to the Township Engineer that he or his agent has informed the immediate downstream property owner of the effects of the stormwater discharge from the proposed development.

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.

Existing on-site natural and man-made SWM facilities shall be used to the maximum extent practicable.

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.

Minimum floor elevations and/or any wall penetrations 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.

All stormwater conveyance facilities (excluding detention, retention, and wetland basin outfall structures) shall be designed to convey a twenty-five-year storm event.* All stormwater conveyance facilities (excluding detention, retention, and wetland basin outfall structures) conveying water originating from offsite shall be designed to convey a fifty-year storm event.* Safe conveyance of the one-hundred-year runoff event* to appropriate peak rate control BMPs must be demonstrated in the design. Easements shall begin at the furthest upstream property line of the proposed development site in a watershed.

	NOTE:
	*	A twenty-four-hour SCS Type II storm or an IDF Curve Rational Method storm.

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.

§ 270-35 Calculation methodology.

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.

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 III-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 III-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
WinTR-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 minutes 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

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.

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, 2008 Edition, or latest, 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.

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.[1]

Editor's Note: See § 270-97K.

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 pre-development and post-development rates for peak discharge of stormwater runoff from all discharge points.

For the purpose of calculating pre-development 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 on-site discharges used to design conveyance facilities shall be based on actual land use assuming winter or poor land conditions.

Criteria and assumptions to be used in the determination of stormwater runoff and design of management facilities are as follows:

Runoff coefficients shall be based on the information contained in Appendix B-1 and B-2[2] if the actual land use is listed in those appendices. If the actual land use is not listed in these appendices, runoff coefficients shall be chosen from other published documentation, and a copy of said documentation shall be submitted with the SWM site plan.

Editor's Note: Said appendixes are included as attachments to this chapter.

A sample worksheet for calculating Tc is provided in Appendix B-4.[3] Times of concentration (Tc) shall be based on the following design parameters:

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 B-3,[4] or the Manning's kinematic solution shown in the Sheet Flow section of Worksheet No. 1 in Appendix B-4.[5]

[4]

Editor's Note: Said appendix is included as an attachment to this chapter.

[5]

Editor's Note: Said appendix is included as an attachment to this chapter.

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 B-5.[6]

[6]

Editor's Note: Said appendix is included as an attachment to this chapter.

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 included as an attachment to this chapter.

The developer may use stormwater credits for nonstructural BMPs in accordance with the BMP Manual. The allowable reduction will be determined by the Township.

Peak rate control is not required for off-site runoff. Off-site runoff may be by-passed 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.

The sizing of SWM facilities for residential subdivisions shall be based on the maximum impervious coverage permitted by Chapter 340, Zoning, for those lots that are 15,000 square feet or less in size. Within residential subdivisions where lots are greater than 15,000 square feet in size the design for the stormwater management facilities must specifically identify the square footage of impervious coverage assumed for each lot and a note must be provided on the plan to be recorded that indicates the amount used for the design of the SWM facilities within the subdivision.

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.

§ 270-36 Riparian corridors.

In order to protect and improve water quality, a riparian corridor easement shall be created and recorded as part of any subdivision or land development that encompasses a riparian corridor.

Except as otherwise required by Chapter 102, the riparian corridor easement shall be measured to be the greater of the limit of the one-hundred-year floodplain or 35 feet from the top of streambank (on each side).

Minimum management requirements for riparian corridors.

Existing native vegetation shall be protected and maintained within the riparian corridor easement.

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.

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.

Any permitted use within the riparian corridor easement shall be conducted in a manner that will maintain the extent of the existing one-hundred-year floodplain, improve or maintain the stream stability, and preserve and protect the ecological function of the floodplain.

The following conditions shall apply when public and/or private recreation trails are permitted within Riparian Corridors:

Trails shall be for nonmotorized use only.

Trails shall be designed to have the least impact on native plant species and other sensitive environmental features.

Septic drainfields and sewage disposal systems shall not be permitted within the Riparian Corridor Easement and shall comply with setback requirements established under 25 Pa. Code Chapter 73.

§ 270-37 Stormwater management facility design requirements.

Above-ground storage facilities. Above-ground storage facilities consist of all SWM facilities which store, infiltrate/evaporate/transpire, clean or otherwise affect stormwater runoff and the top of which is exposed to the natural environment. Above-ground storage facilities are located above the finished ground elevation. Above-ground storage facilities do not include SWM facilities designed for conveyance or cisterns.

Design criteria. Above-ground storage facilities shall comply with the design criteria in the following table:

Above-ground storage facility design criteria
			Facility Depth
			Less than 2 feet	2 feet to 8 feet	Greater than 8 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	SLHDPE, 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	3 inches	6 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

	NOTES:
	*	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

Facility depth.

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, the bottom elevation of the basin shall be used.

Facilities with a facility depth greater than eight feet shall not be permitted in residential areas.

Facilities with a facility depth greater than 15 feet require a dam permit from DEP.

Embankment construction.

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.

Materials. 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.

Dimensions.

[a]

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.

[b]

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.

[c]

The core shall extend four feet below any pipe penetrations through the impervious core. The core shall be installed along or parallel to the centerline of the embankment.

[3]

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.

[c]

The trench shall be dewatered during backfilling and compaction operations.

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.

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.

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.

Internal construction.

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.

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 timeframe.

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.

