Article III Stormwater Management Standards

§ 270-15 General requirements.

Any person or entity intending to engage in a regulated activity shall, prior to commencing the regulated activity, apply for and obtain a stormwater management permit and comply with the requirements of this chapter. An application for stormwater management permit shall be submitted on a form prescribed by the Township accompanied by the required fee. Applicants proposing regulated activities in the Township which are not exempt under § 270-6 shall submit a stormwater management site plan (SWM site plan) to the Township for review and approval in accordance with Articles III and IV. SWM site plans approved by the Township shall be on site throughout the duration of the regulated activity.

The stormwater management and runoff control criteria and standards in this chapter shall apply to the total proposed regulated activity, even if it is to take place in stages. The measurement of impervious surfaces shall include all of the impervious surfaces in the total proposed regulated activity even if the development is to take place in stages.

No regulated activity within the Township shall commence until:

(1)

The Township issues the stormwater management permit and approval of a SWM site plan, which demonstrates compliance with the requirements of this chapter; and

(2)

The applicant has received a letter of adequacy or approval for the erosion and sediment control plan reviewed by the Township and the Conservation District (if required), and has received all other local, state and federal permit approvals required for the project involving the regulated activity.

Neither submission of a SWM site plan under the provisions herein nor compliance with the provisions of this chapter shall relieve any person from responsibility for damage to any person or property otherwise imposed by law.

The applicant shall design the site to minimize disturbances to land, site hydrology, and natural resources, and to maintain the natural hydrologic regime, drainage patterns and flow conditions. The applicant shall apply the procedures set forth in § 270-18 for the overall site design and for selection, location and design of features and BMPs to be used to comply with the requirements of this chapter.

To the maximum extent practicable, post-construction stormwater shall be discharged within the drainage area of the same stream or water body receiving the runoff prior to construction of the proposed regulated activity.

For regulated activities with one acre or more of proposed earth disturbance, existing drainage peak rate discharges up to and including the 100-year storm and the volume of runoff up to and including the two-year storm onto or through adjacent property(ies) or downgradient property(ies), including diffuse drainage discharge, shall not be altered in any manner by regulated activities under this chapter without written permission from, and, where applicable as determined by the Township an easement and agreement with, the affected landowner(s) for conveyance of discharges onto or through their property(ies). Altered stormwater discharges shall be subject to any applicable discharge criteria specified in this chapter.

Areas located outside of the site (i.e., areas outside of the regulated activity) that drain through a proposed site are not subject to water quality and volume control, infiltration, stream channel protection, or peak flow rate control requirements (as presented in §§ 270-19, 270-20, 270-21 and 270-22). Drainage facilities located on the site shall be designed to safely convey flows from outside of the site through the site.

If site conditions preclude capture of runoff from limited portions of the disturbed area for achieving water quality volume control standards, stream channel protection standards, and the ten-year storm event peak runoff rate reduction standards for new development required by this chapter, the applicant shall propose alternate methods to mitigate the bypass of the BMPs, subject to the approval of the Township. In no case shall resulting peak rate be greater than the predevelopment peak rate for the equivalent design storm.

For all regulated activities, erosion and sediment control BMPs shall be designed, implemented, operated, and maintained during the regulated activities (i.e., during construction) as required to meet the purposes and requirements of this chapter, to meet the erosion and sediment control requirements of the Township, if applicable, and to meet all requirements under Title 25 of the Pa. Code and the Clean Streams Law.[1]

[1]

Editor's Note: See 35 P.S. § 691.1 et seq.

For all regulated activities, permanent BMPs and conveyances shall be designed, implemented, operated, and maintained to meet the purposes and requirements of this chapter and to meet all requirements under Title 25 of the Pennsylvania Code, the Clean Streams Law, and the Stormwater Management Act.[2]

[2]

Editor's Note: See 35 P.S. § 691.1 et seq. and 32 P.S. § 680.1 et seq., respectively.

The design of all BMPs and conveyances shall incorporate sound engineering principles and practices in a manner that does not aggravate existing stormwater problems as identified by the Township. The Township reserves the right to disapprove any design that would result in construction in an area affected by existing stormwater problem(s) or continuation of an existing stormwater problem(s).

Existing wetlands, either on the site or on an adjacent property, shall not be used to meet the minimum design requirements for stormwater management or stormwater runoff quality treatment. Stormwater discharges to existing wetlands shall not degrade the quality or hydrologic integrity of the wetland.

Hotspot runoff controls. Specific structural or pollution prevention practices may be required, as determined to be necessary by the Township Engineer, to pretreat runoff from hotspots prior to infiltration. Following is a list of examples of hotspots:

(1)

Vehicle salvage yards and recycling facilities;

(2)

Vehicle fueling stations;

(3)

Vehicle service and maintenance facilities;

(4)

Vehicle and equipment cleaning facilities;

(5)

Fleet storage areas (bus, truck, etc.);

(6)

Industrial sites based on Standard Industrial Classification Codes;

(7)

Marinas (service and maintenance areas);

(8)

Outdoor liquid container storage;

(9)

Outdoor loading/unloading facilities;

(10)

Public works storage areas;

(11)

Facilities that generate or store hazardous materials;

(12)

Commercial container nursery;

(13)

Contaminated sites/brownfields;

(14)

Other land uses and activities as designated by the Township.

Contaminated and brownfield sites. Where BMPs may contribute to the migration of contaminants in groundwater, the water quality and runoff volume, stream channel protection, and peak rate control standards shall be met; however, at the Township Engineer's discretion, the minimum infiltration requirement may be reduced or eliminated commensurate with the contaminated area and the required water quality and runoff control measures may be increased to mitigate the reduced infiltration requirement for the contaminated area.

Additional water quality requirements. The Township may require additional stormwater control measures for stormwater discharges to special management areas including, but not limited to:

(1)

Water bodies listed as "impaired" by PADEP.

(2)

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

(3)

Areas of known existing flooding problems.

(4)

Critical areas with sensitive resources (e.g., state-designated special-protection waters, cold-water fisheries, carbonate geology or other groundwater recharge areas that may be highly vulnerable to contamination, drainage areas to water supply reservoirs, etc.).

Applicants shall utilize the Pennsylvania Stormwater Best Management Practices Manual (PA BMP Manual), as amended, or other sources acceptable to the Township for soil testing and design standards for BMPs, and where there is a conflict with the provisions of this chapter, the most restrictive applies as determined by the Township.

For areas underlain by karst or carbonate geology that may be susceptible to the formation of sinkholes and other karst features, the location, type, and design of infiltration BMPs shall be based on a site evaluation conducted by a qualified licensed professional and based on the PA BMP Manual (as amended) or other design guidance acceptable to the Township.

All regulated activities located within a special flood hazard area designated by the Federal Emergency Management Agency (FEMA) shall comply with Chapter 325, Article XIII, Floodplain Conservation, and shall be designed to maintain the flood-carrying capacity of the floodway such that the base flood elevations are not increased, either upstream or downstream. The natural conveyance characteristics of the site and the receiving floodplain shall be incorporated into the stormwater management practices proposed for the site.

Riparian buffer area (RBA).

(1)

Areas immediately adjacent to the Township's perennial streams, intermittent streams, and waterways are defined as the riparian buffer areas (RBA). In the RBA, special requirements as set forth in this section shall apply for the purpose of maintaining natural functions. These requirements are established due to:

(a)

The environmental sensitivity of the RBA and the potential for adverse impact when disturbed by construction and other earthmoving activity; and

(b)

The potential of the RBA to mitigate the negative effects of development on lands adjacent to the stream.

(2)

The RBA includes three subzones, Zones 1 through 3, extending landward from the top of the stream bank where different requirements are imposed.

(a)

Zone 1: a minimum twenty-five-foot setback, plus wetlands, measured from the top of the stream bank. No disturbance of vegetation or soil shall be permitted except for restoration or reforestation activities. The purpose of this limitation is to promote shading of the stream with natural vegetation, to provide a source of organic inputs to the aquatic system, to anchor the stream bank and floodplain area, and to remove nitrogen, sediment, and other substances that can adversely affect stream systems. Where prohibitive slopes (25% and steeper) are located within 35 feet of the stream bank, Zone 1 shall extend the entire distance of this sloped area.

(b)

Zone 2: a 125-foot buffer zone extending outward from Zone 1; the combined width of Zone 1 and Zone 2 shall not be less than 150 feet. Within Zone 2, disturbance of natural vegetative cover shall be limited to selective logging (not to exceed 25%) and other activities that minimally disrupt the existing tree and soil cover. The purpose of this limitation is to maximize filtering and overall physical removal of particulate-form pollutants from runoff generated upgradient and to promote subsurfance vegetative uptake of nitrogen and other nonparticulate elements from stormwater generated upgradient.

