Borough of Derry, PA Article III Stormwater Management Performance Standards

§ 352-14 Stormwater management performance districts.

A. For purposes of stormwater management, the majority of the Borough is located in the Conemaugh River Watershed, which includes the Stormwater Management Performance Districts shown on the Westmoreland County Integrated Water Resources Plan interactive watershed resource map, which is hereby adopted as a portion of the Ordinance. The stormwater release rates for real property located within the Borough shall be as set forth on the Performance District Maps. Extreme southwest portions of the Borough are within the Loyalhanna Creek Watershed. Stormwater performance districts have not yet been finalized for these areas. For areas not covered by a stormwater performance district, the release rate shall be 80%* of the predevelopment peak flow as set by the municipality. For more information, refer to www.westmorelandstormwater.org and the Westmoreland County Conservation District's interactive watershed resource map.

B. *In areas not covered by a Stormwater Performance District, an 80% release rate or less is recommended as a standard. This is to apply a "factor of safety" to stormwater calculations; to account for the many variables in site design and stormwater management; and to avoid a land development project inadvertently increasing runoff and causing harm downstream.

§ 352-15 General requirements.

A. Preparation and implementation of a stormwater management site plan is required for all regulated activities, unless preparation of a SWM site plan is specifically exempted.

B. Projects that propose greater than one acre of earth disturbance are subject to NPDES permit requirements and will require a stormwater management plan.

C. No regulated activities, unless exempted, shall commence until the Borough issues written approval of an SWM site plan, which demonstrates compliance with the requirements of this chapter.

D. Regulated development activities shall be as follows:

Regulated Development Activity Table
SWM Plan Requirement	New Impervious Area for New and Redevelopment	Disturbed Area*	Next Steps
Exempt	Less than 500 square feet	AND less than 1 acre	Comply with Exemption section of this chapter[1]
Waiver/modification/demonstrated equivalency	Less than 1/2 acre, subject to municipal approval	AND less than 1 acre	Comply with Waiver/Modification/Demonstrated Equivalency section of this chapter[2]
Small project (per definition), refer to Appendix C	500 square feet to 2,000 square feet	OR 2,000 square feet to 4,000 square feet	Submit small project site plan complete with all attachments
Stormwater management plan meeting the ordinance requirements	Greater than 2,000 square feet if exempt and small project criteria are not met	OR greater than 4,000 square feet	Consult a qualified professional

NOTES:
*	The above table is only applicable for projects with earth disturbance less than 1 acre and that have not had cumulative impacts, within 5 years preceding the permit application date, that are in excess of the square foot limits.

[1] Editor's Note: See § 352-16.

[2] Editor's Note: See § 352-17.

§ 352-16 Exemption from performance standards.

A. The following regulated activities are specifically exempt from the stormwater management plan preparation and submission requirements articulated in this chapter:

(1) Agricultural activity limited to plowing or tilling activities, for animal concentrated (heavy) use areas provided the activities are performed according to the requirements of Chapter 102,[1] or conservation practices being installed as part of the implementation of a Conservation Plan written by an NRCS or SCS-certified planner. This exemption does not include any other type of earth disturbance subject to NPDES permit requirements such as earth disturbance equal to or greater than one acre.

[1] Editor's Note: See 25 Pa. Code Chapter 102.

(2) A high tunnel, if proof is provided that the high tunnel is exempt pursuant to the provisions of Act 15 of 2018. Such an exemption does not exempt high tunnels from other requirements applicable under federal, state or municipal laws.

(3) Forest management and timber operations, provided the activities are performed according to the requirements of Chapter 102.[2]

[2] Editor's Note: See 25 Pa. Code Chapter 102.

(4) Resource extraction activities, provided they are done in accordance with applicable PADEP regulations.

(5) Roadway resurfacing and maintenance projects, which do not increase impervious area, and underground infrastructure projects are exempt from the provisions of this chapter, provided the activities meet the requirements of all other municipal, state and federal requirements,

(6) Domestic landscaping and/or vegetable gardening.

(7) Voluntary green infrastructure (GI) or the retrofit of stormwater management infrastructure as conversion to green infrastructure BMPs to correct existing problems, that are solely intended to better manage runoff from existing development, are not part of new development or redevelopment, and that do not fall under the requirements of this or other development ordinances.

B. A property or a site plan for which an exemption was granted in the past may not be issued subsequent exemption(s) with further developments if the cumulative increase in impervious area is greater than 500 square feet.

C. The Borough may deny or revoke any exemption pursuant to this section at any time for any project that the Borough believes may pose a threat to public health, safety, property or the environment.

§ 352-17 Waivers/modifications/demonstrated equivalency.

