Borough of Adamstown, 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.

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

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

No stormwater facilities shall be placed in, over or immediately adjacent to the following features:

Sinkholes.

Closed depressions.

Lineaments in carbonate areas.

Fracture traces.

Caverns.

Intermittent lakes.

Ephemeral streams.

Bedrock pinnacles (surface or subsurface).

Stormwater management basins shall not be located closer than 100 feet from the rim of sinkholes or closed depressions, nor within 100 feet from disappearing streams; nor shall these basins be located closer than 50 feet from lineaments or fracture traces; nor shall these basins be located close than 25 feet from surface or identified subsurface pinnacles, unless lined with an impermeable liner or equivalent design as signed and sealed by a professional geologist.

Stormwater resulting from regulated activities shall not be discharged into sinkholes.

It shall be the developer's responsibility to verify if the development is underlain by carbonate geology. The following certificate shall be included on all stormwater site plans and shall be signed and sealed by the developer's professional geologist: "I, _____________________, certify that the proposed stormwater/BMP facility (circle one) is/is not underlain by carbonate geology."

Whenever a stormwater facility will be located in an area underlain by carbonate geology, a geological evaluation of the proposed location by a registered professional geologist shall be conducted to determine susceptibility to sinkhole formation. The evaluation may include the use of impermeable liners to reduce or eliminate the separation distances listed in Subsection J(1) and (2).

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 Borough 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 § 251-17.

A planting plan is required for all vegetated stormwater BMPs.

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.

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 tress on the outside edge need to be effectively marked to promote protection of the trees during construction. 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. 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 twenty-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 Borough 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 Sections 303(d) and 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 surface infiltration or vegetative BMPs. Sump pumps shall not be tributary to any subsurface facility. Use of catchment facilities for the purpose of reuse is also permitted. When it is more advantageous to connect directly to streets or storm sewers, roof drain connections to streets or roadside ditches may be permitted on a case-by-case basis by the Borough. It shall be the burden of the person seeking to make the connection to demonstrate to the Borough that such connection is more advantageous and such connection shall not violate any state or federal statute, rule or regulation. Proposed storm sewer piping may connect to an existing storm sewer piping system, provided the existing storm sewer is adequate.

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

§ 251-14 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 Subsection B 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 Borough of Adamstown 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.

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 §§ 251-13J and 251-36A 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 § 251-13.

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.

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

§ 251-15 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 predevelopment hydrograph. 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 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. "Volume control" volumes as given in § 251-14 above may be used as part of this option.

Where the predevelopment hydrograph cannot be matched, one of the following shall apply:

Where the predevelopment hydrograph cannot be matched, per the Cocalico Creek Watershed Act 167 Stormwater Management Plan, the post-development peak discharge rates shall not exceed 50% of the peak rates of runoff prior to development for the two-, ten-, twenty-five-, fifty- and one-hundred-year storm event.* Refer to the Release Rate Map in Appendix D.[1]

A twenty-four-hour SCS type II storm or an IDF Curve Rational Method storm. See Table III-1 in § 251-17.

Editor's Note: The Appendixes to the Stormwater Management Ordinance are on file in the Borough offices.

Where the predevelopment hydrograph cannot be matched, per the Conestoga Creek Watershed Act 167 Stormwater Management Plan, the post-development peak discharge rates shall not exceed 50% of the peak rates of runoff prior to development for the two-, ten-, twenty-five-, fifty- and one-hundred-year storm event.* Refer to the Release Rate Map in Appendix D.[2]

A twenty-four-hour SCS type II storm or an IDF Curve Rational Method storm. See Table III-1 in § 251-17.

Editor's Note: The Appendixes to the Stormwater Management Ordinance are on file in the Borough offices.

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.

Bioretention areas (rain gardens).

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.

Amended soils.

Retention basins.

Detention basins.

Other methods as may be found in the BMP Manual.

Small projects are not required to provide for rate control.

§ 251-16 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: Reference CSN Technical Bulletin No. 5, Stormwater Design for High Intensity Redevelopment Projects in the Chesapeake Bay Watershed, version 2.0. Chesapeake Stormwater Network, January 5, 2011 - page 43.