[1]

Where the Zone 2 riparian buffer is not wooded, it shall be maintained as a filter strip of dense grass and forbs or other features that will provide sediment filtering, nutrient uptake, and will convert concentrated flow to uniform, shallow, sheet flow. When a land development is proposed where there is no established vegetated or wooded buffer (such as in areas previously cultivated for agriculture), Zone 1 and Zone 2 combined shall be not less than 150 feet wide and shall be established and maintained in accordance with the following guidelines:

[a]

Forested and unforested vegetation shall be established through natural succession. Selective planting shall be incorporated on sites devoid of vegetation to stimulate native species and to discourage invasive species.

[b]

Plant selection and planting shall be consistent with Chester County Conservation District, PADEP, or USDA riparian forested buffer guidance.

[2]

Reforestation is a natural BMP and is encouraged; it may be considered in the stormwater design calculations for stormwater quantity and quality control.

(c)

Zone 3: a zone of varying width extending outward from Zone 2, defined as upslope areas adjacent to Zone 2 that are being disturbed during the land development process and where direct discharge of stormwater would otherwise occur. Zone 3 is intended to include level spreading or similar devices as necessary to ensure that any direct discharge flows are properly distributed as sheet flow and channelization and point source discharges are avoided. Zone 3 shall include a minimum twenty-five-foot setback, measured from Zone 2, from impervious surfaces.

(3)

The RBA may be included in net density calculations with uses permitted in the West Whiteland Township Zoning Ordinance of 1998,[3] unless this RBA is required to be subtracted out as the result of other applicable regulations of the said ordinance, such as regulation of the floodplain.

[3]

Editor's Note: See 35 P.S. § 691.1 et seq. and 32 P.S. § 680.1 et seq., respectively.

(4)

An RBA adjacent to "high-quality waters" and "exceptional-value waters" designated under PADEP, Chapter 93, Rules and Regulations, shall be subject to the provisions of the PADEP Special Protection Waters Implementation Handbook and its amendments.

(5)

RBAs shall be restricted to the following uses:

(a)

Zone 1 (very restricted): flood control; utility rights-of-way; pervious footpaths.

(b)

Zone 2 (restricted): pervious surfaces; passive recreational uses; natural stormwater BMPs; tree removal by permit.

(c)

Zone 3 setback (limited restrictions): lawn and garden areas; sheds and similar structures covering less than 150 square feet; composting of yard wastes; nonstructural stormwater BMPs.

(6)

RBAs shall be preserved or restored with native vegetation that can be maintained through the delineation, plan review, construction, and occupancy stages of development.

§ 270-16 Permit requirements by other governmental entities.

The following permit or other regulatory requirements may apply to certain regulated activities and shall be met prior to (or as a condition of) final approval by the Township of the SWM site plan and prior to commencement of any regulated activities, as applicable:

All regulated activities subject to permit or regulatory requirements by PADEP under regulations at Title 25 Pennsylvania Code Chapter 102, or erosion and sediment control requirements of the Township.

Work within natural drainageways subject to permit by PADEP under Title 25 Pennsylvania Code Chapter 105.

Any BMP or conveyance that would be located in or adjacent to surface waters of the commonwealth, including wetlands, subject to permit by PADEP under Title 25 Pennsylvania Code Chapter 105.

Any BMP or conveyance that would be located on or discharge to a state highway right-of-way, or require access to or from a state highway and be subject to approval by PennDOT.

Culverts, bridges, storm sewers, or any other facilities which must pass or convey flows from the tributary area and any facility which may constitute a dam subject to permit by PADEP under Title 25 Pennsylvania Code Chapter 105.

§ 270-17 Erosion and sediment control.

No regulated activity within the Township shall commence until:

(1)

The Township receives documentation that the applicant has received:

(a)

A "letter of adequacy" from the Conservation District or other approval from PADEP in compliance with Title 25 Chapter 102 of the Pennsylvania Code of an erosion and sediment control plan for construction activities, if applicable;

(b)

A PADEP NPDES Construction Activities Permit as required under Title 25 Pennsylvania Code Chapter 92,[1] if applicable;

[1]

Editor's Note: Chapter 92 of Title 25 of the Pennsylvania Code was reserved 10-8-2010, effective 10-9-2010. It was replaced by Chapter 92a, National Pollutant Discharge Elimination System (NPDES) Permitting, Monitoring and Compliance.

(c)

Evidence of any other permit(s) or approvals required for the regulated activities; and

(2)

An erosion and sediment control plan has been approved by the Township, if required.

A copy of the erosion and sediment control plan and any required permit(s), as required by PADEP regulations, shall be available on the site at all times.

Additional erosion and sediment control measures shall be applied where infiltration BMPs are proposed, at a minimum including those required in § 270-20M.

§ 270-18 Site design process.

The applicant shall design the site to minimize the disturbances to land, site hydrology, and natural resources, and to maintain the natural hydrologic regime, drainage patterns and flow conditions. For regulated activities with 10,000 or more square feet of proposed earth disturbance or 2,000 or more square feet of proposed impervious surfaces, the applicant shall demonstrate in its SWM site plan (as required in § 270-32C) that the design sequence, objectives and techniques described below were applied to the maximum extent practicable in the site design of the regulated activity while complying with all other requirements of this chapter. The site design shall:

First, identify and delineate all existing natural resources and natural and man-made hydrologic features listed in § 270-32B(8) that are located within the site, or receive discharge from, or may be impacted by the proposed regulated activity.

Second, provide a prioritized listing of these resources and features to identify:

(1)

Those to be incorporated into the site design in a manner that provides protection from any disturbance or impact from the proposed regulated activity;

(2)

Those to be protected from further disturbance or impact but for which the proposed regulated activity will provide improvement to existing conditions;

(3)

Those that can be incorporated into and utilized as components of the overall site design in a manner that protects or improves their existing conditions while utilizing their hydrologic function within the limits of their available capacity (e.g., for infiltration, evapotranspiration, or reducing pollutant loads, runoff volume or peak discharge rates, etc.) to reduce the need for or size of constructed BMPs; and

(4)

Those that may be considered for alteration, disturbance or removal.

Third, develop the site design to achieve the following:

(1)

Recognize and incorporate the priorities identified in § 270-18B as the basis for the proposed site layout, grading, construction, and permanent ground cover design;

(2)

Minimize earth disturbance (both surface and subsurface);

(3)

Maximize protection of or improvement to natural resources and special management areas;

(4)

Minimize the disturbance of natural site hydrology, in particular natural drainage features and patterns, discharge points and flow characteristics, natural infiltration patterns and characteristics, and natural channel and floodplain conveyance capacity;

(5)

Incorporate natural hydrologic features and functions identified in § 270-18B into the site design to protect and utilize those features and their hydrologic functions to reduce the need for or size of constructed BMPs;

(6)

Maximize infiltration and the use of natural site infiltration features, patterns and conditions, and evapotranspiration features;

(7)

Apply selective grading design methods to provide final grading patterns or preserve existing topography in order to evenly distribute runoff and minimize concentrated flows;

(8)

Minimize the cumulative area to be covered by impervious surfaces and:

(a)

Minimize the size of individual impervious surfaces;

(b)

Separate large impervious surfaces into smaller components;

(c)

Disconnect runoff from one impervious surface to another; and

(d)

Utilize porous materials in place of impervious wherever practicable;

(9)

Minimize the volume and peak discharge rates of stormwater generated;

(10)

Avoid or minimize stormwater runoff pollutant loads and receiving stream channel erosion;

(11)

Locate infiltration and other BMPs:

(a)

At or as near to the source of generation as possible; and

(b)

At depths that are as shallow as possible;

(12)

Prioritize the selection and design of BMPs as follows:

(a)

Nonstructural and vegetation BMPs; then

(b)

Structural (surface and subsurface) BMPs;

(13)

For flow volumes requiring conveyance from the source of generation to a BMP for management, give preference to open-channel conveyance techniques that provide infiltration and water quality benefits, and landscaped- based management in common open space areas, where practicable; and

(14)

Consider additional guidance for incorporating natural hydrology into the site and BMP designs, methods and techniques that support the objectives of § 270-18B and C. Appendix B[1] presents additional discussion of natural hydrology site design and sources of information for conservation design, low-impact development, and sustainable design.

[1]

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

The procedures set forth above shall be utilized to the maximum extent practicable for the overall site design and selection, location and design of features and BMPs to be used to comply with the requirements of §§ 270-19, 270-20, 270-21 and 270-22.

§ 270-19 Water quality and runoff volume requirements.

To control post construction stormwater impacts from regulated activities and meet state water quality requirements, BMPs shall be provided in the site design that replicate predevelopment stormwater infiltration and runoff conditions, such that post construction stormwater discharges do not degrade the physical, chemical, or biological characteristics of the receiving waters. The green infrastructure and low-impact development (LID) practices provided in the PA BMP Manual, as well as the guidance on green infrastructure, LID and conservation design (CD) provided in Appendix B,[1] shall be utilized for all regulated activities wherever possible. The applicant shall comply with the following water quality and runoff volume requirements for all regulated activities, including all new development and redevelopment activities:

The post construction total runoff volume shall not exceed the predevelopment total runoff volume for all storms equal to or less than the two-year, twenty-four-hour-duration precipitation (design storm). The water quality and runoff volume to be managed shall consist of any runoff volume generated by the proposed regulated activity over and above the predevelopment total runoff volume and shall be captured and permanently retained or infiltrated on the site. Permanent retention options may include, but are not limited to, reuse, evaporation, transpiration, and infiltration.

