Borough of Spring Grove, PA Stormwater Management Standards

For all regulated activities, unless preparation of an SWM site plan is specifically exempted in § 339-14:

Preparation and implementation of an approved SWM site plan is required.

No regulated activities shall commence until:

The municipality issues written approval of an SWM site plan, which demonstrates compliance with the requirements of this chapter; and

(b)

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

SWM site plans approved by the municipality, in accordance with § 339-29, shall be on site throughout the duration of the regulated activity.

The municipality may, after consultation with DEP, approve measures for meeting the state water quality requirements other than those in this chapter, provided that such alternative measures meet the minimum requirements of, and do not conflict with, state law including, but not limited to, the Clean Streams Law.[1]

[1]

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

For all regulated earth disturbance activities, erosion and sediment control BMPs shall be designed, implemented, operated, and maintained during the regulated earth disturbance activities (e.g., during construction) to meet the purposes and requirements of this chapter and to meet all requirements under Title 25 of the Pennsylvania Code and the Clean Streams Law.[2] Various BMPs and their design standards are listed in the Erosion and Sediment Pollution Control Program Manual (E&S Manual) No. 363-2134-008, as amended and updated.

[2]

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

Impervious areas:

(1)

The measurement of impervious areas shall include all of the impervious areas in the total proposed development even if development is to take place in stages.

(2)

For development taking place in stages, the entire development plan shall be used in determining compliance with this chapter.

(3)

For projects that add impervious area to a parcel, the total impervious area on the parcel is subject to the requirements of this chapter; except that the volume controls in § 339-15 and the peak rate controls of § 339-18 do not need to be retrofitted for existing impervious areas that are not being altered by the proposed regulated activity.

Stormwater flows onto adjacent property shall not be created, increased, relocated, or otherwise altered without written notification to the adjacent property owner(s). Such stormwater flows shall be subject to the requirements of this chapter.

All regulated activities shall include such measures as necessary to:

(1)

Protect health, safety, and property.

(2)

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

(a)

Minimize disturbance to floodplains, wetlands, and wooded areas.

(b)

Maintain or extend riparian buffers.

(c)

Avoid erosive flow conditions in natural flow pathways.

(d)

Minimize thermal impacts to waters of this commonwealth.

(e)

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

(3)

Incorporate methods described in the Pennsylvania Stormwater Best Management Practices Manual (NOTE: see § 339-44C). If methods other than green infrastructure and LID 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 and green infrastructure is not practicable.

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 Municipal Engineer.

Infiltration BMPs should be spread out, made as shallow as practicable, and located to maximize use of natural on-site infiltration features while still meeting the other requirements of this chapter.

Normally dry, open top, storage facilities should completely drain both the volume control and rate control capacities over a period of time not less than 24 hours and not more than 72 hours from the end of the design storm.

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 municipality. The municipality 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).

The design storm volumes to be used in the analysis of peak rates of discharge should 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's Atlas 14 (NOTE: see § 339-44E) can be accessed at: http://hdsc.nws.noaa.gov/hdsc/pfds/.

For all regulated activities, SWM BMPs shall be designed, implemented, operated, and maintained to meet the purposes and requirements of this chapter and to meet all requirements under Title 25 of the Pennsylvania Code, the Clean Streams Law, and the Storm Water Management Act.[3]

[3]

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

Various BMPs and their design standards are listed in the BMP Manual (NOTE: See § 339-44C).

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 §§ 339-15, 339-16, 339-17, and 339-18). Drainage facilities located on the site shall be designed to safely convey flows from outside of the site through the site.

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.

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

(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 municipality.

Additional water quality requirements. The municipality 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.).

All regulated activities located within a special flood hazard area designated by the Federal Emergency Management Agency (FEMA) shall comply with Chapter 213 of the Spring Grove Borough Code of Ordinances, more specifically known as the "Spring Grove Borough Floodplain Management Ordinance," 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.

§ 339-14 Exemptions and modified requirements.

General exemptions. Regulated activities that involve less than or equal to 1,000 square feet of proposed impervious surfaces and less than or equal to 5,000 square feet of earth disturbance or are listed in Subsection H are exempt from those (and only those) requirements of this chapter that are included in the sections and articles listed in Table 339-14F. Exemptions are for the items noted in Table 339-14F only and shall not relieve the landowner from other applicable requirements of this chapter. Exemption shall not relieve the applicant from implementing such measures as are necessary to protect health, safety, and welfare, property, and water quality.

Agricultural activity is exempt from the SWM site plan preparation requirements of this chapter, provided the activities are performed according to the requirements of 25 Pa. Code Chapter 102.