A. If the Borough, in conjunction with the Borough Engineer, WCD, or PADEP, as applicable, determines that any requirement under this chapter cannot be achieved for a particular regulated activity, the Borough may, after an evaluation of alternatives, approve measures other than those in this chapter, subject to Subsections B, C and D and in compliance with all commonwealth laws and regulations. The request for a waiver, modification, or demonstrated equivalency shall originate with the landowner, shall be in writing, include a study of downstream effects, and accompany the stormwater management plan submission to the Borough. The request shall provide the facts on which the request is based, the provision(s) of the chapter involved and the proposed modification or demonstrated equivalency. The Borough Engineer and WCD shall review the request to determine if it meets the requirements of the chapter including Subsections B, C and D. If acceptable to the Borough and WCD and the regulated stormwater activity involving earth disturbance is less than one acre, the Borough may grant the waiver or modification. If the regulated stormwater activity involving earth disturbance is equal to or greater than one acre, the plan will be subject to the NPDES requirements of PADEP.

B. Waivers, modifications, or demonstrated equivalency of the requirements of this chapter may be approved by the Borough if enforcement will exact undue hardship because of unique physical circumstances or preexisting site conditions peculiar to the land in question, provided that the modifications or demonstrated equivalency will not be contrary or detrimental to the public interest and shall achieve the intended outcome, and that the purpose of the chapter is preserved. Hardship must be due to such unique physical circumstances or preexisting site conditions and not the circumstances or conditions generally created by the provisions of the Stormwater Management Ordinance; and there is no possibility that the property can be developed in strict conformity with the provisions of the Stormwater Management Ordinance. Cost or financial burden shall not be considered a hardship. Hardship cannot have been created by the landowner or developer. Modification or demonstrated equivalency shall not substantially or permanently impair the appropriate use or development of adjacent property(s) not under the landowner's control. Modification or demonstrated equivalency may be considered if an alternative standard or approach will provide equal or better achievement of the purpose of the chapter.

C. No waiver, modification or demonstrated equivalency of any regulated stormwater activity involving earth disturbance greater than or equal to one acre may be granted by the Borough unless that action is approved in advance by the PADEP or the WCD.

D. Applicants may request approval of a demonstrated equivalent stormwater activity for their project in lieu of performing traditional stormwater management. Demonstrated equivalent stormwater activity requests will be evaluated by the Borough Engineer and the WCD on a case-by-case basis. Prior approval of a demonstrated equivalent stormwater activity on a site does not set a precedent for future approval of the same or other alternative activities on any site. The approval of a demonstrated equivalent stormwater activity does not excuse the applicant from following standard E&S and SWM practices as applicable on the original site.

(1) Demonstrated equivalent stormwater activities shall only be approved when the following criteria are met:

(a) Traditional stormwater management activities on the site are precluded by a particular site limitation, such as contaminated soil, steep slopes, existing buildings/infrastructure, combined sewer;

(b) Construction of traditional stormwater management activities on the site would require extra permits or lead to excessive permitting activities and delays;

(c) The site in question does not already have a stormwater management problem; and

(d) The site in question is not already contributing to water quality problems in the receiving stream.

(2) Approvable demonstrated equivalent stormwater activities may include the following:

(a) Restoration of an existing degraded wetland, stream channel, floodplain, or riparian buffer, including daylighting of a stream.

(b) Restoration, retrofit or upgrade an existing stormwater management feature (inadequate detention pond, for example).

(c) Creation of new stormwater management features, especially green infrastructure, for a previously unmanaged site.

(d) Provide a water-based benefit to the public other than stormwater management (for example, extend a public sewer to an area not already served).

(e) Treatment of abandoned mine drainage.

(3) The proposal for demonstrated equivalency shall be accompanied by documentation or methodology quantifying the equivalency of the proposed project to what would have been originally required. Acceptable documentation or methodology may include use of the worksheets and checklist found in PADEP NPDES permit application, Appendix D[1] or approved method showing the proposed equivalency:

(a) Controls approximately the same amount of runoff volume as what would originally have been proposed.

(b) Improves approximately the same amount of runoff quality as would have been originally proposed.

(c) Is located within an impaired watershed or stream segment which will benefit from the proposed project. Impairment may include stream impairment, reduced stream buffer, and pollutant loading. Refer to the Westmoreland County Integrated Water Resources Plan at www.westmorelandstormwater.org.

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

(4) The demonstrated equivalent stormwater activity shall:

(a) Be constructed concurrently with the project for which it is being applied;

(b) Be constructed according to plans approved by the Borough and the WCD including any erosion control and stormwater management practices as applicable;

(c) Obtain all necessary permits;

(d) Be located on land owned by or controlled by the applicant or by a cooperating public or private entity(s) (school, church, club, Borough, etc.);

(e) Be protected by a perpetual easement or deed restriction, or landowner agreement;

(f) Be located in the same general watershed as the project for which it is being applied; and

(g) Have an operation and maintenance plan (O&M) specifying who is responsible for what tasks.

§ 352-18 Small project.

A. When a regulated development activity (refer to Table in § 352-15 of this chapter) creates impervious area between 500 square feet and 2,000 square feet, or total earth disturbance between 2,000 square feet and 4,000 square feet, the stormwater management requirements are as follows. Refer also to Appendix C.[1]

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

B. For new impervious surfaces, the first one inch of runoff shall be permanently removed from the runoff flow, and shall not be released to waters of the commonwealth. Removal options include reuse, evaporation, transpiration and infiltration.