For modeling purposes, the predevelopment ground cover conditions shall be determined using the corresponding ground cover assumptions presented in § 270-23D of this chapter.

The design of the stormwater management facility outlet shall provide for protection from clogging and unwanted sedimentation.

BMPs that moderate the temperature of stormwater shall be used to protect the temperature of receiving waters. The applicant shall fulfill the requirements of the PADEP "Thermal Impact Analysis" for the "PAG-02 Stormwater Discharges Associated with Construction Activities, NOI for Coverage under General or Individual Permit" if they cannot meet the volume control requirements. The Applicant shall fulfill the requirements of the PADEP "Thermal Impact Analysis" for the "PAG-02 Stormwater Discharges Associated with Construction Activities, NOI for Coverage under General or Individual Permit if they cannot meet the volume control requirements."

Water quality improvement shall be achieved in conjunction with achieving the infiltration requirements of § 270-20. The infiltration volume required under § 270-20 may be included as a component of the water quality volume. If the calculated water quality and runoff volume is greater than the volume infiltrated, then the difference between the two volumes shall be managed for water quality and runoff volume control through other techniques or practices but shall not be discharged from the site.

Runoff from the disturbed area shall be treated for water quality prior to entering existing waterways or water bodies. If a stormwater management practice does not provide water quality treatment, then water quality BMPs shall be utilized to provide pretreatment prior to the runoff entering the stormwater management practice.

The Township may require additional water quality and runoff control measures for stormwater discharging to special management areas such as those listed in § 270-15P.

When the regulated activity contains or is divided by multiple drainage areas, the water quality and runoff volume shall be separately addressed for each drainage area.

Weighted averaging of runoff coefficients shall not be used for manual computations or input data for water quality and runoff volume calculations.

Areas located outside of the site (i.e., areas outside of the regulated activity) may be excluded from the calculation of the water quality and runoff volume requirements.

Water quality and volume control practices shall be selected and designed to meet the criteria of § 270-18C that apply to water quality and volume control.

Evapotranspiration may be quantified and credited towards meeting volume requirements according to the PADEP Post Construction Stormwater Management (PCSM) Spreadsheet and Instructions (December 2020) or the most recent guidance from PADEP.

[1]

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

§ 270-20 Infiltration requirements.

Providing for infiltration consistent with the natural hydrologic regime is required to compensate for the reduction in the recharge that occurs when the ground surface is disturbed or impervious surface is created or expanded. The applicant shall achieve the following infiltration requirements:

For regulated activities involving both new development and redevelopment, infiltration should be designed to accommodate the entire water quality and runoff volume required is § 270-19.

(1)

Infiltration BMPs should be consistent with the design and infiltration period guidelines included in the PA BMP Manual, other PADEP design guidance or as noted elsewhere in this chapter. If the runoff volume required by § 270-19 cannot be infiltrated, then alternative methods consistent with the PA BMP Manual (as amended) or other PADEP guidance, such as the managed release concept, may be used to manage this volume with approval from the Municipal Engineer.

For regulated activities involving both new development and redevelopment, the volume of a minimum of one inch of runoff from all regulated impervious surfaces shall be infiltrated.

If the requirements of § 270-20A or B cannot be physically accomplished, then the applicant shall be responsible for demonstrating with data or calculations to the satisfaction of the Township Engineer why this infiltration volume cannot be physically accomplished on the site (e.g., shallow depth to bedrock or limiting zone, open voids, steep slopes, etc.) and what alternative volume can be infiltrated; however in all cases at least the first 0.5 inch of runoff volume shall be infiltrated.

Only if a minimum infiltration of the first 0.5 inch of runoff volume cannot be physically accomplished on the site shall a waiver from § 270-20 be considered by the Township in accordance with § 270-11.

If site conditions preclude capture of runoff from portions of the impervious surfaces, the infiltration volume for the remaining area shall be increased an equivalent amount to offset the loss.

When a project contains or is divided by multiple watersheds, the infiltration volume shall be separately addressed for each watershed.

Existing impervious surfaces located in areas outside of the site (i.e., outside of the regulated activity) may be excluded from the calculation of the required infiltration volume.

A detailed soils evaluation of the site shall be conducted by a qualified licensed professional and at a minimum shall address soil permeability, depth to bedrock, and subgrade stability. The general process for designing the infiltration BMP shall be conducted by a qualified licensed professional and shall be consistent with the PA BMP Manual (as amended) (or other guidance acceptable to the Township Engineer) and in general shall:

(1)

Analyze hydrologic soil groups as well as natural and man-made features within the site to determine general areas of suitability for infiltration practices. In areas where development on fill material is under consideration, conduct geotechnical investigations of subgrade stability; infiltration may not be ruled out without conducting these tests.

(2)

Provide field tests such as double ring infiltrometer or other hydraulic conductivity tests (at the elevation of the proposed infiltration surface) to determine the appropriate hydraulic conductivity rate. Standard septic/sewage percolation tests are not acceptable for design purposes.

(3)

Design the infiltration facility for the required retention (infiltration) volume based on field-determined infiltration capacity (and apply safety factor as per applicable design guidelines) at the elevation of the proposed infiltration surface.

(4)

On-lot infiltration features are encouraged; however, it shall be demonstrated to the Township Engineer that the soils are conducive to infiltration on the identified lots.

Infiltration BMPs shall be selected based on suitability of soils and site conditions and shall be constructed on soils that have the following characteristics:

(1)

A minimum depth of 24 inches between the bottom of the BMP and the top of the limiting zone. Additional depth may be required in areas underlain by karst or carbonate geology (see § 270-20M).

(2)

An infiltration rate sufficient to accept the additional stormwater volume and drain completely as determined by field tests conducted by the applicant.

(3)

The infiltration facility shall completely drain the retention (infiltration) volume within three days (72 hours) from the end of the design storm.

All infiltration practices shall:

(1)

Be selected and designed to meet the criteria of § 270-18C that are applicable to infiltration;

(2)

Be set back at least 20 feet from all buildings and features with subgrade elements (e.g., basements, foundation walls, etc.) and 20 feet from all property lines or right-of-way boundaries unless otherwise approved by the Township;

(3)

For any infiltration practice that collects runoff from shared or multiple features and that is located within 20 feet of a building or feature with subgrade elements (e.g., basements, foundation walls, etc.), the bottom elevation shall be set below the elevation of the subgrade element.

Infiltration facilities shall, to the maximum extent practicable, be located to avoid introducing contaminants to groundwater:

(1)

When a hotspot is located in the area draining to a proposed infiltration facility, an evaluation of the potential of groundwater contamination from the proposed infiltration facility shall be performed, including a hydrogeologic investigation (if necessary) by a qualified licensed professional to determine what, if any, pretreatment or additional design considerations are needed to protect groundwater quality.

(2)

When located within a "wellhead protection area" of a public water supply well, infiltration practices shall be in conformance with the applicable approved source water protection assessment or source water protection plan.

(3)

The applicant shall provide appropriate safeguards against groundwater contamination for land uses that may cause groundwater contamination should there be a mishap or spill.

During site construction, all infiltration practice components shall be protected from compaction due to heavy equipment operation or storage of fill or construction material. Infiltration areas shall also be protected from sedimentation. Areas that are accidentally compacted or graded shall be remediated to restore soil composition and porosity. Adequate documentation to this effect shall be submitted to the Township Engineer for review. All areas designated for infiltration shall not receive runoff until the contributory drainage area has achieved final stabilization.

Consideration of infiltration BMPs for areas underlain by karst or carbonate geology is encouraged, but only where the design, supporting calculations, results of soils or other site investigations or other documentation are provided to the Township demonstrating that the potential or likelihood of subsidence or sinkholes is minimal. Evaluation of site conditions and infiltration design shall rely on guidance in the PA BMP Manual (as amended) or other guidance acceptable to the Township.

Groundwater quality of the carbonate aquifer shall be protected from infiltration of pollutants. Stormwater runoff from hotspots (i.e., sources of significant pollutant runoff) shall first be discharged through a water quality BMP(s) to remove pollutants prior to infiltration. Where soil characteristics are insufficient to provide removal of pollutants from sources other than hotspots, stormwater runoff shall first be discharged through a water quality BMP(s) to remove pollutants prior to infiltration.

Where sediment transport in the stormwater runoff is anticipated to reach the infiltration system, appropriate permanent measures to prevent or collect sediment shall be installed prior to discharge to the infiltration system.

Where roof drains are designed to discharge to infiltration practices, they shall have appropriate measures to prevent clogging by unwanted debris (for example, silt, leaves and vegetation). Such measures shall include but are not limited to leaf traps, gutter guards and cleanouts.

All infiltration practices shall have appropriate positive overflow controls.