Forest management and timber operations are exempt from the SWM site plan preparation requirements of this chapter, provided the activities are performed according to the requirements of 25 Pa. Code Chapter 102.

Exemptions from any provisions of this chapter shall not relieve the applicant from the requirements in § 339-13E through M.

The municipality may deny or revoke any exemption or modified requirements pursuant to this section at any time for any project that the municipality believes may pose a threat to public health and safety or the environment.

Requirements for exempt activities.

(1)

An exemption from any requirement of this chapter shall not relieve the applicant from implementing all other applicable requirements of this chapter or from implementing such measures as are necessary to protect public health, safety, and welfare, property and water quality.

(2)

An exemption shall not relieve the applicant from complying with the requirements for state-designated special protection waters designated by PADEP as high quality (HQ) or exceptional value (EV) waters, or any other current or future state or municipal water quality protection requirements.

(3)

An exemption under this chapter shall not relieve the applicant from complying with all other applicable municipal ordinances or regulations.

(4)

Any applicant desiring exemption from design, plan submission, and plan processing requirements shall complete an application for exemption in the form available at the municipality's office and pay any applicable filing fee.

Table 339-14F Thresholds for Regulated Activities Exempt From Chapter Provisions
Chapter Article/Section	Activities Listed In § 339-14H	Less Than 1,000 Square Feet of Proposed Impervious Surfaces AND Less Than 5,000 Square Feet of Proposed Earth Disturbance	More Than 1,000 Square Feet of Proposed Impervious Surfaces OR More Than 5,000 Square Feet of Proposed Earth Disturbance
Article I	Not exempt	Not exempt	Not exempt
Article II	Not exempt	Not exempt	Not exempt
§§ 339-22 and 339-13E	Not exempt	Not exempt	Not exempt
§§ 339-13, 339-15, 339-16, 339-17, 339-18, 339-19, and 339-20	Exempt	Exempt	Not exempt
Article IV	Exempt	Exempt	Not exempt
Article V	Exempt	Exempt	Not exempt
Article VI	Exempt	Exempt	Not exempt
Article VII	Not Exempt	Not exempt	Not exempt
Article VIII	Not exempt	Not exempt	Not exempt
Other erosion, sediment and pollution control requirements	Must comply with Title 25, Chapter 102, of the Pa. Code and other applicable state and municipal codes, including the Clean Streams Law[1]

[1]

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

Exemptions for specific activities. The following specific regulated activities are exempt from the requirements of §§ 339-13, 339-15, 339-16, 339-17, 339-18, 339-19, and 339-20, and Article IV, Article V, and Article VI of this chapter (as shown in Table 339-14F), unless otherwise noted below. All other conveyance and system design standards established by the municipality in other codes or ordinances shall be required, and all other provisions of this chapter shall apply.

(1)

Emergency exemption. Emergency maintenance work performed for the protection of public health, safety and welfare. This exemption is limited to repair of an existing stormwater management facility; upgrades, additions or other improvements are not exempt. A written description of the scope and extent of any emergency work performed shall be submitted to the municipality within two business days of the commencement of the activity. A detailed plan shall be submitted no later than 30 calendar days following commencement of the activity. If the municipality finds that the work is not an emergency, then the work shall cease immediately and the requirements of this chapter shall be addressed as applicable.

(2)

Maintenance. Any maintenance to an existing stormwater management facility, BMP or conveyance made in accordance with plans and specifications approved by the Municipal Engineer or municipality.

(3)

Existing landscaping. Use of land for maintenance, replacement or enhancement of existing landscaping.

(4)

Gardening. Use of land for gardening for home consumption.

(5)

Agricultural related activities.

(a)

Agricultural activities (as defined in Article II), when performed in accordance with the requirements of 25 Pa. Code Chapter 102.

(b)

High tunnel if:

[1]

The high tunnel or its flooring does not result in an impervious surface exceeding 25% of all structures located on the landowner's total contiguous land area under common ownership; and

[2]

The high tunnel meets one of the following:

[a]

The high tunnel is located at least 100 feet from any perennial stream or watercourse, public road or neighboring property line.

[b]

The High Tunnel is located at least 35 feet from any perennial stream or watercourse, public road or neighboring property line and located on land with a slope not greater than 7%.

[c]

The High Tunnel is supported with a buffer or diversion system that does not directly drain into a stream or other watercourse by managing stormwater runoff in a manner consistent with the requirements of Pennsylvania Act 167.[2]

[2]

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

(6)

Forest management. Forest management operations, which are consistent with a sound forest management plan as filed with the municipality and which comply with the Pennsylvania Department of Environmental Protection's management practices contained in its publication "Soil Erosion and Sedimentation Control Guidelines for Forestry" (as amended or replaced by subsequent guidance). Such operations are required to have an Erosion and Sedimentation Control Plan, which meets the requirements of 25 Pa. Code Chapter 102 and meets the erosion and sediment control standards of § 339-15 of this chapter.