C. Facilities, to the greatest extent possible and subject to Borough approval, shall be designed to drain the permanently removed runoff volume in a period no greater than 72 hours. Runoff volumes in excess of one inch shall be safely conveyed to a stable vegetated area, natural watercourse, the curb or gutter line of roadway or existing storm collection/conveyance/control system as applicable.

D. This method is exempt from the requirements of § 352-25 of this chapter.

E. A small project stormwater management plan must be submitted to the Borough and the WCD and shall consist of the following items and related support material needed to determine compliance with §§ 352-20 to 352-23 of this chapter. The applicant can also use protocols listed in Appendix C.[2]

(1) Narrative: General description of proposed stormwater management techniques, including calculations, assumptions and criteria used in the design of the stormwater management facilities and BMPs, and construction specifications of the materials to be used for stormwater management facilities and BMPs;

(2) Stormwater management plan: Showing locations of all stormwater management facilities and BMPs, especially green infrastructure, limits of disturbance, including the type and amount of proposed impervious area, structures, roads, paved areas and buildings;

(3) Small project stormwater management worksheet;

(4) Signed agreement page for operation and maintenance of stormwater facilities and BMPs (Refer to Appendix B[3]); and

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

(5) Erosion and sediment control plan: including all reviews and letters of adequacy from the Conservation District.

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

§ 352-19 General standards.

A. Proposed land development must consider avoiding, minimizing, and mitigating impacts to the site that may increase stormwater runoff from the proposed project. Applied sequentially, these three low-impact development strategies should be an overall guide as a project is planned and carried out.

B. The Westmoreland County Integrated Water Resources Plan provides an online decisionmaking tool to assist developers, designers, property owners in addressing all water resources during development and redevelopment and should be consulted. Refer to www.paiwrp.com and www.westmorelandstormwater.org.

C. The following provisions shall be considered the overriding performance standards against which all proposed stormwater control measures shall be evaluated and shall apply throughout the Borough.

(1) Any landowner and any person engaged in the alteration or development of land which may affect stormwater runoff characteristics shall implement such measures as are reasonably necessary to prevent injury to health, safety or other property. For alteration or development taking place in stages, the cumulative development must be used in determining conformance with this chapter. Such measures shall include such actions as are required:

(a) To assure that the maximum rate of stormwater runoff is no greater after development as calculated using the standards in this chapter than prior to development activities for the one-, two-, five-, ten-, twenty-five-, fifty-, 100-year storms having rainfall durations of 24 hours. Rainfall data shall be obtained from NOAA Atlas 14 or other source approved by the Borough.

(b) To manage the water quality, rate and volume and direction of resulting stormwater runoff in a manner which otherwise adequately protects health and property from possible injury.

(c) To notify adjacent property owners or owners of affected properties of any alteration or increase of stormwater flows.

(2) Runoff treatment BMPs must be employed where necessary to ensure the water quality, rate and volume requirements are met.

(3) Volume control BMPs shall be used to maintain existing hydrologic conditions for small storm events by promoting groundwater recharge and/or evapotranspiration. Runoff volume controls shall be implemented using the PADEP Stormwater BMP Manual 2006 or other approved method such as those listed in the following chart:

Acceptable Computation Methodologies for Stormwater Management Plans
Method	Developed by	Applicability
Win TR-20 (or commercial computer package based on TR-20)	USDA NRCS	Applicable where use of full hydrologic computer model is desirable or necessary.
Win TR-55 (or commercial computer package based on TR-55 i.e., VT/PSUHM)	USDA NRCS	Applicable for land development plans within limitations described in TR-55.
HEC-1, HEC-HMS	U.S. Army Corps of Engineers	Applicable where use of full hydrologic computer model is desirable or necessary.
PennDOT 584 (based on rational method)	PennDOT	Applicable under standards established by PennDOT. NOT to be used for runoff volume and pond sizing computations.
EFH2	USDA NRCS	Applicable in agricultural areas subject to the program limits.
SWMM	EPA	Applicable in urban and suburban areas subject to limits established by EPA.
PADEP BMP Manual 2006	PADEP	Applicable under standards established by PADEP.
Other methods	Varies	Other methodologies approved by the Borough.

D. The project plan shall specify permanent stormwater BMPs to be implemented, operated and maintained to meet legal water quality, rate and volume requirements. If methods other than low-impact development (LID) and green infrastructure 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.

E. In order to protect and maintain water quality, additional stormwater runoff created by the development project must be captured, stored and treated. In addition, post construction stormwater infiltration of runoff must replicate preconstruction infiltration of runoff to the maximum extent possible with the exception of hot spots. As a minimum, this shall be a volume of additional runoff generated by a two-year, twenty-four-hour storm. Preferred BMPs for a hot spot include storm inlet filters, proprietary stormwater quality devices, underground detention tanks, detention ponds with forebays, tree planting, green roof. Permeable pavement, infiltration BMPs, and rain gardens are not recommended for hotspots.

