Township of Shrewsbury, PA Part 3 STORMWATER MANAGEMENT STANDARDS

§ 25-301 General Requirements.

[Ord. 2011-09, 9/7/2011, § 301]

1. For all regulated activities, unless preparation of an SWM site plan is specifically exempted in § 25-302:

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

B. No regulated activities shall commence until the Township issues written approval of a SWM site plan, which demonstrates compliance with the requirements of this chapter.

2. SWM site plans approved by the Township, in accordance with § 25-406, shall be on site throughout the duration of the regulated activity.

3. The Township may, after consultation with DEP, approve measures for meeting the state water quality requirements other than those in this chapter, provided that they meet the minimum requirements of, and do not conflict with, state law, including, but not limited to, the Clean Streams Law. The Township shall maintain a record of consolations with DEP pursuant to this subsection.

4. 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. Various BMPs and their design standards are listed in the Erosion and Sediment Pollution Control Program Manual (E&S Manual),² No. 363-2134-008 (April 15, 2000), as amended and updated.

5. For all regulated activities, implementation of the volume controls in § 25-304 is required.

6. Impervious Areas.

[Amended by Ord. No. 2018-03, 5/2/2018]

A. For development taking place in stages, the entire development plan must be used in determining conformance with this chapter.

B. 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 § 25-304 and the peak rate controls of § 25-305 do not need to be retrofitted to existing impervious areas that are not being altered by the proposed regulated activity. An existing impervious area is considered to be altered if the regulated activity changes the runoff characteristics (flow, speed, direction, location) across and/or from the impervious area.

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

8. All regulated activities shall include such measures as necessary to:

A. Protect health, safety, and property.

B. Meet the water quality goals of this chapter, as stated in § 25-103, Purpose, by implementing measures to:

(1) Minimize disturbance to floodplains, wetlands, critical environmental areas and wooded areas.

(2) Maintain or extend riparian buffers.

(3) Avoid erosive flow conditions in natural flow pathways.

(4) Minimize thermal impacts to waters of this commonwealth.

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

(6) Minimize soil disturbance and compaction. Topsoil, if removed, shall be replaced to a minimum depth equal to its depth prior to removal.

C. To the maximum extent practicable, incorporate the techniques for low impact development practices described in the Pennsylvania Stormwater Best Management Practices Manual (BMP Manual).¹

9. The design of all facilities within an area of karst topography and/or carbonate geology shall include an evaluation of measures to minimize adverse effects, including hydrogeologic studies if required by the Township.

10. All natural streams, channels, swales, seeps, springs, drainage systems and/or areas of surface water concentration shall be maintained in their existing condition. All encroachment activities shall comply with the requirements of PA DEP 25 Pa. Code, Chapter 105, Water Obstructions and Encroachments. Any approvals or permits issued do not relieve compliance as referenced in § 25-106, Compatibility with Other Permit and Ordinance Requirements.

11. All stormwater management facilities (excluding individual residential underground infiltration facilities) are considered structures and must comply with building setback requirements. The outside toe of slope of the embankment in a fill condition or the top of embankment in a cut condition shall be considered as the point that must meet the setback requirements. Individual residential underground infiltration facilities shall be a minimum of 10 feet from the property line. Discharge of controlled flows can be no closer to an adjacent property than two times the length of the required discharge rip-rap apron. This requirement applies to discharge aprons that do not outlet to a defined waterway or an existing storm sewer. Minimum distance is 20 feet.

12. 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.

13. All storage facilities should completely drain both the volume control and rate control capacities over a period of time not less than 24 and not more than 72 hours from the end of the design storm. However, any designed infiltration at such facilities is exempt from the minimum twenty-four-hour standard, i.e., may infiltrate in a shorter period of time, so long as none of the exempted stormwater intended for infiltration is discharged into the surface waters of the commonwealth.

14. The design storm values to be used in the analysis of peak rates of discharge should be obtained from the Precipitation-Frequency Atlas of the United States, Atlas 14, Volume 2, Version 3.0, U.S. Department of Commerce, National Oceanic and Atmospheric Administration (NOAA), National Weather Service, Hydrometeorological Design Studies Center, Silver Spring, Maryland. NOAA's Atlas 145 can be accessed at: http://hdsc.nws.noaa.gov/hdsc/pfds/.

15. 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.