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.

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 subwatershed to another unless they are subwatersheds 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.

The SWM and BMP facility site design shall prevent the mixing of off-site and on-site runoff, unless the upstream drainage area is less than 5% of the total on-site area.

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.

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 off site 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.

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.

Roof drains shall not be connected to streets, sanitary or storm sewers or roadside ditches. Roof drains shall discharge to infiltration areas or vegetative BMPs to the maximum extent practicable.

Stormwater management facilities which involve a state highway shall be subject to the approval of PennDOT.

A concentrated discharge of stormwater to an adjacent property shall be within an existing watercourse or otherwise an easement shall be required.

§ 251-17 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 Borough Engineer. 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
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	U.S. 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 Borough
EFH2	USDA NRCS	Applicable in rural and undeveloped areas subject to the program limits
Other methods	Varies	Other methodologies approved by the Borough

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 item B above) or PennDOT Publication 584 PennDOT Drainage Manual 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. (Refer to Appendix B-3.[1])

Editor's Note: The Appendixes to the Stormwater Management Ordinance are on file in the Borough offices.

Hydrographs may be obtained from NRCS methods such as TR-55, TR-20 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.[2]

Editor's Note: Reference: Penn State Urban Hydrology Model User Manual by Thomas A. Seybert, PE, David F. Kibler, PE, and Elizabeth 1. White, PE, August 1993 page 70 and VT/PSUHM help screen.

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.

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.

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[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.

Editor's Note: The Appendixes to the Stormwater Management Ordinance are on file in the Borough offices.

A sample worksheet for calculating Tc is provided in Appendix B-5.[4] Time 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-4 or the Manning's kinematic solution shown in the sheet flow section of Worksheet No. 1 in Appendix B-5.[5]

Editor's Note: The Appendixes to the Stormwater Management Ordinance are on file in the Borough offices.

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-6.[6]

Editor's Note: The Appendixes to the Stormwater Management Ordinance are on file in the Borough offices.

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 the 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.

Editor's Note: The Appendixes to the Stormwater Management Ordinance are on file in the Borough offices.

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.

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.

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.

§ 251-18 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 stream bank (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 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 the continued private ownership and shall count toward the minimum lot area as required by Chapter 320, Zoning.

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.

§ 251-19 Stormwater management facility design standards.

General.

For all aboveground storage facilities, the bottom of the excavated basin shall be a minimum of two feet or 24 inches above the seasonal high-water table or bedrock. Soil sampling, test pits or auger testing must be completed in the proposed location of the facilities in support of the design.

Aboveground storage facilities located in areas with unrestricted access to the public shall be enclosed with fencing to deter access for safety reasons.

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

Aboveground storage facilities with a facility depth greater than 15 feet require a dam permit from DEP.

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 approved by the Borough Engineer around the outfall barrel and shall be watertight.

(6)

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.

(7)

When required, embankments shall be compacted by sheep's-foot or pad roller. The loose-lift thickness shall be nine inches or less, depending on roller size, and the maximum particle size six inches or less (two-thirds 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.

(8)

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.

(9)

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.

(10)

When required, pretreatment elements shall consist of forebays, filter strips or alternate approved by the Borough Engineer, to keep silt to a smaller portion of the facility for ease of maintenance.

(11)

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 § 251-13N and the BMP Manual which is designed to promote infiltration.

(12)

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

(13)

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

(14)

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

(15)

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

(16)

The spillway shall be designed to convey the one-hundred-year peak inflow when required.

(17)

Freeboard shall be measured from the top of the water surface elevation for emergency use.

(18)

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 U.S. Army Corps of Engineers methodology (HEC-1) or other methodologies as approved by the Borough.

(19)

Embankment construction.

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 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.

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.

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.

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.

(f)

Compaction requirements shall be the same as those for the embankment to assure maximum density and minimum permeability.

(g)

The core shall be constructed concurrently with the outer shell of the embankment.

(h)

The trench shall be dewatered during backfilling and compaction operations.

Aboveground storage facilities. 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 stormwater management facilities designed for conveyance or cisterns.

Design criteria. Refer to Table 3-2.