No sand, salt or other particulate matter may be applied to a porous surface material for winter ice conditions.

The following procedures and materials shall be required during the construction of all subsurface facilities:

(1)

Excavation for the infiltration facility shall be performed with equipment that will not compact the bottom of the seepage bed/trench or like facility.

(2)

The bottom of the bed and/or trench shall be scarified prior to the placement of aggregate.

(3)

Only clean aggregate with documented porosity, free of fines, shall be allowed.

(4)

The tops, bottoms and sides of all seepage beds, trenches, or like facilities shall be covered with drainage fabric. Fabric shall be nonwoven fabric acceptable to the Township Engineer.

(5)

Stormwater shall be distributed throughout the entire seepage bed/trench or like facility and provisions for the collection of debris shall be provided in all facilities.

§ 270-21 Stream channel protection requirements.

For regulated activities involving new development with one or more acres of earth disturbance, the applicant shall comply with the following stream channel protection requirements to minimize stream channel erosion and associated water quality impacts to the receiving waters:

The peak flow rate of the post construction two-year, twenty-four-hour design storm shall be reduced to the predevelopment peak flow rate of the one-year, twenty-four-hour-duration precipitation, using the SCS Type II distribution.

To the maximum extent practicable, and unless otherwise approved by the Township Engineer, the post construction one-year, twenty-four-hour storm flow shall be detained for a minimum of 24 hours and a maximum not to exceed 72 hours from a point in time when the maximum volume of water from the one-year, twenty-four-hour storm is stored in a proposed BMP (i.e., when the maximum water surface elevation is achieved in the facility). Release of water can begin at the start of the storm (i.e., the invert of the orifice is at the invert of the proposed BMP).

For modeling purposes, the predevelopment ground cover conditions shall be determined using the corresponding ground cover assumptions presented in § 270-23D of this chapter.

The minimum orifice size in the outlet structure to the BMP shall be three inches in diameter unless otherwise approved by the Township Engineer, and a trash rack shall be installed to prevent clogging. For sites with small drainage areas contributing to the BMP that do not provide enough runoff volume to allow a twenty-four-hour attenuation with the three-inch orifice, the calculations shall be submitted showing this condition.

When the calculated orifice size is below three inches, gravel filters (or other methods) are recommended to discharge low-flow rates subject to the Township Engineer's satisfaction. When filters are utilized, maintenance provisions shall be provided to ensure filters meet the design function.

All proposed stormwater facilities shall make use of measures to extend the flow path and increase the travel time of flows in the facility.

When a regulated activity contains or is divided by multiple drainage areas, the peak flow rate control shall be separately addressed for each drainage area.

§ 270-22 Stormwater peak rate control requirements.

The applicant shall comply with the following peak flow rate control requirements for all regulated activities including those that involve new development and redevelopment.

Post-construction peak flow rates from any regulated activity shall not exceed the predevelopment peak flow rates as shown for each of the design storms specified in Table 22.1.

Table 22.1 Peak Rate Control Standards
The limits of each district (watershed) are illustrated on the Official Stormwater Management Release Rate Map, provided in Appendix D.[1]

For the Chester Creek District, the peak post-development runoff rates for the two-, five-, ten-, twenty-five-, fifty-, and 100-year design storms shall be no greater than 50% of the predevelopment peak discharges. For all other districts, the peak flow rate of the post-construction design storm shall be reduced to the peak flow rate of the corresponding predevelopment design storm as shown in the table below.

Post-Construction Design Storm Frequency (24-Hour Duration)	Predevelopment Design Storm
Post-Construction Design Storm Frequency (24-Hour Duration)	New Development Regulated Activities	Redevelopment Regulated Activities
2-year	1-year	2-year
5-year	5-year	5-year
10-year	10-year	10-year
25-year	25-year	25-year
50-year	50-year	50-year
100-year	100-year	100-year

[1]

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

For modeling purposes, the predevelopment ground cover conditions shall be determined using the corresponding ground cover assumptions presented in § 270-23D of this chapter.

For regulated activities involving only redevelopment, no peak flow rate controls are required when and only if the total regulated impervious surface area is at least 20% less than the total existing impervious surface area to be disturbed by the regulated activity. In all cases where this requirement is not met, the redevelopment regulated activity shall achieve the peak flow rate controls presented in Table 22.1, using the redevelopment ground cover assumptions presented in § 270-23D. This design criterion for redevelopment is only permitted with approval of Municipal Engineer. It shall result in no impact on downstream properties.

Only the area of the proposed regulated activity shall be subject to the peak flow rate control standards of this chapter. Undisturbed areas for which the discharge point has not changed are not subject to the peak flow rate control standards.

Areas located outside of the site (i.e., areas outside of the regulated activity) that drain through a proposed site are not subject to peak flow rate control requirements. Drainage facilities located on the site shall be designed to safely convey flows from outside of the site through the site.

When a regulated activity contains or is divided by multiple drainage areas, the peak flow rate controls shall be separately addressed for each drainage area.

The effect of structural and nonstructural stormwater management practices implemented as part of the overall site design may be taken into consideration when calculating total storage volume and peak flow rates.

§ 270-23 Calculation methodology.

Stormwater runoff from all regulated activity sites with a drainage area of greater than five acres shall be calculated using a generally accepted calculation technique(s) that is based on the NRCS Soil Cover Complex Method. Table 23.1 summarizes acceptable computation methods. The method selected for use shall be based on the individual limitations and suitability of each method for a particular site. The use of the Rational Method to estimate peak discharges for drainage areas greater than five acres shall be permitted only upon approval by the Township Engineer.

Table 23.1 Acceptable Computation Methodologies for SWM Site Plan
Method	Developed By	Applicability
TR-20 (or commercial computer package based on TR-20)	USDA NRCS	Applicable where use of full hydrology computer model is desirable or necessary
TR-55 (or commercial computer package based on TR-55)	USDA NRCS	Applicable for land development plans where limitations described in TR-55 are met
HEC-1/HEC-HMS	US Army Corps of Engineers	Applicable where use of a full hydrologic computer model is desirable or necessary
Rational Method (or commercial computer package based on rational method)	Emil Kuichling (1889)	For sites up to 5 acres, or as approved by the municipality
Other methods	Varies	Other computation methodologies approved by the municipality

All calculations using the Soil Cover Complex Method shall use the appropriate design rainfall depths for the various return period storms consistent with this chapter. Rainfall depths used shall be obtained from the latest version of the Precipitation-Frequency Atlas of the United States, National Oceanic and Atmospheric Administration (NOAA), National Weather Service, Hydrometeorological Design Studies Center, Silver Spring, Maryland (NOAA Atlas 14) values consistent with a partial duration series. When stormwater calculations are performed for routing procedures or infiltration, water quality and runoff volume functions, the duration of rainfall shall be 24 hours.

All calculations using the Rational Method shall use rainfall intensities consistent with appropriate times of concentration (duration) and storm events with rainfall intensities obtained from NOAA Atlas 14 partial duration series estimates, or the latest version of the PennDOT Drainage Manual (PDM Publication 584). 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.

The applicant shall utilize the following ground cover assumptions for all predevelopment water quality and runoff volume, infiltration volume and peak flow rate calculations:

(1)

For regulated activities involving new development, the following ground cover assumptions shall be used:

(a)

For areas that are woods (as defined in Article II of this chapter), predevelopment calculations shall assume ground cover of "woods in good condition."

(b)

For all other areas (including all impervious surfaces), predevelopment calculations shall assume ground cover of "meadow."

(2)

For regulated activities involving redevelopment, the following ground cover assumptions shall be used:

(a)

For areas that are woods (as defined in Article II of this chapter), predevelopment calculations shall assume ground cover of "woods in good condition."

(b)

For areas that are not woods or not impervious surfaces, predevelopment calculations shall assume ground cover of "meadow."

(c)

For areas that are impervious surfaces, predevelopment calculations shall assume at least 20% of the existing impervious surface area to be disturbed as "meadow" ground cover.

(3)

The applicant shall determine which stormwater standards apply to the proposed regulated activity as follows:

(a)

Stormwater standards for new development shall apply to all proposed regulated activities that involve only new development activities as defined in this chapter.

(b)

Stormwater standards for redevelopment shall apply to all proposed regulated activities that involve only redevelopment activities as defined in this chapter.

(c)

Stormwater standards for new development shall apply to regulated activities that involve a combination of both new development and redevelopment activities, as defined in this chapter.

The applicant shall utilize the following ground cover assumptions for all predevelopment water quality and runoff volume, infiltration volume and peak flow rate calculations:

(1)

For regulated activities involving new development, the following ground cover assumptions shall be used:

(a)

For areas that are woods (as defined in Article II of this chapter), predevelopment calculations shall assume ground cover of "woods in good condition."

(b)

For all other areas (including all impervious surfaces), predevelopment calculations shall assume ground cover of "meadow."

(2)

For regulated activities involving redevelopment, the following ground cover assumptions shall be used:

(a)

For areas that are woods (as defined in Article II of this chapter), predevelopment calculations shall assume ground cover of "woods in good condition."