16. Various BMPs and their design standards are listed in the BMP Manual.¹

17. All regulated activities within wellhead protection zones, as established by the Township, shall be designed in accordance with the regulations contained in the Wellhead Protection Ordinance [Chapter 26, Part 1B] or other current land use regulations.

18. All work shall be in accordance with the Township's Construction and Material Specifications.

§ 25-302 Exemptions.

[Ord. 2011-09, 9/7/2011, § 302; as amended by Ord. No. 2018-03, 5/2/2018]

Any regulated activity that meets the following exemption criteria is exempt from the Part(s) of this chapter as specified herein. However, the requirements of the chapter shall otherwise remain in effect. The criteria for exemption in this section apply to the total development proposed, including instances in which the development is proposed to take place in phases. The date of enactment of this chapter shall be the starting point from which future development and the respective proposed impervious surface computations shall be cumulatively considered and regulated. Exemption shall not relieve an applicant from implementing such measures as necessary to meet the intent of this chapter, or compliance with any NPDES permit requirements.

A. For Residential Uses Within Residential Zoning Districts. Regulated activities that create impervious areas less than 500 square feet are exempt from the peak rate control and the SWM site plan preparation requirements of this chapter.

B. Regulated activities that create DIAs smaller than 5,000 square feet are exempt from the peak rate control and the SWM site plan preparation requirements of this chapter.

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

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

E. The use of land for domestic (i.e., for a single individual dwelling unit) gardening and landscaping is exempt from specific approval and permitting under this chapter so long as such activities comply with all other applicable ordinances and statutes.

F. Exemptions from any provisions of this chapter shall not relieve the applicant from the requirements in § 25-301, Subsections 4 through 14.

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

H. For all regulated activities that are exempt from the SWM site plan preparation, the applicant shall submit a site plan in sufficient detail to show the existing conditions and proposed improvements as well as design calculations.

§ 25-303 Volume Controls.

[Ord. 2011-09, 9/7/2011, § 303]

The low impact development practices provided in the BMP Manual¹ shall be utilized for all regulated activities to the maximum extent practicable. Water volume controls shall be implemented using the Design Storm Method in paragraph .A or the Simplified Method in paragraph .B below. For regulated activity areas equal or less than one acre that do not require hydrologic routing to design the stormwater facilities, this Chapter establishes no preference for either methodology; therefore, the applicant may select either methodology, with Township concurrence.

A. The Design Storm Method (CG-1 in the BMP Manual¹) is applicable to any size of regulated activity. This method requires detailed modeling based on site conditions.

(1) Do not increase the post development total runoff volume for all storms equal to or less than the two-year twenty-four-hour duration precipitation.

(2) For modeling purposes:

(a) Existing (predevelopment) nonforested pervious areas must be considered meadow.

(b) Twenty percent of existing impervious area, when present, shall be considered meadow in the model for existing conditions.

B. The Simplified Method (CG-2 in the BMP Manual¹) provided below is independent of site conditions and should be used if the Design Storm Method is not followed. This method is not applicable to regulated activities greater than one acre.

For new impervious surfaces:

(1) Stormwater facilities shall capture at least the first two inches of runoff from all new impervious surfaces.

(2) At least the first one inch of runoff from new impervious surfaces shall be permanently removed from the runoff flow; i.e., it shall not be released into the surface waters of this Commonwealth. Removal options include reuse, evaporation, transpiration, and infiltration.

(3) Wherever possible, infiltration facilities should be designed to accommodate infiltration of the entire permanently removed runoff; however, in all cases at least the first 0.5 inch of the permanently removed runoff should be infiltrated.

§ 25-304 Rate Controls.

[Ord. 2011-09, 9/7/2011, § 304]

1. For computation of pre-development peak discharge rates, 20% of existing impervious areas, when present, shall be considered meadow.

2. Post development discharge rates shall not exceed the pre-development discharge rates for the one-, two-, five-, ten-, twenty-five-, fifty- and one-hundred-year twenty-four-hour storms. If it is shown that the peak rates of discharge indicated by the post-development analysis are less than or equal to the peak rates of discharge indicated by the pre-development analysis for the one-, two-, five-, ten-, twenty-five-, fifty- and one-hundred-year twenty-four-hour storms, then the requirements of this Section have been met. Otherwise, the applicant shall provide additional controls as necessary to satisfy the peak rate of discharge requirement.

§ 25-305 Stormwater Management Facilities for Pennsylvania Department of Transportation and Pennsylvania Turnpike Commission Roadways and Associate Facilities.