Aboveground storage facility with facility depth of less than two feet:

The minimum top of embankment width shall be two feet.

Maximum interior side slope 2:1.

Maximum exterior side slope 2:1.

Minimum outlet pipe diameter shall be six inches.

Outlet pipe material: PVC, HDPE or RCP.

Anti-clogging device is required.

Watertight joints shall be provided in karst areas.

The spillway freeboard shall be a minimum three inches.

The spillway may be used to route the one-hundred-year storm.

Aboveground storage facility with depth of two to eight feet:

Embankment minimum top width of five feet.

Maximum interior side slope 5:1.

Maximum exterior side slope 3:1.

A key trench and anti-seep collar shall be provided.

Compaction density of the embankment is required.

A dewatering feature is required.

Pretreatment filtering of runoff is required.

Minimum outlet pipe diameter of 12 inches.

Outlet pipe material: HDPE or RCP.

An anti-clogging device is required.

An antivortex design is required.

Watertight joints shall be provided.

The spillway freeboard shall be a minimum six inches.

The minimum spillway width is 10 feet.

[15]

The maximum spillway width is 50 feet.

[16]

The downstream channel into which the spillway discharges shall be checked for adequate capacity and stability.

[17]

The spillway shall not be considered to function as part of the primary outlet structure and shall be only for emergency situations.

Aboveground storage facility with depth greater than eight feet.

Embankment minimum top width of eight feet.

Maximum interior side slope 5:1.

Maximum exterior side slope 3:1.

A key trench and anti-seep collar shall be provided.

Compaction density of the embankment is required.

A dewatering feature is required.

Pretreatment filtering of runoff is required.

Minimum outlet pipe diameter of 15 inches.

Outlet pipe material: RCP.

An anti-clogging device is required.

An antivortex design is required.

Watertight joints shall be provided.

The spillway freeboard shall be a minimum 12 inches.

The minimum spillway width is 20 feet.

[15]

The maximum spillway width is 50 feet.

[16]

The downstream channel into which the spillway discharges shall be checked for adequate capacity and stability.

[17]

The spillway shall not be considered to function as part of the primary outlet structure and shall be only for emergency situations.

General requirements for aboveground storage facilities:

Where practical, the spillway shall be constructed in undisturbed ground.

The effect on the downstream areas if the facility embankment fails shall be considered in the design of all facilities. Where possible, the facility shall be designed to minimize the potential damage caused by such failure of the embankment.

For all aboveground facilities that do not rely on infiltration to dewater the runoff, a flow path length-to-width ratio of 2:1 shall be provided to maximize the treatment time between the inflow point and the outlet structure.

Table 3-2. Aboveground Storage Facility Design Criteria
	Facility Depth
	Less than 2 Feet	2 Feet to 8 Feet	Greater than 8 Feet
Embankment Geometry
Top width (minimum)	2 feet	5 feet	8 feet
Interior side slope (maximum)	2:1	5:1	5:1
Exterior side slope (maximum)	2:1	3:1	3:1
Embankment Construction
Key trench	Not required	Required	Required
Pipe collar	Not required	Required	Required
Compaction density	Not required	Required	Required
Internal Construction
Dewatering feature	N/A	Required	Required
Pretreatment elements	Not required	Required	Required
Outlet Structure
Pipe size (minimum)	6 inches	12 inches	15 inches
Pipe material	HDPE, PVC, RCP	HDPE, RCP	RCP
Anti-clogging devices	Required	Required	Required
Anti-vortex design	Not required	Required	Required
Watertight joints in piping	Yes	Yes	Yes
Spillway Requirements
Spillway freeboard (minimum)	3 inches	6 inches	12 inches
Width (minimum)	Not required	10 feet	20 feet
Width (maximum)	Not required	50 feet	50 feet
Spillway channel design	Not required	Required	Required
Routing of one-hundred-year storm	Permitted	Not permitted	Not permitted

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.

General.

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

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 nonaggregate 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 governing municipal road/street or subdivision and land development ordinances. 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.

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.

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 or as required to establish vegetation.

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

Design criteria. Refer to Table 3-3.

Infiltration and storage facility.