(b)

For areas that are not woods or not impervious surfaces, predevelopment calculations shall assume ground cover of "meadow."

(c)

For areas that are impervious surfaces, predevelopment calculations shall assume at least 20% of the existing impervious surface area to be disturbed as "meadow" ground cover.

(3)

The applicant shall determine which stormwater standards apply to the proposed regulated activity as follows:

(a)

Stormwater standards for new development shall apply to all proposed regulated activities that involve only new development activities as defined in this chapter.

(b)

Stormwater standards for redevelopment shall apply to all proposed regulated activities that involve only redevelopment activities as defined in this chapter.

(c)

Stormwater standards for new development shall apply to regulated activities that involve a combination of both new development and redevelopment activities, as defined in this chapter.

Runoff coefficients and curve numbers shall be consistent with those provided in Appendix C, or as otherwise approved by the Township Engineer.

§ 270-24 Other requirements.

Any BMP intended to hold standing water for four days or longer shall be designed to incorporate biologic controls consistent with the West Nile Guidance found in PADEP document 363-0300-001"Design Criteria — Wetlands Replacement/Monitoring" (as amended), or contact the Penn State Cooperative Wetland Center or the Penn State Cooperative Extension Office for design information.

Any stormwater basin required or regulated by this chapter designed to store runoff and requiring a berm or earthen embankment shall be designed to provide an emergency spillway to safely convey flow up to and including the 100-year proposed conditions. The height of embankment shall provide a minimum one foot of freeboard above the maximum pool elevation computed when the facility functions for the 100-year proposed conditions inflow. Should any BMP require a dam safety permit under PA Chapter 105 regulations,[1] the facility shall be designed in accordance with and meet the regulations of PA Chapter 105 concerning dam safety. PA Chapter 105 may require the safe conveyance of storms larger than 100-year event.

[1]

Editor's Note: See 25 Pa. Code Chapter 105.

Any drainage conveyance facility and/or channel not governed by PA Chapter 105 regulations shall be designed to convey, without damage to the drainage facility or roadway, runoff from the twenty-five-year storm event. Larger storm events (fifty-year and 100-year storms) shall also be safely conveyed in the direction of natural flow without creating additional damage to any drainage facilities, nearby structures, or roadways.

Roadway crossings or structures located within designated floodplain areas shall be able to convey runoff from a 100-year design storm consistent with Federal Emergency Management Agency National Flood Insurance Program — Floodplain Management Requirements.

Any stormwater management facility located within a PennDOT right-of-way shall comply with PennDOT minimum design standards and permit submission and approval requirements.

Adequate erosion protection and energy dissipation shall be provided along all open channels and at all points of discharge. Design methods shall be consistent with the Federal Highway Administration Hydraulic Engineering Circular Number 11 (Publication No. FHWA-IP-89-016, as amended) and the PADEP Erosion and Sediment Pollution Control Program Manual (Publication No. 363-2134-008, as amended), or other design guidance acceptable to the Municipal Engineer.

§ 270-25 Design criteria for stormwater management facilities.

Any stormwater management facility (i.e., detention basin) designed to store runoff regulated by this chapter shall be designed to provide an emergency spillway to handle flow up to and including the 100-year post development conditions. The height of the embankment (top of berm) must be set so as to provide a minimum one foot of freeboard above the maximum pool elevation computed when the facility functions for the 100-year post development inflow. Should any stormwater management facility require a dam safety permit under PADEP Chapter 105, the facility shall be designed in accordance with Chapter 105[1] and meet the regulations of Chapter 105 concerning dam safety which may be required to pass storms larger than the 100-year event. Any inspections of this dam required by any regulatory agency shall be prepared by competent consultants and at the cost of the maintainer of the facility.

[1]

Editor's Note: See 25 Pa. Code Chapter 105.

Any facilities that constitute water obstructions (e.g., culverts, bridges, outfalls, or stream enclosures) and any work involving wetlands as directed in PADEP Chapter 105 regulations (as amended or replaced from time to time by PADEP) shall be designed in accordance with Chapter 105[2] and will require a permit from PADEP. Any other drainage conveyance facility that does not fall under Chapter 105 regulations shall be designed to convey, without damage to the drainage structure or roadway, runoff from a minimum twenty-five-year design storm. The Township may require design based on a larger storm event. Any facility that constitutes a dam as defined in PADEP Chapter 105 regulations may require a permit under dam safety regulations. Any facility located within a PennDOT right-of-way must meet PennDOT minimum design standards and permit submission requirements. If the primary drainage facilities do not have capacity for future flows, then a safe drainage path must be provided to convey up to the 100-year design storm without impacting structures.

[2]

Editor's Note: See 25 Pa. Code Chapter 105.

Storm sewers must be able to convey post development runoff from a minimum twenty-five-year design storm without surcharging inlets.

Adequate erosion protection shall be provided along all open channels and at all points of discharge.

The design of all stormwater management facilities shall incorporate sound engineering principles and practices. The Township shall reserve the right to reject any design that would result in the occurrence or continuation of an adverse hydrologic, hydraulic, or water quality condition within the watershed.

Stormwater drainage systems shall be provided in order to permit unimpeded flow along natural watercourses, except as modified by stormwater management facilities or open channels consistent with this chapter.

The existing points of concentrated drainage that discharge onto adjacent property shall not be altered without permission of the adjacent property owner(s) and shall be subject to applicable discharge criteria specified in this chapter.

Areas of existing diffused drainage discharge shall be subject to applicable discharge criteria in the general direction of existing discharge, whether proposed to be concentrated or maintained as diffused drainage areas, except as otherwise provided by this chapter. If diffused flow is proposed to be concentrated and discharged onto adjacent property, the applicant must document to the Township in accordance with § 270-25I that adequate downstream conveyance exists to safely transport the concentrated discharge, or the applicant must obtain drainage easements from affected downstream property owners and provide the facilities to safely convey the flow.

Downstream hydraulic capacity analysis. Any downstream capacity hydraulic analysis conducted in accordance with this chapter shall use the following criteria for determining adequacy for accepting increased peak flow rates:

(1)

Natural or man-made channels or swales must be able to convey the increased runoff associated with a two-year return period event within their banks at velocities consistent with protection of the channels from erosion. Acceptable velocities shall be based upon criteria included in the PADEP Erosion and Sediment Pollution Control Program Manual.

(2)

Natural or man-made channels or swales must be able to convey the increased twenty-five-year return period runoff without creating any hazard or adverse effects to adjacent or downstream persons or property.

(3)

Culverts, bridges, storm sewers, and all other facilities that must pass or convey flows from the tributary area must be designed in accordance with PADEP Chapter 105 regulations (if applicable) and, at a minimum, pass the increased twenty-five-year return period runoff.

Where a development site is traversed by watercourses, riparian buffers shall be provided conforming to the line of such watercourses. The width of the buffers shall be determined as set forth in § 270-15T, here above. Excavating, placing of fill, building structures, or making any alterations that may adversely affect the flow of stormwater within any portion of the riparian buffer shall be prohibited unless the proposed work is associated with a regulated wetlands mitigation program. The buffer must be defined and restricted by a restrictive covenant.

When it can be shown that, due to topographic conditions, natural drainage ways on the site cannot adequately provide for drainage, open channels may be constructed conforming substantially to the line and grade of such natural drainage ways. Work within natural drainage ways shall be subject to approval by PADEP through the joint permit application process, or, where deemed appropriate by PADEP, through the general permit process.

Any stormwater management facilities regulated by this chapter that would be located in wetlands or within the floodway (as determined by FEMA or 50 feet from top of bank) of waters of the commonwealth shall be subject to approval by PADEP through the joint permit application process, or, where deemed appropriate by PADEP, the general permit process. When there is a question as to whether wetlands may be involved, it is the responsibility of the applicant or his agent to show that the land in question cannot be classified as wetlands; otherwise, approval to work in the area must be obtained from PADEP.

Any stormwater management facilities regulated by this chapter located on or flowing into state highway rights-of-way or stormwater systems shall be subject to approval by PennDOT.

Minimization of impervious surfaces and infiltration of runoff through seepage beds, infiltration trenches, or other BMPs are required, where soil conditions permit, to reduce the size or eliminate the need for detention facilities.

In order to promote overland flow and infiltration/percolation of stormwater, roof drains must discharge into an accepted BMP providing infiltration and filtering of the stormwater.

§ 270-26 Floodplain methodology.

All existing and proposed 100-year floodplains shall be delineated on the drainage plan (see § 270-32). If the 100-year floodplain is not mapped by the Federal Emergency Management Agency as part of the National Flood Insurance Program, the horizontal and vertical limits of the floodplain shall be determined utilizing the standard step method (i.e., HEC-RAS or similar approved computer model). If the HEC-RAS model is used, the applicant shall submit a computer disc containing all input files for the calculations, in order to expedite the floodplain review. If the drainage area is less than 100 acres, the Manning's equation shall be used.

