Township of Easttown, PA Stormwater Management Standards

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

The applicant has received a letter of adequacy or approval for the erosion and sediment control plan review 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.

[Amended 10-17-2022 by Ord. No. 452-22]

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 § 388-16 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 or more acre 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.

[Amended 12-15-2014 by Ord. No. 424-14; 10-17-2022 by Ord. No. 452-22]

For regulated activities with one acre or less proposed earth disturbance, the applicant shall provide written notification to the affected landowner(s) describing the proposed regulated activity and proposed discharge(s), unless otherwise required by the Township.

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 §§ 388-17, 388-18, 388-19 and 388-20). 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 two-, five- and 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 Engineer. In no case shall resulting peak rate be greater than the predevelopment peak rate for the equivalent design storm.

[Amended 12-15-2014 by Ord. No. 424-14]

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 Pennsylvania Code and the Clean Streams Law.[1]

[Amended 12-15-2014 by Ord. No. 424-14]

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,[2] and the Storm Water Management Act.[3]

[2]

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

[3]

Editor's Note: See 32 P.S. § 680.1 et seq.

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).

[Amended 12-15-2014 by Ord. No. 424-14]

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.

Hotspots 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:

[Amended 12-15-2014 by Ord. No. 424-14]

Vehicle salvage yards and recycling facilities;

Vehicle fueling stations;

Vehicle service and maintenance facilities;

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.

[Amended 12-15-2014 by Ord. No. 424-14]

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

[Amended 12-15-2014 by Ord. No. 424-14]

Water bodies listed as impaired by PADEP.

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

Areas of known existing flooding problems.

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 Engineer, for testing and design standards for BMPs, and where there is a conflict with the provisions of this chapter, the most restrictive applies. If methods other than green infrastructure, LID, and CD methods are proposed to achieve the volume and rate controls required under this chapter, the SWM site plan must include a detailed justification demonstrating that the use of LID, green infrastructure, and CD are not practicable.

[Amended 12-15-2014 by Ord. No. 424-14; 10-17-2022 by Ord. No. 452-22]

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 Engineer.

[Amended 12-15-2014 by Ord. No. 424-14; 10-17-2022 by Ord. No. 452-22]

All regulated activities located within an identified floodplain area shall comply with Chapter 220, Floodplains, of this Code. The natural conveyance characteristics of the site and the receiving floodplain shall be incorporated into the stormwater management practices proposed for the site.

[Amended 5-15-2017 by Ord. No. 428-17]

Riparian buffer zones.

[Amended 12-15-2014 by Ord. No. 424-14; 10-17-2022 by Ord. No. 452-22]

All regulated activities located within riparian buffer zones shall comply with Article V of Chapter 274, Natural Resources Protection, of this Code.

For regulated activities within the Darby Creek, Trout Creek, and/or Crum Creek Watersheds (refer to Appendix F), the riparian buffer shall be planted with native vegetation and maintained in a vegetated state (refer to Appendix B, Native Plant List, contained in the PA BMP Manual). If an existing buffer is legally prescribed (i.e., deed, covenant, easement, etc.) and it exceeds the requirements of this chapter, the existing buffer shall be maintained.

§ 388-14 Permit requirements by other governmental entities.

[Amended 12-15-2014 by Ord. No. 424-14]

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 25 Pa. Code Chapter 102, or erosion and sediment control requirements of the Township.

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

§ 388-15 Erosion and sediment control.

No regulated activity within the Township shall commence until:

[Amended 12-15-2014 by Ord. No. 424-14]

The Township receives documentation that the applicant has received:

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, for projects where the area of disturbance exceeds one acre, where pond dredging is involved, or when the disturbance is associated with activities described under Title 25, Chapter 105 of the Pennsylvania Code permits;

[Amended 10-17-2022 by Ord. No. 452-22]

A PADEP NPDES permit for stormwater discharges associated with construction activities as required under 25 Pa. Code Chapter 92a, if applicable;

[Amended 6-2-2014 by Ord. No. 422-14; 10-17-2022 by Ord. No. 452-22]

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

An erosion and sediment control plan has been approved by the Township, in the case of projects with less than one acre of earth disturbance. The erosion and sediment control plan shall be included with the stormwater management site plan. Measures to minimize soil erosion and sedimentation shall meet the standards and specifications contained in the Pennsylvania Department of Environmental Protection "Soil Erosion and Sediment Pollution Control Manual," as amended, and the Pennsylvania Clean Streams Law, Chapter 102, Erosion and Sedimentation Control Rules and Regulations, as amended, and the specifications contained therein. The Township Engineer shall ensure compliance with the appropriate standards and specifications.

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 § 388-18M.