F. In addition to the provisions set forth in Subsections A through C, inclusive, as set forth above, all regulated development activities within the Borough shall be designed, implemented, operated and maintained to meet the purposes of this article, through these two elements:

(1) Erosion and sediment control during the earth disturbance activities (e.g., during construction).

(2) Water quality, rate and volume protection measures after completion of earth disturbance activities (e.g., postconstruction stormwater management), including operations and maintenance.

G. No regulated development activities within the Borough shall commence until the requirements of this chapter are met.

H. All best management practices (BMPs) used to meet the requirements of this chapter shall conform to the state water quality requirements, and any more stringent requirements as determined by the Borough.

I. LID and green infrastructure techniques described in the PADEP Stormwater BMP Manual 2006 or most current edition are encouraged.

J. Proposed projects must comply with the Borough's approved MS4 permit, including compliance with the six minimum control measures (MCMs) and with the Borough's Pollutant Reduction Plan (PRP).

§ 352-20 Watershed standards.

A. The stormwater management performance standards in this article are intended to implement the provisions, standards and criteria contained in the Pennsylvania Stormwater Management Act (Act 167), 32 P.S. § 680.1 et seq. If there is any discrepancy between the provisions of this article and the provisions, standards and criteria of the Act, or if a stormwater management plan is subsequently approved and adopted by the appropriate governmental agency or body, then the provisions, standards and criteria of the current watershed plan shall govern.

B. Management of stormwater runoff is key objective of 25 Pa. Code, Chapter 93, of the PADEP regulations, because runoff can change the physical, chemical and biological integrity of water bodies thereby impacting rate, volume and water quality.

C. The project plan shall describe how these rate, volume and water quality protection requirements will be met. Infiltration BMPs shall be evaluated and utilized to the maximum extent possible to manage the net change in stormwater runoff generated so that postconstruction discharges do not degrade the physical, chemical or biological characteristics of the receiving waters. These BMPs may be used to satisfy all or part of the requirements found within this chapter.

D. Refer to the Stormwater Performance Districts outlined in Appendix A,[1] and the Westmoreland County Integrated Water Resources Plan (IWRP) at www.westmorelandstormwater.org. The project plan shall describe how the proposed project will address performance standards, impairments, and pollutant loading found in the IWRP. For areas not covered by a stormwater performance district, the release rate shall be 80% of the predevelopment peak flow as set by the Borough.

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

§ 352-21 Design criteria for stormwater management facilities and BMPs.

A. General criteria.

(1) Applicants may select runoff control techniques, or a combination of techniques, which are most suitable to control stormwater runoff from the development site. Refer to the Acceptable Computation Methodologies table of this article.[1] All controls must be subject to approval of the Borough Engineer and the WCD. The Borough Engineer may request specific information on design and/or operating features of the proposed stormwater controls in order to determine their suitability and adequacy in terms of the standards of this chapter.

[1] Editor's Note: See § 352-19.

(2) If the proposed development site is located in an impaired watershed according to Category 4 of the Pennsylvania Integrated Water Quality Monitoring and Assessment Report, or in a watershed with a TMDL according to Category 5 of the same report, the applicant shall identify the source and cause of impairment and shall propose, if required or applicable, the use of BMPs to mitigate any impacts to the waters.

(3) The applicant should consider the effect of the proposed stormwater management techniques on any special soil conditions or geological hazards which may exist on the development site. In the event such conditions are identified on the site, the Borough Engineer may require in depth studies by a competent geotechnical engineer. Not all stormwater control methods may be advisable or allowable at a particular development site.

(4) The applicant shall consider the effect of the proposed stormwater management techniques on existing stream impairments and pollutant loading. Refer to the Westmoreland County Integrated Water Resources Plan (IWRP) at www.westmorelandstormwater.org.

(5) The applicant shall consider existing conditions on the site for the prior five years to determine prevailing land use and impervious cover, and shall consider 20% of existing impervious cover as meadow for pre-existing conditions on redevelopment sites.

(6) The hydrologic soil group (HSG) used for existing soils shall be assumed to be for drained conditions. As an example, a soil identified in the USDA NRCS soil survey as HSG B/D shall be assumed to be HSG B soil unless information, such as infiltration testing and groundwater monitoring data, is presented that justifies the use of HSG D soil.

(7) For postdevelopment conditions of sites that are undeveloped, the HSG for each disturbed soil shall be reduced by one HSG level relative to its HSG reported in the USDA NRCS soil survey in order to account for compaction of the soil due to earthmoving activities, unless specific accommodations are employed, such as deep-tilling (or subsoiling) and soil amendment, to counter against the soil compaction that normally results due to typical earthmoving activities.

(8) The stormwater management practices to be used in developing a stormwater management plan for a particular site shall be selected according to the following order of preference:

(a) Site planning for locating proposed buildings, impervious areas and grading which minimizes disruption of the natural site characteristics especially utilizing low-impact development techniques.

(b) Minimization of impervious areas and promotion of retentive grading.

(c) Implementation of nonstructural measures (refer to the PADEP Stormwater BMP Manual 2006 or current edition).

(d) Implementation of innovative/green infrastructure structural measures (refer to the PADEP Stormwater BMP Manual 2006 or current edition).