(7)

Maintenance of existing gravel and paved surfaces. Replacement of existing gravel and paved surfaces shall meet the erosion and sediment control requirements of 25 Pa. Code Chapter 102 and § 339-13E of this chapter, and is exempt from all other requirements of this chapter listed in Subsection H above. Resurfacing of existing gravel and paved surfaces is also exempt from the requirements of this chapter listed above. Paving of existing gravel surfaces is exempt from the requirements of this chapter listed above. Construction of new or additional impervious surfaces shall comply with all requirements of this chapter as indicated in Table 339-14F.

(8)

Municipal roadway shoulder improvements. Shoulder improvements conducted within the existing roadway cross section of municipal owned roadways, unless an NPDES permit is required, in which case the proposed work must comply with all requirements of this chapter.

(9)

In-place replacement of residential dwelling unit. The replacement in the exact footprint of an existing one- or two-family dwelling unit.

(10)

In-place replacement, repair, or maintenance of residential impervious surfaces. The replacement of existing residential patios, decks, driveways, pools, garages, and/or sidewalks that are accessory to an existing one- or two-family dwelling unit in the exact footprint of the existing impervious surface.

Modified requirements for small projects.

(1)

Regulated activities that involve both of the following: 1) 1,000 to 2,500 square feet of proposed impervious surfaces and 2) 1,000 to 5,000 square feet of proposed earth disturbance may apply the modified requirements presented in the "Simplified Approach to Stormwater Management for Small Projects" (Simplified Approach) (Appendix A)[3] to comply with the requirements of §§ 339-13, 339-15, 339-16, 339-17, 339-18, 339-19, and 339-20, and Article IV, Article V, and Article VI of this chapter (as shown in Table 339-14H). The applicant shall first contact the Municipal Engineer to confirm that the proposed project is eligible for use of the Simplified Approach and is not otherwise exempt from the provisions of this chapter; to determine what components of the proposed project are to be considered as impervious surfaces; and to determine if other known site or local conditions exist that may preclude the use of any techniques included in the Simplified Approach. Appendix A includes instructions and procedures for preparation, submittal, review and approval of documents required when using the Simplified Approach and shall be adhered to by the applicant. All other provisions of this chapter shall apply.

Table 339-14H Thresholds for Regulated Activities That Are Eligible for "Modified" Requirements for the Provisions of this Chapter That Are Listed Below
Chapter Article/Section	Activities Listed in § 339-14H
Article I	All provisions apply
Article II	All provisions apply
§§ 339-22 and 339-13E	All provisions apply
§§ 339-13, 339-15, 339-16, 339-17, 339-18, 339-19, and 339-20	Exempt if modified requirements of § 339-14I are applied
Article IV	Exempt if modified requirements of § 339-14I are applied
Article V	Exempt if modified requirements of § 339-14I are applied
Article VI	Exempt if modified requirements of § 339-14I are applied
Article VII	Exempt if modified requirements of § 339-14I are applied
Article VIII	All provisions apply
Other erosion, sediment and pollution control requirements	Must comply with Title 25, Chapter 102, of the Pa. Code and other applicable state and municipal codes, including the Clean Streams Law[4]

NOTES:

"Modified requirements" - Regulated activities listed within the subsections of this chapter noted in Table 339-14H are eligible for exemption only from the indicated sections and subsections of this chapter and only if the modified requirements of § 339-14I are met to the satisfaction of the municipality; all other provisions of this chapter apply.

[3]

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

[4]

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

§ 339-15 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) or a minimum of 1.5 inches of runoff from all regulated impervious surfaces shall be managed, whichever volume to be managed is greater. 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 § 339-19D 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.

Water quality improvement shall be achieved in conjunction with achieving the infiltration requirements of § 339-16. The infiltration volume required under § 339-16 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 municipality may require additional water quality and runoff control measures for stormwater discharging to special management areas such as those listed in § 339-13S.

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.

[1]

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

§ 339-16 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 either new development or redevelopment, infiltration should be designed to accommodate the entire water quality and runoff volume required in § 339-15. If the runoff volume required by § 339-15 cannot be infiltrated, then alternative methods consistent with the PA BMP Manual (as amended) may be used to manage this volume with approval from the Municipal Engineer. If the requirements of any subsection of § 339-16 cannot be physically accomplished, then the applicant shall be responsible for demonstrating with data or calculations to the satisfaction of the Municipal Engineer why the required infiltration volume controls 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.