§ 388-16 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. The applicant shall demonstrate in its SWM site plan (as required in § 388-25C) 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 § 388-25B(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:

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

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

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

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

Third, develop the site design to achieve the following:

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

Minimize earth disturbance (both surface and subsurface);

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

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 Subsection B 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:

Minimize the size of individual impervious surfaces;

Separate large impervious surfaces into smaller components;

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:

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

At depths that are as shallow as possible;

(12)

Prioritize the selection and design of BMPs as follows:

Nonstructural and vegetation BMPs; then

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 Subsections B and C. Appendix B presents additional discussion of "conservation design" and "low-impact development."[1]

[Amended 10-17-2022 by Ord. No. 452-22]

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 §§ 388-17, 388-18, 388-19 and 388-20.

§ 388-17 Water quality and runoff volume requirements.

[Amended 12-15-2014 by Ord. No. 424-14; 10-17-2022 by Ord. No. 452-22]

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 amended, as well as the guidance on green infrastructure, LID and conservation design (CD) provided in Appendix B, 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 § 388-21D 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.

Water quality improvement shall be achieved in conjunction with achieving the infiltration requirements of § 388-18. The infiltration volume required under § 388-18 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 § 388-13P.

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 § 388-16C 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.

§ 388-18 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:

Wherever possible, infiltration should be designed to accommodate the entire water quality and runoff volume required in § 388-17. Infiltration BMPs should be consistent with the design and infiltration period guidelines included in the PA BMP Manual or other PA DEP design guidance. If the runoff volume required by Section 305 cannot be infiltrated, then alternative methods consistent with the PA BMP Manual (as amended) or other PA DEP guidance, such as the managed release concept, may be used to manage this volume with approval from the municipal engineer.

[Added 10-17-2022 by Ord. No. 452-22]

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

(Reserved)[1]

Editor's Note: Former Subsection C was repealed 10-17-2022 by Ord. No. 452-22.

If the requirements of Subsection 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.

[Amended 12-15-2014 by Ord. No. 424-14; 10-17-2022 by Ord. No. 452-22]

Only if a minimum infiltration of the first 0.5 inch of runoff volume cannot be physically accomplished on the site, a waiver from § 388-18 shall be considered by the Township, in accordance with § 388-9.

[Amended 12-15-2014 by Ord. No. 424-14; 10-17-2022 by Ord. No. 452-22]

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 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:

[Amended 12-15-2014 by Ord. No. 424-14]

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.

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.

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.

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:

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 Subsection N.)

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

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:

Be selected and designed to meet the criteria of § 388-16C that are applicable to infiltration;

Be set back at least 25 feet from all buildings and features with subgrade elements (e.g., basements, foundation walls, etc.), unless otherwise approved by the Township Engineer;

[Amended 12-15-2014 by Ord. No. 424-14]

For any infiltration practice that collects runoff from shared or multiple features and that is located within 50 feet of a building or feature with subgrade elements (e.g., basements, foundation walls, etc.), the bottom elevation shall be set below the bottom elevation of the subgrade element;

Have the following minimum setbacks:

Ten feet from all residential structures.

Ten feet from all property lines.

Comply with setback requirements established under 25 Pa Code Chapter 73.

[Amended 10-17-2022 by Ord. No. 452-22]

(d)

One hundred feet from any well.

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

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.

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.

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.

[Amended 12-15-2014 by Ord. No. 424-14]

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 Engineer.

[Amended 12-15-2014 by Ord. No. 424-14]

Groundwater quality of the carbonate aquifer shall be protected from infiltration of pollutants. At a minimum, 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:

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

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

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

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.

[Amended 12-15-2014 by Ord. No. 424-14]

(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.

§ 388-19 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).

[Amended 12-15-2014 by Ord. No. 424-14]

For modeling purposes, the predevelopment ground cover conditions shall be determined using the corresponding ground cover assumptions presented in § 388-21D 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.

[Amended 12-15-2014 by Ord. No. 424-14]

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.

[Amended 12-15-2014 by Ord. No. 424-14]

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

[Amended 10-17-2022 by Ord. No. 452-22]

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.

§ 388-20 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 that are not located in the Darby Creek or Crum Creek Watersheds:

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 388-20.1.

Table 388-20.1 Peak Rate Control Standards
Post-Construction Design Storm Frequency (24-hour duration)	Predevelopment Design Storm
	New Development Regulated Activities	Redevelopment Regulated Activities
2-year	1-year	2-year
5-year	2-year	5-year
10-year	2-year	10-year
25-year	25-year	25-year
50-year	50-year	50-year
100-year	100-year	100-year

NOTE: Peak flow rate of the post-construction design storm shall be reduced to the peak flow rate of the corresponding predevelopment design storm shown in the table.

For modeling purposes, the predevelopment ground cover conditions shall be determined using the corresponding ground cover assumptions presented in § 388-21D of this chapter.

For regulated activities involving only redevelopment, no peak flow rate controls are required when and only if the 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 388-20.1, using the redevelopment ground cover assumptions presented in § 388-21D. This design criterion for redevelopment is only permitted with approval of the Township Engineer. It shall result in no impact on downstream properties.