(e) Stormwater detention/retention structures.

(9) Any BMP which is a dam, culvert, stream obstruction or encroachment or outfall as defined in 25 Pa. Code Chapter 105 shall be designed according to the requirements in those regulations.

(10) Drainage easements shall be provided for all stormwater conveyance and BMPs serving multiple properties and not located within a public right-of-way. Easements shall include ingress and egress to a public right-of-way, and shall be recorded at the county with the final plan. Terms of easement shall prohibit excavation or placement of fill or structures and any alteration that may adversely affect the flow of stormwater within any portion of the easement.

(11) No person shall install, create, modify, remove, fill, landscape or otherwise alter or place any structure, soil, rock, material or vegetation in or on, or otherwise adversely affect, any stormwater management facility or any area within a stormwater easement without the written approval of the Borough and approval of the WCD.

(12) Persons engaged in land development activities shall provide the required financial Security and O&M agreements to the Borough as outlined in Appendix B of this chapter.[2]

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

B. Criteria for stormwater management facilities and BMPs.

(1) If stormwater management facilities and BMPs are utilized for the development site, the facility(s) shall be designed such that postdevelopment peak runoff rates from the developed site are controlled to those rates defined by a Stormwater Management Performance District for the one-, two-, five-, ten-, twenty-five-, fifty-, 100-year storm frequencies having rainfall durations of 24 hours. Rainfall data shall be obtained from NOAA Atlas 14 or other source as approved by the Borough.

(2) All stormwater management facilities and BMPs shall be equipped with outlet/overflow structures to provide rate discharge control for the designated storm frequencies. Provision shall also be made to safely pass the entire postdevelopment 100-year storm without breaching or otherwise damaging the facilities, downstream or neighboring properties.

(3) Release of stormwater flow from a development site must be to an existing stormwater conveyance or easement, whether natural or man-made, that has defined bed and banks. Calculations and information shall be presented as to the ownership, responsible party, capacity, and stability of such conveyance. Release of "sheet flow" as from a level spreader, will be permitted on a case-by-case basis as approved by the Borough Engineer and the WCD.

(4) All stormwater management facilities and BMPs shall be designed to control volume and water quality as defined by the Stormwater Management Performance District. Refer to Appendix A.[3]

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

(5) Shared stormwater management facilities and BMPs, which provide control of runoff for more than one development site within a single subarea, may be considered and are encouraged. Such facilities shall meet the criteria contained in this section. In addition, runoff from the development sites involved shall be conveyed to the facility in a manner that avoids adverse impacts (such as flooding or erosion) to channels and properties located between the development site and the shared storage facilities.

(6) Where stormwater management facilities and BMPs will be utilized, multiple-use facilities, such as wetlands, lakes, ball fields or similar recreational/open space uses are encouraged wherever feasible, subject to the approval of the Borough.

(7) Other considerations which shall be incorporated into the design of the stormwater management facilities and BMPs include:

(a) Inflow and outflow structures shall be designed and installed to prevent erosion and embankments, cuts, fills and bottoms of impoundment-type structures should be protected from soil erosion.

(b) Control and removal of debris both in the storage structure and in inlet or outlet devices shall be a design consideration.

(c) Inflow and outflow structures, pumping stations and other structures shall be designed and protected, using safety benches, trash racks, energy dissipaters and other means to minimize safety hazards.

(d) Access restrictions include fencing, with at least one lockable ingress and egress gate, in a minimum height of six feet shall be required at the discretion of the Borough and the Borough Engineer.

(e) Interior slopes of storage ponds shall not exceed a ratio of three to one horizontal to vertical dimension with a combination of interior and exterior slopes not exceeding five. Steeper slopes may be approved by the Borough Engineer if documented to be stable by a geotechnical analysis prepared by, or under the direction of, and sealed by a licensed geotechnical engineer.

(f) Pond principal spillways shall consist of a solid reinforced concrete box (riser) that is designed and constructed to control the flow rate out of the detention facility. Noncorrugated plastic pipe may be used for the riser where the riser height is less than three feet and the plastic pipe is properly anchored to prevent horizontal or vertical displacement. The riser shall be joined by a watertight connection to a horizontal reinforced concrete pipe or high-performance polypropylene pipe (barrel) extending through the embankment and outletting beyond the downstream toe of the fill. The maximum capacity of the barrel will be the twenty-five-year postdevelopment flow. The construction materials must be approved by the Borough Engineer.

(g) Pond dam embankments shall consist of all excavation on the project, except such material as may be determined unsuitable by the Borough Engineer, which includes but is not limited to frozen material, organic material, rock in excess of three inches in diameter, construction debris, sandy or other coarse grained soils, noncompactable soils and excessively wet or dry material. Acceptable material shall conform to the current PennDOT Publication 408 section on embankment material or shall be as approved by the Borough Engineer. The pond dam embankment shall be compacted to a minimum 95% standard proctor maximum dry density as per ASTM D698. The embankment shall be constructed and compacted in maximum eight-inch lifts and the principal spillway outfall barrel and anti-seep collar(s) shall be constructed integral with the dam embankment construction.