Maximum depth from surface: two feet less than limiting zone.

Loading ratio. Per December 2006 BMP Manual, as amended. The maximum impervious loading ratio of 5:1 relating impervious drainage area to infiltration area. The maximum total loading ratio of 8:1 relating total drainage area to infiltration area. In areas of karst geology, the maximum impervious drainage area to infiltration area is 3:1.

Minimum distribution pipe size shall be four inches. Distribution system piping may be PVC or HDPE.

Pretreatment of runoff to the facility is required to provide a method to eliminate solids, sediment and other debris from entering the subsurface facility.

Observation/access ports shall be provided in the facility. 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. 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. The number of access points shall be sufficient to flush or otherwise clean out the system.

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.

Storage without infiltration facility.

Minimum distribution pipe size shall be four inches. Distribution system piping may be PVC, HDPE or RCP.

Pretreatment of runoff to the facility is required to provide a method to eliminate solids, sediment and other debris from entering the subsurface facility.

Table 3-3. Subsurface Storage Facility Design Criteria
	Facility Type
	Infiltration and Storage	Storage Without Infiltration
Facility Geometry
Depth from surface (maximum)	2 feet less than limiting zone	N/A
Loading ratio (maximum)	Per BMP Manual*	N/A
Distribution System Requirements
Pipe size (minimum)	4 inches	4 inches
Pretreatment	Required	Required
Loading/balancing	Required	Not required
Observation/access ports	Required	Required

Unless otherwise determined by professional geologic evaluation.

Conveyance facilities. 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 stormwater management facilities which store, infiltrate/evaporate/transpire, or clean stormwater runoff.

General.

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.

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 nonaggregate 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 governing municipal road/street or subdivision and land development ordinances.

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 which may either be an inlet or manhole.

(f)

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

(g)

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

(h)

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

(i)

The roughness coefficient (Manning "n" values) used for conveyance pipe capacity calculations should be determined in accordance with the manufacturer's specifications or with PennDOT Publication 584, PennDOT Drainage Manual (Appendix B-7[1]).

Editor's Note: The Appendixes to the Stormwater Management Ordinance are on file in the Borough offices.

(j)

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

(k)

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.

(l)

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.

(m)

Inlets in streets shall be located along the curbline.

(n)

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

(o)

All inlets placed in paved areas shall have heavy-duty bicycle-safe grating consistent with PennDOT Publication 72M. A note to this effect shall be added to the SWM site plan or inlet details therein.

(p)

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.

(q)

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

(r)

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

(s)

Swale capacities and velocities shall be computed using the Manning's 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 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.

(t)

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.

(u)

Headwalls and endwalls shall be constructed of concrete.

(v)

Flared end sections shall be of the same material as the connecting pipe and be designed for the size of the connecting pipe.

(w)

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.

(x)

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

(y)

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.

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.

(z)

In addition to the material requirements in this section, culverts designed to convey waters of the commonwealth may be constructed with either a corrugated metal arch or a precast concrete culvert.

Design criteria. Refer to Table 3-4.

Within public street right-of-way:

Conveyance system material shall consist of HDPE or RCP pipe.

The minimum pipe slope shall be 0.5%.

A minimum one foot of cover to the stone subgrade shall be provided over the conveyance pipes.

The minimum pipe diameter shall be 15 inches.

The minimum street crossing angle for the conveyance system shall be 75° to 90°.

Maximum spacing between access or maintenance ports shall be 400 feet.

Inlets and manholes shall be concrete.

Inlets shall be depressed a minimum of two inches below the surface grade to provide positive flow.

Swales shall be provided with a minimum freeboard of six inches.

The maximum swale velocity shall be determined based on the stability of the channel.

The minimum swale slope shall be 1%.

Side slopes in residential areas shall be a maximum of 4:1. Side slopes in nonresidential areas shall be a maximum of 4:1.

The bottom width to flow depth ratio shall be 12:1.

Pipe entrances/discharges in public street rights-of-way shall be provided with a headwall/endwall treatment.

[15]

The pipe discharge locations shall be provided with an energy dissipater designed to handle the anticipated flow conditions.