The methods below shall be used to compute the design flow(s) in the drainage course although other methods may be used with approval of the Township. A conservative average of two methods shall be used, and the design flow is subject to approval by the Township.

(1)

The graphical and tabular methods in TR-55. The graphical method may be used for streams whose drainage area at the point of interest is no larger than 2,000 acres, and the tabular hydrograph method may be used for drainage areas up to 20 square miles (12,800 acres).

(2)

The Rational Method may be used for streams whose drainage area at the point of interest is no larger than 320 acres.

(3)

The method in Water Resources Bulletin Number 13, Floods in Pennsylvania, issued by the Pennsylvania Department of Environmental Resources (now PADEP), may be used for streams whose drainage area at the point of interest is larger than two square miles.

(4)

The Procedure PSU-IV for Estimating Design Flood Peaks on Ungaged Pennsylvania Watersheds.

(5)

The Penn State Runoff Model (PSRM).

All development activity within a special flood hazard area designated by the Federal Emergency Management Agency shall comply with Article XIII of the West Whiteland Township Zoning Ordinance[1] and this subsection. All development shall be designed to maintain the flood-carrying capacity of the floodway such that the base flood elevations are not increased, either upstream or downstream. The natural conveyance characteristics of the site and the receiving floodplain shall be incorporated into the stormwater management practices proposed for the site.

[1]

Editor's Note: See Ch. 325, Zoning.

§ 270-27 Additional stormwater management system design criteria for selected BMPs.

Infiltration devices shall be selected based on suitability of soils and site conditions. Suitability of soils shall be determined by soil infiltration testing, with suitability defined as having minimum percolation rates of 0.5 inch per hour at the elevation of the bottom of the facility; lower rates may be acceptable if approved by the Township.

(1)

Soil infiltration testing shall be performed for all proposed infiltration areas; soil testing shall include evaluation of appropriate soil horizons with deep pits and percolation measurements, making sure to assess percolation rates at the proposed infiltration device bed bottom. Soil testing, including the methodology, frequency, and locations of the tests, shall be based on the type of proposed BMP and on the known geology of the area and be reviewed and approved by the Township as advised by the Township Engineer. The design soil infiltration rate shall be the average infiltration rate measured at each proposed area. Soil infiltration tests shall be performed within the soil horizon in which the bottom elevation of the proposed infiltration areas lie. The location and number of test pits and percolation holes shall be determined based on the type(s) of stormwater management practices being designed in consultation with and guidance from the Township Engineer.

(2)

The Township Engineer shall be given a minimum of three working days' notice of when the testing will be completed.

The lowest elevation of the infiltration area shall be at least two feet above the seasonal high-water table ("SHWT") and bedrock, except in areas of carbonate geology, in which case the distance shall be four feet.

All roof drains that discharge to infiltration systems shall have appropriate measures to prevent clogging by vegetation and to prevent sinkhole formation.

All infiltration systems shall have appropriate positive overflow controls within one foot of the finished surface or grade.

All infiltration systems shall have a setback of 15 feet from all residential structures and property lines. Negative impact to below-ground structures shall be prevented.

All infiltration systems shall be designed to infiltrate the stored volume within 48 hours or whatever longer period may be specified by the Township.

All surface inflows shall be designed to minimize the discharge of sediment into the infiltration system in order to prevent sediment accumulation, which reduces stormwater storage capacity and ultimately clogs the infiltration mechanism.

Special provisions are required when using infiltration BMPs in carbonate areas in order to avoid groundwater contamination and the formation of solution channels and sinkholes.

(1)

Because the potential for these problems to develop increases as soil thickness decreases, the soil mantle should be determined to be a minimum of four feet in thickness, in order both to remove pollutants and to disperse uniformly groundwater movement, which is important to avoid the formation of solution channels.

(2)

In carbonate areas, BMPs that disperse stormwater over the largest feasible area shall be used (e.g., subtle berms and level spreaders) so as not to modify the natural hydrologic regime significantly. Use of infiltration BMPs that result in significant increases (more than 100%) in the predevelopment rate of infiltration per unit area shall be avoided in carbonate areas, unless a detailed geologic evaluation (see below) demonstrates that the potential for sinkhole formation is minimal.

(3)

Stormwater runoff from significant pollutant-producing sources ("hotspots," as defined in § 270-15N) shall be filtered and/or pretreated using a water quality BMP before being discharged in carbonate areas.

(4)

In these cases, the Township may require that a detailed geologic evaluation of the project site be performed to determine the suitability for recharge, including both the potential for groundwater contamination and potential for sinkhole formation. The evaluation shall be performed by a registered professional geologist licensed in the Commonwealth of Pennsylvania and/or any other Township-approved professional, and shall, at a minimum, address soil permeability, depth to bedrock, susceptibility to sinkhole formation, and subgrade stability.

The following procedures and materials shall be required during the construction of all subsurface facilities and shall be noted on the plans:

(1)

The area of the subsurface facilities shall be protected by orange safety fence prior to the start of construction; equipment and traffic shall be prohibited from traveling over the proposed location(s) of the facilities.

(2)

Excavation shall be performed with equipment that will not compact the bottom of the bed. Construction equipment shall not be allowed on excavated areas of the seepage beds at any time during its construction.

(3)

The bottom of the bed shall be scarified immediately prior to the placement of the geotextile fabric on the bottom of the bed.

(4)

The bed shall be protected during construction. Sediment shall not be allowed to be washed back into the bed both during the time when the bottom of the bed is open and once the stone is in place.

(5)

Only uniformly graded, clean aggregate, free of fines, slate, shale, clay, silt, and vegetative material shall be used. The porosity of the stone must meet the plan specification, and the supplier of the stone shall provide certification of such for the stone delivered to the site. The design engineer shall verify that the porosity meets or exceeds the requirements of the design and submit such verification to the Township.

(6)

Compaction of the infiltration bed is prohibited. Excavation of the final two feet of the bed shall be completed from outside the bed or by other means approved by the Township to prevent compaction of the bed floor.

(7)

Construction equipment no heavier than light-tracked vehicles shall be allowed on the top of the stone only after it has been placed to its full depth and solely for the purpose of completion of the placement of the stone and backfill.

(8)

Drainage filter fabric shall be placed in accordance with manufacturer's directions, including pipe penetrations, and shall overlap a minimum of 18 inches.

(9)

During site construction, all facility components shall be protected from sedimentation using storm inlet protection in conformance with the PADEP Chapter 102 regulations, as amended, and the Erosion and Sediment Pollution Control Manual, as amended. Inlet protection shall remain until the contributory drainage area has achieved full stabilization. If sediment enters the system, the contractor shall be required to clean out the sediment to the satisfaction of the Township; this may require the reconstruction of the infiltration facility in whole or in part.

(10)

The geotextile fabric around the infiltration bed shall be PennDOT nonwoven Class 1 and shall completely enclose the stone (all sides, top and bottom). Other materials may be used if approved by the Township; documentation supporting the use of such other materials shall be submitted to the Township for review.

(11)

Upon completion, the applicant shall be responsible for post-construction testing of the facilities sufficient to demonstrate that such facilities function as intended, such testing shall be coordinated with the Township Engineer and shall be included in any developer agreement required for a land development.

BMP design.

(1)

The applicant shall submit designs for water quality facilities to the Township for review and approval. Such designs shall achieve the water quality objectives through a combination of BMPs. The use of multiple, small, unconnected BMPs on a site, rather than one large stormwater management facility, shall be required.

(2)

All BMPs, including structural and nonstructural BMPs, shall be designed to assure that they function in accordance with the standards set forth in the latest version of the Pennsylvania Stormwater BMP Manual. The specifications and construction of BMPs shall be in accordance with the 2000 Maryland Stormwater Design Manual.

(3)

In selecting the appropriate BMPs or combinations thereof, the applicant shall consider the following:

(a)

A total contributing area.

(b)

Permeability and infiltration rate of the site soils.

(c)

Slope and depth to bedrock.

(d)

Seasonal high-water table.

(e)

Proximity to building foundations and wellheads.

(f)

Erodibility of soils.

(g)

Land availability and configuration of the topography.

(h)

Consistency with approved watershed and stormwater management plans or regulations.

(i)

Impact on neighboring and downstream properties and facilities.

(4)

The following additional factors shall be considered when evaluating the suitability of BMPs used to control water quality at a given development site: Peak discharge and required volume control.

(a)

Peak discharge and required volume control.

(b)

Stream bank erosion.

(c)

Efficiency of the BMPs to mitigate potential water quality problems.

(d)

The volume of runoff that will be effectively treated.

(e)

The nature of the pollutant being removed.

(f)

Maintenance requirements.

(g)

Creation or protection of aquatic and wildlife habitat.

(h)

Recreational value.

(i)

Enhancement of aesthetic and property value.

(j)

Impact on neighboring and downstream properties and facilities.

§ 270-28 Need for basins, storm sewers, culverts, bridges and other structural installations.