(h) Landscaping shall be provided for the facility which stabilizes disturbed areas and preserves the natural and beneficial values of the surrounding area. Landscaping shall consist of plant species native to southwestern Pennsylvania. No trees or other woody vegetation shall be planted on the fill areas of the pond dam embankment that may result in loosening of the compacted soils of the embankment. Detention basin bottoms shall be vegetated with a diverse native planting mix and may include trees, woody shrubs and meadow/wetland herbaceous plants. Noxious and invasive plants that inhibit plant species diversity shall not be included in the plant mix. PennDOT seed mix Formulas B, C, D and L as per PennDOT Publication 408 latest revision, or approved equivalent mix shall be used for revegetation of embankment areas. Crownvetch and birdsfoot trefoil shall not be used due to their interference with performance of inspections of the dam embankment. For other suitable landscape plant species, refer to Appendix B of the Pennsylvania Stormwater Best Management Practices Manual, latest edition.

(i) Facilities shall be located to facilitate maintenance, considering the frequency and type of equipment that will be required.

(j) Underground detention/retention/infiltration facilities shall be equipped with open grate inlet(s), inspection ports and/or manhole access to facilitate visual inspections and maintenance.

(k) Other considerations which should be incorporated into the design of the detention facilities include the following:

[1] Inflow and outflow structures shall be designed and installed to prevent erosion, and bottoms of impoundment-type structures should be protected from soil erosion.

[2] All inflow structures into detention/retention ponds shall discharge into a pond forebay designed in accordance with the Pennsylvania Stormwater Best Management Practices Manual.

[3] Control and removal of debris both in the storage structure and in all inlet or outlet devices shall be a design consideration.

[4] Inflow and outflow structures, pumping stations and other structures shall be designed and protected to minimize safety hazards.

[5] The water depth at the perimeter of a storage pond should be limited to that which is safe for children. This is especially necessary if bank slopes are steep or if ponds are full and recirculating in dry periods. Restrictions of access (fence, walls, etc.,) shall be necessary, depending on the location of the facility.

[6] A safety bench with a minimum width of eight feet shall be provided around the entire interior perimeter of the pond at an elevation of six inches minimum above the two-year peak water surface elevation in the pond.

[7] An anti-vortex device and trash rack shall be attached to the top of the principal spillway to improve the flow of water into the spillway and prevent floating debris from being carried out of the basin.

[8] The base of the principal spillway must be firmly anchored to prevent its floating. Computations must be made to determine the anchoring requirements. As a minimum, a factor of safety of 1.25 shall be used (downward forces = 1.25 x upward forces).

[9] The barrel of the principal spillway, which extends through the embankment, shall be designed to carry the twenty-five-year predevelopment flow provided by the riser of the principal spillway with the water level at the crest of the emergency spillway. The barrel shall consist of a reinforced concrete pipe or high-performance polypropylene pipe and shall be fully supported along its entire length by a concrete cradle extending to the center line of the pipe and projecting horizontally a minimum of six inches beyond the outside wall of the pipe and vertically six inches below the bottom of the pipe. The connection between the riser and the barrel must be watertight. The outlet of the barrel must be protected to prevent erosion or scour of downstream area. This will include an end section or end wall with a designed rip-rap apron. For principal spillway barrels that pass through compacted fill pond dam embankments, the barrel, cradle and anti-seep collar(s) shall be constructed integral with the dam embankment construction as opposed to cutting into the completed dam embankment to construct the barrel, etc.

[10] Anti-seep collars.

[a] Anti-seep collars shall be used on the barrel of the principal spillway within the normal saturation zone of the embankment to increase the seepage length by at least 10% if either of the following conditions is met:

[i] The settled height of the embankment exceeds 10 feet.

[ii] The embankment has a low silt clay content and the barrel is greater than 10 inches in diameter.

[b] The anti-seep collars shall be installed within the saturated zone. The maximum spacing between collars shall be 14 times the projection of the collar above the barrel. Collars shall not be closer than two feet to a pipe joint. Collars should be placed sufficiently far apart to allow space for hauling and compacting equipment. Collars shall consist of reinforced concrete with a minimum thickness of eight inches. Connections between the collars and the barrel shall be watertight.

[11] The emergency spillway shall consist of an open channel having a control section at least 20 feet in length. The control section is a level portion of the spillway channel at the highest elevation in the channel. The emergency spillway shall be designed to carry the peak rate of runoff expected from the entire postdevelopment 100-year storm to the stormwater facility. The spillway channel shall be located so as to avoid sharp turns or bends. The channel shall return the flow of water to a defined channel downstream from the embankment. At the discretion of the Borough Engineer and/or WCD, the emergency spillway shall include a reinforced concrete-grade control wall extending perpendicular to the flow through the spillway with the top of the wall flush with the spillway crest. This control wall shall have a minimum width of eight inches, extend a minimum of 18 inches below the spillway crest and have a length no less than the width of the emergency spillway opening at the top of dam. The emergency spillway shall provide a minimum one foot of freeboard between the water surface of the full 100-year postdevelopment discharge through the spillway and the top of the dam embankment.