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 § 270-31N and the BMP Manual which is designed to promote infiltration.

Outlet configuration.

For facilities with a depth of two feet or greater, a type D-W endwall or riser box outlet structure shall be provided.

For facilities with a depth less than two feet, no outlet structure is required.

All discharge control devices with appurtenances shall be made of reinforced concrete and stainless steel. Bolts/fasteners shall be stainless steel.

Spillway.

Material. The spillway shall be designed to provide a nonerosive, stable condition when the project is completed.

Nonemergency use. Use of the spillway to convey flows greater than the fifty-year design storm is permitted.

Emergency use. The spillway shall be designed to convey the one-hundred-year peak inflow into the basin when routed through the basin with all primary outlet facilities clogged.

When required, freeboard shall be measured from the top of the water surface elevation for emergency use.

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 US Army Corps of Engineers methodology (HEC-1) or other methodologies as approved by the Township.

Subsurface storage facilities. 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 stormwater management facilities designed for conveyance.

Design criteria. 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.

Distribution system requirements.

Pretreatment requirements. The facility shall be designed to provide a method to eliminate solids, sediment, and other debris from entering the subsurface facility.

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.

Observation/access ports.

For facilities with the bottom less than five feet below the average grade of the ground surface, a clean-out shall be an acceptable observation port.

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.

[3]

The number of access points shall be sufficient to flush or otherwise clean out the system.

Materials.

Pipe material. Distribution system piping may be SLHDPE, or RCP.

Stone for infiltration beds. The stone used for infiltration beds shall be clean washed, uniformly graded coarse aggregate (AASHTO No. 3 or equivalent approved by the Township). The void ratio for design shall be assumed to be 0.4.

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 nonvehicular areas. Backfill within vehicular areas shall comply with this section unless otherwise specified in Chapter 285, Subdivision and Land Development. 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.

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.

Cover.

When located under pavement, the top of the subsurface facility shall be a minimum of three inches below the bottom of pavement subbase.

Where located under vegetative cover, the top of the subsurface facility shall be a minimum of 12 inches below the surface elevation.

Subsurface facilities shall be designed to safely convey and/or bypass flows from storms exceeding the design storm.

Location.

Infiltration systems greater than three feet deep shall be located no less than 30 feet from basement walls.

Infiltration systems designed to handle runoff from commercial or industrial impervious parking areas shall be no closer than 100 feet from any water supply well.

Overflow. The stormwater management facility design shall provide an emergency overflow system with measures to provide nonerosive velocity along its length at the outfall.

Conveyance facilities. Conveyance facilities consist of all SWM 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.

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	Nonvehicular loading
(a)	Pipe design
	[1]	Material	SLHDPE, RCP	SLHDPE, RCP	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	15 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	2 inches	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

	NOTES:
	N/A =	Not applicable or no criteria specified
	SLHDPE =	Smooth lined high density polyethylene pipe;
	RCP =	Reinforced concrete pipe

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. Conveyance pipes, culverts, manholes, inlets and endwalls which are otherwise subject to vehicular loading shall be designed for the HS-25 loading condition.

Conveyance pipes.

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 nonvehicular areas. Backfill within vehicular areas shall comply with this section unless otherwise specified in Chapter 285, Subdivision and Land Development.

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.

Access/maintenance ports. An access/maintenance port is required may either be an inlet or manhole.

Watertight joints shall be provided where pipe sections are joined, except for perforated pipe installed as pavement base drain.

(e)

The street crossing angle shall be measured between the pipe centerline and the street centerline.

(f)

Elliptical pipe of an equivalent cross-sectional area may be substituted in lieu of circular pipe where cover or utility conflict conditions exist.

(g)

The roughness coefficient (Manning "n" values) used for conveyance pipe capacity calculations should be determined in accordance with PennDOT Publication 584, PennDOT Drainage Manual, or per the manufacturer's specifications.

(h)

No double piping shall be permitted.

Inlets.

All pipes must enter inlets completely through one of the sides. No corner entry of pipes is permitted.

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 curb line. 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.

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.

Curbed streets.

Inlets in streets shall be located along the curb line.

Top units shall be PennDOT Type "C." The hood shall be aligned with the adjacent curb height.

(e)

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.

(f)

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)

All risers used for adjusting inlets shall be cast-in-place or precast concrete. Risers created from any other material are unacceptable.

Swales.

A swale shall be considered as any man-made ditch designed to convey stormwater directly to another SWM facility or surface waters.

Inlets within swales shall have PennDOT Type "M" top units or equivalent approved by the Township Engineer.

Swale capacities and velocities shall be computed using the Manning equation using the following design parameters:

Vegetated swales.

[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.

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.

All swales shall be designed to maximize infiltration and concentrate low flows to minimize siltation and meandering, unless geotechnical conditions do not permit infiltration.

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.

Level spreaders.

Shall discharge at existing grade onto undisturbed vegetation.

Discharge at a depth not exceeding 3.0 inches for a fifty-year, twenty-four-hour design storm.

(8)

Energy dissipaters. Energy dissipaters shall be designed in accordance with the requirements in the E&S Manual.

(9)

End treatments.

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.

Headwalls and endwalls shall be constructed of concrete.

Flared end sections shall be of the same material as the connecting pipe and be designed for the size of the connecting pipe. (See appendix for detail.[1])

Editor's Note: The Appendix is included as an attachment to this chapter.

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:

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.