Basins, storm sewers, culverts, bridges and other structural installations shall be provided to accommodate stormwater safely where natural nonstructural practices are not feasible and where stormwater flows otherwise would have adverse impact on the environment and the general welfare of the Township and its citizens in order to:

Maintain natural hydrologic regimes of streams and watercourses. Such flows may be redirected as required, subject to the approval of PADEP.

Promote drainage of all low points along the line of streets. Overflow swales shall be designed to convey the full 100-year storm flows away from all street low points. These swales shall be located to prevent flooding of the downslope lots.

Intercept stormwater runoff along streets at intervals reasonably related to the extent and grade of the area drained, and to prevent substantial flow of water across intersections or flooding of intersections during the design storm stipulated elsewhere in this section.

Insure adequate and unimpeded flow of stormwater under driveways in, near, or across natural watercourses or drainage swales. Suitable pipes or other waterway openings shall be provided as necessary.

§ 270-29 Design of basins (detention and retention), storm sewers, culverts, bridges and other structural installations.

All detention and retention basins shall meet the requirements of this section.

(1)

Basins shall be installed prior to any earthmoving or land disturbances that they will serve. The phasing of their construction shall be noted in the erosion and sediment pollution control ("E&S") plan and accompanying narrative. Permanent vegetation shall be established prior to denuding any other land, unless the basin functions as an E&S device.

(2)

Energy dissipaters and/or level spreaders shall be installed at points approved by the Township where pipes or drainageways discharge from basins. Multiple outlet structures and multiple outlet piping from the basin may be required by the Township to reduce the impact of point discharges.

(3)

The following slope restrictions shall apply to basins.

(a)

Exterior slopes of compacted soil shall not exceed one foot vertical for three feet horizontal, and may be further reduced if the soil has unstable characteristics.

(b)

Interior slopes of the basin shall not exceed one foot vertical in three feet horizontal.

(c)

Concrete, stone, and brick walls are prohibited as elements of a basin.

(4)

Basins shall also be designed to satisfy the following requirements:

(a)

The minimum top of berm width shall be eight feet.

(b)

Outlet pipes shall have a minimum diameter of 12 inches. For pipe lengths exceeding 100 feet, the minimum diameter shall be 15 inches.

(c)

Properly spaced anti-seep collars shall be installed on all basin outlet pipes. Design calculations shall be provided, which shall demonstrate the adequacy of the design.

(d)

All basins shall be constructed with a compacted relatively impervious (Unified Soil Classification CL-ML or CL) key trench and core. The key trench shall extend at least two feet into undisturbed subsoil (below topsoil layer). The minimum bottom width of the trench shall be six feet and the minimum top width of the core shall be four feet. The side slopes of the compacted core and trench shall not exceed one horizontal to one vertical, and the top elevation of the core shall be set at or above the twenty-five-year design water elevation. The Township may require an impermeable liner to be installed up to the 100-year design water surface elevation.

(5)

Basin outlet structures and emergency spillways.

(a)

Outlet structures designed to control peak discharge flows and to distribute the flows by pipes to discharge areas shall be constructed primarily of concrete or masonry material and shall have childproof, nonclogging trash racks over all design openings, except those openings designed to carry perennial stream flows. The preferred trash rack material shall be epoxy-coated, or stainless steel; other materials may be permitted if approved by the Township.

(b)

Six inches of freeboard shall be provided between the crest of the primary outlet structure and the invert of the emergency spillway.

(c)

Emergency spillways shall be constructed in undisturbed earth wherever possible. When constructed in fill, sod, precast concrete paving blocks, concrete, or permanent erosion control matting shall be used. Design calculations shall be submitted demonstrating that the specified material can withstand velocities based on the 100-year design storm event. When using sod, it shall be applied along the inside slope above the twenty-five-year water surface elevation, along the face and sides of the spillway, and down the outside slope to the existing grade. Emergency spillways shall be designed to convey safely the 100-year basin inflow hydrograph through the basin assuming the principal outlet is completely blocked and the basin water surface elevation is equal to the spillway invert elevation.

(d)

Emergency spillways shall be designed with a minimum of one foot of freeboard and shall be designed to convey the 100-year storm event considering that the stormwater management facility is full up to the spillway invert elevation.

(e)

All principal outlet structures shall be built using reinforced concrete with watertight construction joints.

(f)

Facilities shall be both functional and harmonious in design with the surrounding environment. The use of architecturally treated concrete, stucco, or stone facade treatment shall be utilized for enhancing the outlet structure.

(6)

An earthen berm between inlet and outlet areas shall be required when the distance is deemed by the Township to be insufficient for sediment trappings.

(7)

Permanent grasses or stabilization measures shall be established on the sides of all earthen basins by hydroseeding within five days of initial construction (or conversion from sediment basin or sediment trap). The Township may require jute or erosion control matting to be installed inside the basin or on the basin embankment.

(8)

Stormwater runoff shall discharge to a suitable natural drainage course (except where prohibited by the riparian buffer area regulations of § 270-15T, hereinabove) or storm sewer system. Where not possible or not permitted, level spreading devices or other suitable facilities (i.e., swale) shall be designed with sufficient capacity to convey the 100-year storm event without creating any safety, flooding, or property hazard. Securing of necessary drainage easements for this purpose shall be the sole responsibility of the developer.

(9)

The developer shall test soil samples from the site to determine if the soils are suitable for berm embankment construction. If in the opinion of the Township the soils are found to be unsuitable, the developer shall import suitable soils for constructing the basin berm.

(10)

Basin inlet and outlet structures shall be located at maximum distances from one another.

(11)

In residential areas, ponds shall be equipped with management practices that reduce the potential for unauthorized entry and use of the pond by the general public. Preference shall be given to split rail fences equipped with mesh wire or other such practices that are both functional and attractive. A securable gate shall be provided to allow for periodic access by maintenance equipment. Such barriers shall have a minimum height of 42 inches.

(12)

The minimum distance between a proposed basin discharge point (including the energy dissipater) and a downstream property boundary shall in no case be less than 20 feet. Where there is discharge onto or through adjacent properties prior to release to a stream, designers shall demonstrate how downstream properties are to be protected. The Township may require that the setback distance be increased based upon factors such as topography, soil conditions, the size of structures, the location of structures, and discharge rates. A drainage easement shall also be required.

Detention basins shall also meet the following requirements. The minimum bottom slope shall be 2% unless the basin is designed to allow for infiltration or with an underdrain system. Slight variation in elevation (six to 12 inches) is encouraged to promote plant diversity.

Retention basins shall also meet the following requirements:

(1)

The retention basin depth shall average three feet to six feet with no area shallower than three feet.

(2)

An aquatic bench or shelf at least 10 feet wide and with a gentle slope not exceeding 10H:IV shall be provided along the entire perimeter of the retention basin.

(3)

Any side slopes below the permanent water surface level shall not exceed 5H:IV.

Swale design.

(1)

Grass swales not specifically designed as BMP devices shall have a minimum bottom slope of 2% and their depth shall be deep enough to convey the twenty-five-year storm event with a minimum of one foot of freeboard. Swale linings shall be designed based on the ten-year velocity. Swales shall have sufficient capacity to convey the 100-year storm discharge without creating any safety or property hazard.

(2)

Swales, when located outside of the Township right-of-way, shall be located within an easement of sufficient width to allow access for maintenance and to convey all storms up to and including the 100-year storm; in no case shall such a right-of-way be less than 20 feet wide. A note on the plan shall indicate that the easement allows the Township the right, though not the responsibility, to perform needed maintenance and/or repairs and to invoice the property owner for such work. The note shall further state that, in the event that the property owner fails to pay such invoice, the Township shall have the right to lien the subject property.

(3)

Cross country swales shall be designed as bio-swales and include check dams to remove sediment.

Storm sewer design.

(1)

Where storm sewer pipe and inlets are required, they shall be placed immediately in front of the curb within the right-of-way. Any storm sewer pipe which outlets on private residential property shall only discharge to the rear yard and must be a minimum of 50 feet from habitable structures.

(2)

When storm sewers are placed outside of the right-of-way of a street, they shall be placed within an easement not less than 20 feet unless otherwise approved by the Township. Larger easements may be required if site conditions dictate additional area is required as determined by the Township. Open ditches shall be avoided. Diversion terraces and swales can be considered if conditions are warranted (soil erodibility, slope and liner type) and approved by the Township Engineer and/or Conservation District, as applicable.

(3)

Storm sewers shall have a minimum diameter of 15 inches and only reinforced concrete or smooth lined corrugated polyethylene pipe shall be used. The minimum grade of the pipe shall be 1/2%. Smooth bore corrugated polyethylene pipe up to a maximum diameter of 48 inches may be used. Smaller diameter storm sewers may be permitted for BMPs in accordance with DEP guidelines if approved by the Township Engineer.

(4)

All storm sewer utilities beneath a paved surface shall be bedded and backfilled with PennDOT No. 2A stone. This backfill shall be placed in six-inch lifts and solidly compacted to the satisfaction of the Township.