C. Criteria for collection/conveyance facilities.

(1) All stormwater runoff collection or conveyance facilities, whether storm sewers or other open or closed channels, shall be designed in accordance with the following basic standards:

(a) All building sites shall use measures to provide drainage away from and around the structure in order to prevent any potential flooding damage as much as practical. Such measures shall include grading the surrounding lawn or pavement area so that it slopes away from the structure by a minimum of 5% slope over a minimum distance of six feet; raising the floor of the structure so that it is a minimum of six inches above the predominant surrounding land elevation and above the designated floodplain elevation for those located within a floodplain; eliminating or waterproofing penetrations through the structure's walls or foundation; constructing berms, curbs, or swales to divert surface water around the structure; arranging roof and area drains to carry water away from the structure.

(b) Developers proposing a land development or subdivision shall arrange internal drainage within the subdivision so that surface water is safely directed and channeled away from all structures within and adjacent to the development site.

(c) Developers shall provide to all persons constructing a structure within a land development site, standards including drawings and specifications to ensure that those persons adhere to the general site plans and stormwater management plans for the development. Persons constructing a structure within a land development site shall submit to the Borough Engineer and WCD drawings, calculations, and other information to show how they will meet the stormwater management requirements of the development site.

(d) Lots located on the high side or low side of streets shall extend roof, trench and area drains to a stable vegetated area, natural watercourse, the curb or gutter line of roadway or storm collection/conveyance/control system (if applicable) in accordance with the approved stormwater management plan for the development site.

(e) For all building sites and lots, the inclusion of rain barrels, rain gardens, drywells and other strategies for infiltration of roof runoff close to its source is encouraged.

(f) Collection/conveyance facilities should not be installed parallel and less than 10 feet from the top or bottom of an embankment, greater than or equal to 15 feet in height to avoid the possibility of failing or causing the embankment to fail, unless documented to be stable by a geotechnical analysis.

(g) All collection/conveyance facilities shall be designed to convey the twenty-five-year storm peak flow rate from the contributing drainage area to the nearest suitable outlet such as a stormwater control facility, curbed street, storm sewer or natural watercourse. However, the 100-year design storm may be required as determined by the Borough Engineer where it is necessary for the storm system to convey the 100-year design storm to the stormwater management facility.

(h) Where drainage swales or open channels are used, they shall be suitably lined to prevent erosion and designed to avoid erosive velocities.

(2) Wherever storm sewers are proposed to be utilized, they shall comply with the following additional criteria:

(a) Where practical, designed to traverse under seeded and planted areas. If constructed within 10 feet of road paving, walks or other surfaced areas, drains shall have a narrow trench and maximum compaction of backfill to prevent settlement of the superimposed surface or development.

(b) Preferably installed after excavating and filling in the area to be traversed is completed, unless the drain is installed in the original ground with a minimum of three feet cover and/or adequate protection during the fill construction.

(c) Designed:

[1] With cradle when traversing fill areas of indeterminate stability.

[2] With anchors when gradient exceeds 20%.

[3] With encasement or special backfill requirements when traversing under a paved area.

(d) Designed to adequately handle the anticipated stormwater flow and be economical to construct and maintain. The minimum pipe size shall be 15 inches in diameter, with the exception of roof drains, foundation drains, internal french drains and/or sump pumps, or similar conveyances.

(e) Drain pipe, trenching, bedding and backfilling requirements and appropriate grates, catch basins, stormwater inlets, manholes and other appurtenances shall conform to the requirements of the Borough and/or applicable PennDOT specifications, Publication 408.

(f) All corrugated metal pipe shall be polymer coated, and with paved inverts where prone to erode. Pipe within a Borough right-of-way shall be reinforced concrete pipe or high performance polypropylene pipe with a diameter sufficient to accept existing and reasonably anticipated future stormwater capacity and in no event less than the minimum diameter of 15 inches.

(g) Storm inlets and structures shall be designed to be adequate, safe, self-cleaning and unobtrusive and consistent with Borough standards with sufficient capture and conveyance capacity and spacing of inlets and cleanouts for maintenance.

(h) Where a proposed sewer or conveyance connects with an existing storm sewer or conveyance system, the applicant shall demonstrate that sufficient capacity exists in the downstream system to handle the additional flow.

(i) Storm sewer outfalls shall be equipped with energy dissipation devices to prevent erosion and conform with applicable requirements of the Pennsylvania DEP for stream encroachments (Section 7 of the Dam Safety and Encroachments Act, 32 P.S. § 693.1, et seq., the rules and regulations promulgated thereunder at 25 Pa. Code §§ 105.441 to 105.449), and the Pennsylvania DEP Erosion and Sediment Control Manual (current edition). Exit velocities of the design storm for the stormwater conveyance systems must be considered in the design of the energy dissipation devices.

(j) Storm inlets shall include castings with the message "Dump No Waste. Drains to Stream," or similar message as approved by the Borough Engineer, cast into or permanently affixed to the casting and positioned in a manner that is legible to pedestrian traffic.