[Ord. 2011-09, 9/7/2011, § 305]

1. For the purposes of the Act 167 Stormwater Management (Plan) elements, contained within the York County Integrated Water Resources Plan, and this Chapter, design policy pertaining to stormwater management facilities for Pennsylvania Department of Transportation (PennDOT) and Pennsylvania Turnpike Commission (PTC) roadways and associated facilities is provided in § 13.7, "Antidegradation and Post Construction Stormwater Management Policy," of PennDOT Publication No. 13M, Design Manual, Part 2 (August 2009), as developed, updated, and amended in consultation with the Pennsylvania Department of Environmental Protection (DEP). As stated in DM-2.13.7.D, "Act 167 and Municipal Ordinances," PennDOT and PTC roadways and associated facilities shall be consistent with Act 167 Plans. DM-2.13.7.B, "Policy on Antidegradation and Post Construction Stormwater Management," was developed as a cooperative effort between PennDOT and DEP. DM-2.13.7.0, "Project Categories," discusses the anticipated impact on the quality, volume and rate of stormwater runoff.

2. Where standards in the Act 167 elements of the IWRP and this Chapter are impractical, PennDOT or the PTC may request assistance from DEP, in consultation with the County, to develop an alternative strategy for meeting State water quality requirements and the goals and objectives of the Act 167 elements with the IWRP.

§ 25-306 Design Criteria.

[Ord. 2011-09, 9/7/2011, § 306]

1. Off-Site Areas. Off-site areas which drain through a proposed development site are not subject to release rate criteria when determining allowable peak runoff rates. However, on-site drainage facilities shall be designed to safely convey off-site flows through the development site.

2. On-Site Areas. On-site areas proposed to remain undisturbed as part of the regulated activity, including previously developed areas, that are not within the drainage area of the regulated activity shall be considered as existing conditions, without considering any reductions in cover type.

3. Downstream Hydraulic Capacity Analysis. Any existing downstream hydraulic capacity analysis shall be conducted in accordance with this Chapter.

A. All downstream facilities impacted by the total site area of the regulated activity shall be studied to determine if the facility has adequate capacity to handle existing and proposed flows. An impacted downstream facility is one to which the runoff from the total site area of the regulated activity comprises more than 50% of the total flow to such a facility. The study shall end at a perennial stream. Downstream facilities include, but are not limited to, man-made or natural swales and open channels, pipes, inlets, culverts, bridges and roadways.

B. If any private facility is found to be undersized, the applicant shall be responsible for updating the facility in coordination with the regulated activity.

C. If any public facility is found to be undersized or inadequate, the applicant shall work with the appropriate municipality on upgrading the facility in coordination with the regulated activity.

4. Regional Detention Alternatives. For certain areas within the study area, it may be more cost-effective to provide one control facility for more than one development site than to provide an individual control facility for each development site. The initiative and funding for any regional runoff control alternatives are the responsibility of prospective developers. The design of any regional control basins must incorporate reasonable development of the entire upstream watershed. The peak outflow of a regional basin would be determined on a case-by-case basis using the hydrologic model of the watershed consistent with protection of the downstream watershed areas. "Hydrologic model" refers to the calibrated model as developed for the stormwater management plan.

5. Capacity Improvements of Local Drainage Networks. In certain instances, local drainage conditions may dictate more stringent levels of runoff control than those based upon protection of the entire watershed. In these instances, if the developer could prove that it would be feasible to provide capacity improvements to relieve the capacity deficiency in the local drainage network, then the capacity improvements could be provided by the developer in lieu of runoff controls on the development site. Any capacity improvements would be designed based upon development of all areas tributary to the proposed improvement and the capacity criteria specified in § 25-308. In addition, all new development upstream of a proposed capacity improvement shall be assumed to implement the applicable runoff controls consistent with this Chapter except that all new development within the entire subarea(s) within which the proposed development site is located shall be assumed to implement the developer's proposed discharge control, if any.

6. Capacity improvements may also be provided as necessary to implement any regional or subregional detention alternatives.

7. Where the potential for groundwater and/or surface water contamination exists, based on the proposed use of the regulated activity, safeguards shall be incorporated into the site.