A waiver from § 339-16 shall be considered by the municipality only if a minimum of at least 0.5 inch infiltration requirement cannot be physically accomplished on the site.

If site conditions preclude capture of runoff from portions of the impervious surfaces, the infiltration volume for the remaining area shall be increased by 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 Municipal 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 Municipal 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.

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

(2)

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 elevation of the subgrade element.

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

(1)

When a hot spot 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 Municipal Engineer for review. All areas designated for infiltration shall not receive runoff until the contributory drainage area has achieved final stabilization.

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 Municipal 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.

§ 339-17 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 Municipal 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 § 339-19D 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 Municipal 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 Municipal Engineer's satisfaction. When filters are utilized, maintenance provisions shall be provided to ensure filters meet the design function.

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.

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.

§ 339-18 Rate controls.

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 339-18A.

Table 339-18A Peak Rate Control Standards (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)
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

For modeling purposes, the predevelopment ground cover conditions shall be determined using the corresponding ground cover assumptions presented in § 339-19D of this chapter.

For regulated activities involving only redevelopment, no peak flow rate controls are required when and only if the total proposed 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 339-18A, using the redevelopment ground cover assumptions presented in § 339-19D. This design criterion for redevelopment is only permitted with approval of Municipal Engineer. It shall result in no measurable 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.

§ 339-19 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 339-19A 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 Municipal Engineer.

Table 339-19A 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 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)

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

[1]

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 B of this chapter.[1]

[1]

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

Runoff coefficients (C) for both predevelopment and proposed (post-construction) conditions for use in the Rational Method shall be obtained from Table C-2 in Appendix B of 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 Number 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 Number 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 B of this chapter.[2]

[2]

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

Runoff calculations shall include the following assumptions:

(1)

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

(2)

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

§ 339-20 Other requirements.

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 25 Pa. Code Chapter 105 regulations, the facility shall be designed in accordance with and meet the regulations of 25 Pa. Code Chapter 105 concerning dam safety. Title 25 Pa. Code Chapter 105 may require the safe conveyance of storms larger than 100-year event.

Any drainage conveyance facility and/or channel not governed by 25 Pa. Code 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.

Conveyance facilities to or exiting from stormwater management facilities (i.e., detention basins) shall be designed to convey the design flow to or from the facility.

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.

§ 339-21 Riparian buffers.

In order to protect and improve water quality, a riparian buffer easement shall be created and recorded as part of any subdivision or land development that encompasses a riparian buffer. Riparian buffer easement agreements shall be submitted to the municipality's Solicitor for review and approval prior to recording.

Except as required by 25 Pa. Code Chapter 102, the riparian buffer easement shall be measured to be the greater of the limit of the 100-year floodplain or a minimum of 35 feet from the top of the stream bank (on each side).

Minimum management requirements for riparian buffers.

(1)

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

(2)

Whenever practicable invasive vegetation shall be actively removed and the riparian buffer easement shall be planted with native trees, shrubs and other vegetation to create a diverse native plant community appropriate to the intended ecological context of the site.

The riparian buffer easement shall be enforceable by the municipality and shall be recorded in the appropriate County Recorder of Deeds Office, so that it shall run with the land and shall limit the use of the property located therein. The easement shall allow for the continued private ownership and shall count toward the minimum lot area a required by zoning, unless otherwise specified in the Municipal Zoning Ordinance[1].

[1]

Editor's Note: See 400, Zoning.

Any permitted use within the riparian buffer easement shall be conducted in a manner that will maintain the extent of the existing 100-year floodplain, improve or maintain the stream stability, and preserve and protect the ecological function of the floodplain.

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

(1)

Trails shall be for nonmotorized use only.

(2)

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

Septic drain fields and sewage disposal systems shall not be permitted within the riparian buffer easement and shall comply with setback requirements established under 25 Pa. Code Chapter 73.

§ 339-22 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 municipality 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 Pa. Code Chapter 102, or erosion and sediment control requirements of the municipality.

Work within natural drainage ways subject to permit by PADEP under Title 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 Title 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 Title 25 Pa. Code Chapter 105.

§ 339-23 Water quality and runoff volume requirements.

For modeling purposes, the predevelopment ground cover conditions shall be determined using the corresponding ground cover assumptions presented in § 339-19D 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.

Water quality improvement shall be achieved in conjunction with achieving the infiltration requirements of § 309-15. The infiltration volume required under § 309-15 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.

The municipality may require additional water quality and runoff control measures for stormwater discharging to special management areas such as those listed in § 339-13R.

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.

[1]

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