(5)

In carbonate areas, watertight pipe connections are required and appropriate specifications shall be indicated on the plans.

(6)

Headwalls, endwalls, or end sections shall be required on all open pipes, shall be of concrete construction, and shall be set on a minimum of 12 inches of AASHTO No. 57 (PennDOT 2B) coarse aggregate.

(7)

All storm sewers shall be constructed per PennDOT specifications as outlined in Publication 408 Design Manual, Part 2, Highway Design and Standards for Roadway Construction, RC-Series unless otherwise dictated by the Township Code.

(8)

Any changes in alignment shall be straight sections connected by inlets or manholes.

(9)

When there is a change in pipe size through an inlet, the top inside elevation of the outlet pipe shall be at or below the top inside elevations of all incoming pipes.

(10)

Storm sewer sizes shall be determined based upon the following design storm frequencies:

(a)

Twenty-five years, unless otherwise specified by the Township.

(b)

The design of storm sewer systems within the drainage area of detention or retention facilities must be analyzed for adequacy during the 100-year storm, including the effects of the control facility tailwater. This may require a hydraulic grade line analysis. When approved by the Township, overflow swales may be provided at low points in streets to convey safely the full 100-year peak flow to the control facility, in lieu of providing the full capacity in the storm sewer.

(11)

Storm sewer design shall be based upon PennDOT design methods. Inlet efficiency and bypass flow shall be determined for all inlets, and the gutter flow spread shall not exceed 1/2 the travel lane width or, where on-street parking is permitted, a maximum of eight feet. The Township may require that a hydraulic grade line analysis be performed on storm sewer systems.

(12)

Culverts shall be evaluated for inlet and outlet control restrictions.

(13)

Rainfall intensities shall be obtained from the latest version of the Precipitation-Frequency Atlas of the United States, National Oceanic and Atmospheric Administration (NOAA), National Weather Service, Hydrometeorological Design Studies Center, Silver Spring, Maryland (NOAA Atlas 14).

(14)

Manholes and/or inlets shall not be more than 300 feet apart on pipe sizes up to 24 inches and not more than 400 feet apart on larger sizes.

(15)

Inlets, manholes, covers, and frames shall conform to PennDOT specifications. At street intersections, inlets shall be placed along the tangent and not in the curved portion of the curbing.

(16)

When precast concrete inlets or manholes are used within a street, a minimum of two courses of brick masonry or a grade ring shall be placed to bring the grate or cover to proper elevation. If brick is used, every third vertical joint shall be left open; if grade ring(s) are used, the horizontal joint(s) shall be left open to facilitate drainage of the base course. Geotextile fabric shall be wrapped around the outside of the brick masonry or grade ring(s) to prevent fines from washing into the structure.

Roof drains.

(1)

Stormwater roof drains shall not discharge water directly over a sidewalk or into any sanitary sewer line.

(2)

Except for single-family homes, proposed roof drains and collector locations shall be shown on the storm drainage plans. Roof drains and collectors shall meet all appropriate Township Code requirements.

Design of bridges and culverts.

(1)

Bridges and culverts shall have ample waterway opening to carry the design flows, based on a minimum storm frequency of 100 years. One-hundred-year water depths shall not exceed six inches above the roadway center line elevation. Bridge and culvert construction shall be in accordance with PennDOT specifications and shall also meet PADEP requirements. The appropriate permits and approvals must be acquired by the applicant prior to final plan approval.

(2)

Culverts shall be provided with wing walls and constructed for the full width of the right-of-way. If the Township determines that the character of the road is expected to change in the future, the cartway of the bridge shall be made to anticipate this condition. On each side of the bridge cartway, the bridge railing must be set back from the edge of the final cartway for the accommodation of sidewalks, and/or bicycle trails.

§ 270-30 Other site development requirements.

Grading shall not redirect or concentrate surface water onto an adjacent property.

Lots shall be laid out and graded to provide positive drainage away from buildings.

Procedures for protecting soils or geologic structures with water supply potential from contamination by surface water or other disruption by construction activity shall be established in consultation with the Township. Such areas shall include, at minimum, those underlain by carbonate formations. The Township shall require pollution control facilities to be provided on existing or proposed stormwater management systems within or adjacent to the project site.

Provisions for protecting existing wells and other water supplies shall be established.

Graded slopes shall not be steeper than three horizontal units to one vertical unit.

Any approvals required by the steep slope provisions in Article XIV of the West Whiteland Township Zoning Ordinance[1] shall be secured prior to earthmoving or stripping of vegetation.

[1]

Editor's Note: See Ch. 325, Zoning.

A minimum of four inches of topsoil shall be provided on all disturbed areas prior to final seeding and mulching.

Mature healthy trees of at least six inches diameter at breast height along with other significant existing vegetation within the limits of earth disturbance shall be located in the field and on the storm drainage plan and shall be retained and protected. Such trees shall not be removed except as provided on the approved subdivision or land development plan. The filling of soil over the roots of trees to be preserved is prohibited. Roots are presumed to extend out from the tree as far as the tree's branches extend outward.

Landscaping requirements.

(1)

Stormwater management facilities shall be landscaped in accordance with the following standards. Landscape plans shall be prepared by a professional landscape architect licensed in the Commonwealth of Pennsylvania.

(2)

Landscaping shall be required in and around all constructed stormwater management facilities having a surface area of 1,000 square feet or more for the purposes of:

(a)

Assisting in the management of stormwater;

(b)

Stabilizing the soil within such facilities to minimize and control erosion;

(c)

Enhancing the visual appearance of such facilities; and

(d)

Mitigating maintenance problems commonly associated with such facilities.

(3)

A planting plan and planting schedule shall be submitted in accordance with the following:

(a)

For wet meadows, including floors of stormwater management basins:

[1]

Wet meadows and the floors of stormwater management basins shall be planted with native wildflowers and native warm season grasses, the intent being to create a mixed meadow of such plantings, where appropriate. Selection of plantings shall be based on whether the area in question is usually well drained or permanently wet and whether the area will be used for recreation purposes. No woody plants shall be planted within the saturated zone (phreatic line) of a stormwater management basin or on a berm constructed for impounded water.

[2]

Seeding by drills, corrugated rollers, cyclone or drop seeders, or hand seeding is preferred for these areas; however, hydroseeding followed by hydromulching may be used on wet ground and steep slopes.

[3]

Fertilizers, as a nutrient supplement, shall not be used unless it is documented that soil conditions warrant such use and the nutrient applied does not exceed plant uptake. Soil for planting of wildflowers shall contain not less than 3% nor more than 10% organic matter, as determined by an agricultural chemist, with certification of the test before planting.

[4]

Seeding shall take place between April 1 and May 15 or between September 1 and October 15. Planting areas shall be soaked to maintain a consistent level of moisture for at least four to six weeks after planting.

[5]

Once established, only a single annual mowing when plants are dormant is required to maintain the landscaping described hereinabove. Maintenance practices shall include eradication of invasive species as identified by the Pennsylvania Department of Conservation and Natural Resources ("DCNR").

(b)

Wet edges that remain wet all or most of the year shall be planted with wildflowers, grasses, shrubs, or an appropriate combination of these elements. Plants to be located on rims or banks, which remain dry most of the year, shall be planted with species tolerant of dry soil conditions.

(c)

Wooded areas.

[1]

Where stormwater management facilities adjoin wooded areas, native trees and shrubs shall be selected and planted outside the facility so as to blend with existing surroundings.

[2]

Plantings in such areas shall be of sufficient density to eliminate the need for mowing.

[3]

Clusters of trees and shrubs should be planted around stormwater management facilities but well away from outfalls and constructed berms to provide for wildlife habitat, wind control, buffering, and visual screening.

[4]

Vegetation shall be planted during appropriate times of the year, predominantly between late March and mid-May or from early October until evidence of ground freezing, depending upon the species selected. Most deciduous trees and shrubs may be planted in either spring or fall; evergreens are best planted in late summer or early fall.

[5]

Maintenance practices shall include eradication of invasive species as identified by the DCNR.

(d)

Slopes.

[1]

Where slopes are gentle 10H:1V or flatter, a mixture of native meadow grasses and wildflowers (for wet meadows) shall be planted.

[2]

On steep slopes (as defined by the West Whiteland Township Zoning Ordinance), native dense spreading shrubs (tolerant of dry soils) shall be planted. Heavy mat mulch shall be used during the period of establishment.

[3]

No woody plant materials or trees shall be located on a berm acting as an impoundment structure for a stormwater management facility. Trees shall be located downstream of an impoundment berm a sufficient distance from the toe of the constructed slope to assure that the toe of the slope is outside the dripline of the species planted at maturity, but in no case less than 15 feet.

[4]

Maintenance practices shall include eradication of invasive species as identified by the DCNR.

(e)

In cases where stormwater management facilities are to be located in proximity to wetlands or waterways, the applicant shall consider such conditions and the planting plan and schedule and shall reflect consideration of existing flora.

(f)

Stormwater management facilities shall be screened in a manner that complements the existing landscape and provides sufficient access for maintenance.