A. For industrial or commercial sites where it is possible that toxic or hazardous substances may come into contact with stormwater runoff, pretreatment of the first-flush (first 1/2 inch) runoff over areas where industrial and commercial operations take place shall be provided. Pretreatment shall include means for separating light and heavy nonaqueous phase liquids from the stormwater before the stormwater is conveyed to the general stormwater management facility(ies).

B. Infiltration systems may be used to handle runoff from commercial or industrial working or parking areas only after the first-flush stormwater from these areas has been pretreated for removal of toxic and hazardous substances. This prohibition does not extend to roof areas which are demonstrated to be suitably protected from the effects of the commercial/industrial activities.

8. Roof drains and sump pumps shall discharge to infiltration or vegetative BMPs and to the maximum extent practicable satisfy the criteria for DIAs.

§ 25-307 Regulations Governing Stormwater Management Facilities.

[Ord. 2011-09, 9/7/2011, § 307]

1. Any stormwater facility located on State highway rights-of-way shall be subject to approval by the Pennsylvania Department of Transportation (PennDOT).

2. Any stormwater management facilities regulated by this Chapter that would be located in or adjacent to waters of the Commonwealth or wetlands shall be subject to approval by PA DEP through the joint permit application process, or, where deemed appropriate by PA DEP, the general permit process. When there is a question whether wetlands may be involved, it is the responsibility of the developer or his agent to show that the land in question cannot be classified as wetlands, otherwise approval to work in the area must be obtained from PA DEP.

3. Any stormwater management facility located within the vicinity of a floodplain shall be subject to approval in accordance with PA DEP 25 Pa.Code, Chapter 105, "Floodplain Management," of PA DEP's Rules and Regulations.

4. All earthmoving activities must be reviewed and approved by the York County Conservation District prior to commencing work.

5. The design of all stormwater management facilities shall incorporate good engineering principles and practices. The Township shall reserve the right to disapprove any design that would result in the continuation of adverse hydrologic or hydraulic conditions within the watershed.

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

7. Areas of existing diffused drainage discharge shall be subject to any applicable discharge criteria in the general direction of existing discharge, whether proposed to be concentrated or maintained as diffused drainage areas, except as otherwise provided by this Chapter. If diffused flow is proposed to be concentrated and discharged onto adjacent property, the developer must document that adequate downstream conveyance facilities exist to safely transport the concentrated discharge, or otherwise prove that no erosion, sedimentation, flooding or other harm will result from the concentrated discharge.

8. Where a development site is traversed by watercourses, drainage easements shall be provided conforming to the line of such watercourses. The terms of the easement shall prohibit excavation, the placing of fill or structures, and any alterations that may adversely affect the flow of stormwater within any portion of the easement. Also, maintaining of vegetation in a natural state within the easement shall be required, except as approved by the appropriate governing authority.

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

10. Roof drains must not be connected to streets, sanitary or storm sewers or roadside ditches to promote overland flow and infiltration/percolation of stormwater where advantageous to do so. When it is more advantageous to connect directly to streets or storm sewers, then it shall be permitted on a case by case basis by the Township.

11. Special Requirements for Areas Falling Within Defined Exceptional Value and High Quality Subwatersheds. The temperature and quality of water and streams that have been declared as exceptional value and high quality is to be maintained as defined in Chapter 93, "Water Quality Standards," Title 25 of Pennsylvania Department of Environmental Protection Rules and Regulations. Temperature sensitive BMP's and stormwater conveyance systems are to be used and designed with storage pool areas and supply outflow channels and should be shaded with trees. This will require modification of berms for permanent ponds and the relaxation of restrictions on planting vegetation within the facilities, provided that capacity for volumes and rate control is maintained. At a minimum, the southern half on pond shorelines shall be planted with shade or canopy trees within 10 feet of the pond shoreline. In conjunction with this requirement, the maximum slope allowed on the berm area to be planted is 10 to 1. This will lessen the destabilization of berm soils due to root growth. A long term maintenance schedule and management plan for the thermal control BMP's is to be established and recorded for all development sites within defined Exceptional Value and/or High Quality Subwatersheds.

§ 25-308 Calculation Methodology.

[Ord. 2011-09, 9/7/2011, § 308]

1. Stormwater runoff from all development sites shall be calculated using the Rational Method, Modified Rational Method, or a Soil Cover Complex methodology.

A. Any stormwater runoff calculations involving drainage areas greater than 200 acres, including on- and off-site areas, shall use generally accepted calculation technique that is based on the NRCS Soil Cover Complex method. It is assumed that all methods will be selected by the design professional based on the individual limitations and suitability of each method for a particular site.