[Amended 10-17-2022 by Ord. No. 452-22]

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.

Regulated activities located within the Darby Creek Watershed (refer to Appendix F[1]) shall achieve the applicable peak flow release rate control requirements presented in the approved PA Act 167 plan for that watershed and as presented in Table 388-20.2 below:

Table 388-20.2 Peak Rate Control Standards for Darby Creek Watershed
Post-Construction Design Storm Frequency (24-hour duration)	Predevelopment Design Storm
	New Development Regulated Activities	Redevelopment Regulated Activities
2-year	1-year	1-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

NOTE: Peak flow rate of the post-construction design storm shall be reduced to the peak flow rate of the corresponding predevelopment design storm shown in the table.

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

Regulated activities located within the Crum Creek Watershed (refer to Appendix F[2]) shall achieve the applicable peak flow release rate control requirements presented in the approved PA Act 167 plan for that watershed and as presented in Tables 388-20.3 and 388-20.4 below, and as identified in the Crum Creek Management Districts Map in Appendix G of this chapter (the map is Appendix A of the Crum Creek Act 167 Plan Model Ordinance, and "Appendix A" therefore appears in the title block of the map, although it is contained in Appendix G of this chapter).[3]

Table 388-20.3 Peak Rate Control Standards for Crum Creek Watershed, Management District A
Post-Construction Design Storm Frequency (24-hour duration)	Predevelopment Design Storm
	New Development Regulated Activities	Redevelopment Regulated Activities
2-year	1-year	1-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

NOTE: Peak flow rate of the post-construction design storm shall be reduced to the peak flow rate of the corresponding predevelopment design storm shown in the table.

Table 388-20.4 Peak Rate Control Standards for Crum Creek Watershed, Management District B
Post-Construction Design Storm Frequency (24-hour duration)	Predevelopment Design Storm
	New Development Regulated Activities	Redevelopment Regulated Activities
2-year	1-year	1-year
5-year	2-year	2-year
10-year	5-year	5-year
25-year	10-year	10-year
50-year	25-year	25-year
100-year	100-year	100-year

NOTE: Peak flow rate of the post-construction design storm shall be reduced to the peak flow rate of the corresponding predevelopment design storm shown in the table.

[2]

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

[3]

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

§ 388-21 Calculation methodology.

[Amended 12-15-2014 by Ord. No. 424-14]

Stormwater runoff from all regulated activity sites shall be calculated using a generally accepted calculation technique(s) that is based on the NRCS Soil Cover Complex Method. Table 388-21.1 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.

Table 388-21.1 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	United States Army Corps of Engineers	Applicable where use of a full hydrologic computer model is desirable or necessary
Other methods	Varies	Other computation methodologies approved by the Township

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.

[Amended 10-17-2022 by Ord. No. 452-22]

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

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

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

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

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

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

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

For areas that are impervious surfaces, predevelopment calculations shall assume all impervious areas as "meadow" ground cover, except for those existing impervious surfaces which are served by stormwater management facilities designed and constructed in conformance with the current ordinance.

[Amended 1-15-2018 by Ord. No. 433-18]

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

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

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

At the discretion of the Township Engineer, regulated activities that involve a combination of both new development and redevelopment activities, as defined in this chapter, may either:

Apply the stormwater standards (redevelopment or new development) that are associated with the activity that involves the greatest amount of land area; or

[2]

Apply the redevelopment and new development stormwater standards to the corresponding redevelopment and new development portions of the proposed regulated activity.

Runoff curve numbers (CN) for both predevelopment and proposed (post-construction) conditions to be used in the Soil Cover Complex Method shall be obtained from Table C-1 in Appendix C of this chapter.[1]

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

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

Hydraulic computations to determine the capacity of pipes, culverts, and storm sewers shall be consistent with methods and computations contained in the Federal Highway Administration Hydraulic Design Series No. 5 (Publication No. FHWA-NHI-01-020 HDS No. 5, as amended). Hydraulic computations to determine the capacity of open channels shall be consistent with methods and computations contained in the Federal Highway Administration Hydraulic Engineering Circular No. 15 (Publication No. FHWA-NHI-05-114 HEC 15, as amended). Values for Manning's roughness coefficient (n) shall be consistent with Table C-3 in Appendix C of this chapter.

Runoff calculations shall include the following assumptions:

Average antecedent moisture conditions (for the Soil Cover Complex Method only, for example, TR-55, TR-20).

A Type II distribution storm (for the Soil Cover Complex Method only, for example, TR-55, TR-20).

§ 388-22 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 Virus Design Guidance found in Appendix D,[1] PADEP Document 363-0300-001, "Design Criteria - Wetlands Replacement/Monitoring" (as amended), or contact the Pennsylvania State Cooperative Wetland Center or the Penn State Cooperative Extension Office for design information.