Outside public street right-of-way: vehicular loading.

Conveyance system material shall consist of PVC, HDPE or RCP pipe.

The minimum pipe slope shall be 0.5%.

A minimum one foot of cover to the stone subgrade shall be provided over the conveyance pipes.

The minimum pipe diameter shall be 15 inches.

Maximum spacing between access or maintenance ports shall be 400 feet.

Inlets and manholes shall be concrete.

Inlets shall be depressed a minimum of two inches below the surface grade to provide positive flow.

Outside public street right-of-way: nonvehicular loading.

Conveyance system material shall consist of PVC, HDPE or RCP pipe.

The minimum pipe slope shall be 0.5%.

A minimum one foot of cover to the surface shall be provided over the conveyance pipes.

The minimum pipe diameter shall be eight inches.

Maximum spacing between access or maintenance ports shall be 600 feet.

Manholes shall be concrete.

Inlets shall be depressed a minimum of one inch below the surface grade to provide positive flow.

Swales shall be provided with a minimum freeboard of six inches.

The maximum swale velocity shall be determined based on the stability of the channel.

The minimum swale slope shall be 1%.

Side slopes in residential areas shall be a maximum of 4:1. Side slopes in nonresidential areas shall be a maximum of 3:1.

The bottom width to flow depth ratio shall be 12:1.

Pipe entrances/discharges in public street rights-of-way shall be provided with a headwall/endwall or flared end section treatment.

The pipe discharge locations shall be provided with an energy dissipater designed to handle the anticipated flow conditions.

Table 3-4. Conveyance Facility Design Criteria
Location	Within Public Street Right-of-Way	Outside Public Street Right-of-Way
Loading	All	Vehicular loading	Non-vehicular Loading
Pipe Design
Material	HDPE, RCP	PVC, HDPE, RCP	PVC, HDPE, RCP
Slope (minimum)	0.5%	0.5%	0.5%
Cover	1 foot to stone subgrade	1 foot to stone subgrade	1 foot to surface
Diameter (minimum)	15 inches	15 inches	8 inches
Street crossing angle	75° to 90°	N/A	N/A
Access/ maintenance port frequency (maximum)	400 feet	400 feet	600 feet
Inlet Design
Material	Concrete	Concrete	N/A
Grate depression	2 inches	2 inches	1 inch minimum
Manhole design
Material	Concrete	Concrete	Concrete
Swale Design
Freeboard (minimum)	6 inches	N/A	6 inches
Velocity (maximum)	Stability check	N/A	Stability check
Slope (minimum)	1%	N/A	1%
Side slopes (residential area)	4:1 maximum	N/A	4:1 maximum
Side slopes (nonresidential area)	4:1 maximum	N/A	3:1 maximum
Bottom width to flow depth ratio	12:1	N/A	12:1
Outlet Design
End treatment	Headwall/ endwall	N/A	Headwall/ endwall or flared end section
Energy dissipater	Required	N/A	Required

Capture and reuse facilities.

Design requirements:

Calculation of water usage to ensure adequate capacity is available for storage of follow-up rainfall events. The property will draw from the cistern on a daily basis; the daily draw shall be not less than 1% of the total volume of the cistern.

Verification of conveyance pipe capacity in the roof leader design.

The water storage container(s) shall be protected from direct sunlight to minimize algae growth.

An alternative supply of water shall be available for the property use during dry periods.

Water storage containers should be watertight with smooth interior surfaces.

(f)

The cover (or lid) should have a tight fit to keep out surface water, children, animals, dust and light. The cover or lid opening should be a minimum 24 inches in order to access the facility for maintenance and repair.

(g)

Cisterns shall be designed to store the runoff volume of a one-hundred-year storm event for the area served by the water storage facility.

(h)

Every water storage facility (cistern, rain barrel, etc.) shall be provided with an overflow or an emergency spillway. The overflow shall be designed to discharge away from buildings and other structures and towards existing natural or man-made channels, stormwater facilities or vegetated slopes.

(i)

The plans proposing a water storage facility shall include the following:

All calculations and assumptions used in the design.

Sufficient detail showing the proposed method of dewatering (i.e., pump).