D. Criteria for riparian buffers.

(1) If a riparian buffer is required by PADEP as part of an NPDES permit, then PADEP regulations will govern.

(2) If a riparian buffer is used to meet stormwater management requirements and/or MS4 pollutant load reduction credits it shall meet the following requirements:

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

(b) Except as required by Pa. Code Title 25 Chapter 102, the riparian buffer easement shall may be measured to a minimum of 35 feet measured from the top of the nearest bank (on each side), or an average of 35 feet with no distance from top of bank less than 25 feet.

(c) 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.

[3] There shall be no earth disturbance beyond which is necessary to establish or maintain a planted buffer.

(3) The riparian buffer easement shall be enforceable by the Borough and shall be recorded in the Westmoreland 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 required by the Borough's Zoning Ordinance,[4] unless specified otherwise in the Borough ordinance.

[4] Editor's Note: See Ch. 425, Zoning.

(4) 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.

(5) Stormwater drainage pipes and all other linear utility lines as approved by the Borough shall be permitted within the riparian buffer easement, but they shall cross the easement in the shortest practical distance. Other stormwater management facilities and BMPs are not permitted within the riparian buffer easement.

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

(a) Trails shall be for nonmotorized use only.

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

(7) Septic drainfields 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.

E. Criteria for stream restoration project.

(1) A stream restoration project may eligible for stormwater management and/or MS4 load reduction credits if it meets qualifying criteria set by the Borough and as follows:

(a) Existing conditions, such as channel or stream bank erosion and an actively enlarging or incising urban stream condition, shall be documented prior to restoration.

(b) Projects should be located on first to third order (small) streams to be effective.

(c) Project should address a minimum of 100 linear feet of stream channel and both sides where need to do so is evident.

(d) Upstream impervious area should be sufficiently treated to address peak flows that may exceed engineering design thresholds or compromise channel form and function.

(e) Utilize a comprehensive approach employing a mix of techniques appropriate to the site, creating long-term stability of the streambed, stream banks and floodplain.

(f) Armored length of streams (i.e., using riprap or gabions) may be used to maintain channel stability, but the same length shall not be included in the load reduction calculation.

(g) Project shall maximize floodplain reconnection, with minimal channel invert elevation increase (i.e., bank height ratio = 1.0 or less) to achieve the objective.

(h) Project shall include a minimum 35 feet permanent riparian buffer.

(i) Project shall include an O&M plan identifying O&M activities, frequencies and responsible parties.

§ 352-22 Erosion and sedimentation controls.

A. No regulated development activities within the Borough shall commence until approval by the Borough and the WCD of an erosion and sediment control plan for construction activities.

B. Any earth disturbance activity of 5,000 square feet or more requires an erosion and sedimentation control plan under 25 Pa. Code § 102.4(b). Refer to the PADEP Erosion and Sediment Pollution Control Manual (current edition).

C. In addition, under Title 25 Pa. Code, Chapter 92,[1] a PADEP NPDES construction activities permit is required for regulated development activities involving one acre or more of disturbance.

[1] Editor's Note: See 25 Pa. Code Chapter 92a.

D. Evidence of any necessary permit(s) for regulated development activities from the appropriate PADEP regional office or Westmoreland Conservation District must be provided to the Borough. The issuance of an NPDES construction permit [or permit coverage under the Statewide General Permit (PAG-2)] may satisfy the requirements under Subsection A, upon review and approval by the Borough.

E. A copy of the erosion and sediment control plan and any permit required by PADEP or the Borough shall be available at the project site at all times.

§ 352-23 Water obstructions and encroachments.

A. No regulated development activities which require Chapter 105[1] (Water Obstructions and Encroachment) permit from either PADEP or Westmoreland Conservation District shall commence until all permits have received PADEP or WCD approval, and Borough approval.

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

B. Evidence of any necessary Chapter 105 permit from PADEP / WCD shall be provided to the Borough.

C. Proposed development shall avoid the long- and short-term adverse impacts associated with the occupancy and modification of floodplains as designated by FEMA, to the extent possible wherever there is a practicable alternative in order to reduce the risk of flood loss, minimize the impacts of floods on human safety, health and welfare, and restore and preserve the natural and beneficial values served by floodplains.

D. Any proposed development found to be within the base floodplain of a waterway shall be in compliance with Derry Borough Floodplain Management Ordinance, Chapter 205 of the Derry Borough Code, and shall include the identification of impacts, an evaluation of practicable alternatives outside the floodplain, and when impacts cannot be avoided, the development of measures to minimize the impacts and restore and preserve the floodplain as appropriate. Findings shall be presented at a public meeting and a determination made by Borough Council of the Borough of Derry.

E. Any proposed stormwater management plan should be consistent with the provisions of the Pennsylvania Floodplain Management Act 166 of 1978[2] and applicable municipal floodplain ordinances, and when impacts cannot be avoided, the development of measures to minimize the impacts and restore and preserve the floodplain is appropriate.

[2] Editor's Note: See 32 P.S. § 679.101 et seq.