B. The Township may allow the use of the Rational Method or Modified Rational Method to estimate peak discharges from drainage areas that contain less than 200 acres.

C. All calculations consistent with this Chapter using the Soil Cover Complex method shall us the appropriate design rainfall depths. If a hydrologic computer model such as PSRM or HEC-RAS is used for stormwater runoff calculations, then the duration of rainfall shall be 24 hours. The SCS Rainfall Type II curve shall be used for the rainfall distribution.

D. For the purposes of pre-development flow rate determination, undeveloped land, including disturbed areas, shall be considered as "meadow" in good condition, unless the natural ground cover generates a lower curve number or Rational "C" value (i.e., forest), as listed in Tables 25-1 and 25-2, respectively.

E. All calculations using the Rational Method shall use rainfall intensities consistent with appropriate times-of-concentration for overland flow and return periods. Times-of-concentration for overland flow shall be calculated using the methodology presented in Chapter 3 of Urban Hydrology for Small Watersheds, NRCS, TR-55 (as amended or replaced from time to time by NRCS). Time-of-concentration for channel and pipe flow shall be computed using Manning's equation.

F. Runoff Curve Numbers (CN) for both existing and proposed conditions to be used in the Soil Cover Complex method shall be obtained from Table 25-1.

G. Runoff coefficients (c) for both existing and proposed conditions for use in the Rational Method shall be obtained from Table 25-2.

H. Where uniform flow is anticipated, the Manning equation shall be used for hydraulic computations such as the capacity of open channels, pipes, and storm sewers. Values for Manning's roughness coefficient (n) shall be consistent with Table 25-3.

I. The design of any stormwater detention facilities intended to meet the performance standards of this Chapter shall be verified by routing the design storm hydrograph through these facilities, using either manual methods or computerized routing. Routing shall be based upon the modified PULS method; other routing methodologies shall be subject to the approval of the Township Engineer.

J. The stormwater collection system shall be designed using the peak discharge computed using the Rational Formula.

2. Design Standards-Water Carrying Facilities.

A. All storm sewer pipes, streets, and inlets (excluding detention and retention basin outfall structures) shall be designed for a ten-year storm event. Sole access structures (culverts and bridges) shall be designed to convey the twenty-five- year flood without overtopping the roadway.

(1) When a pipe or culvert is intended to convey the discharge from a stormwater management facility, its required capacity shall be computed by the Rational Method and compared to the peak outflow from the stormwater facility for the one-hundred-year storm. The greater flow shall govern the design of the pipe or culvert.

(2) When a pipe is part of a storm sewer system and crosses the roadway, it shall be designed as a storm sewer with the same design storm as the remainder of the drainage system.

(3) Greater design frequencies may be justified on individual projects.

(4) A one-hundred-year storm frequency may be required for design of the stormwater collection system to insure that the resultant stormwater runoff from the post-development storm is directed into the management facility.

B. In general, inlets shall be spaced such that, based upon the Rational Method, tc = five min. and ten-year rainfall intensity, the area contributing to the inlet shall not produce a peak runoff of greater than 4 cfs. Also, inlets shall be spaced so that their efficiency, based upon efficiency curves published by the Pennsylvania Department of Transportation, is not less than 65%.

C. Inlets shall be placed on both sides of the street at low spots and at the upper side of street intersections to prevent stormwater from crossing an intersection. Other devices such as high efficiency grates or perforated pipe may be required if conditions warrant. All inlets at low points along the roadway shall have a ten-inch curb reveal and shall be equipped with pavement base drain extending 50 feet in either direction, parallel to the centerline of the roadway.

D. In all cases where drainage is picked up by means of a headwall, the pipe shall be designed as a culvert. Inlet and outlet conditions shall be analyzed. The minimum diameter of culvert shall be 18 inches. The procedure contained in Hydraulic Engineer Circulars No. 5 and No. 13, as prepared by the U.S. Department of Transportation, Federal Highway Administration, Washington, DC, shall be used for the design of culverts. All culverts shall include concrete headwalls and endwalls.

E. Guards shall be provided on all intake and outfall structures as well as outlet structures. The guard bars shall be 1/2-inch diameter galvanized bars on six-inch centers attached to the structure with 3/8-inch diameter stainless steel anchors. Guards shall also be provided for any pipe opening, 18 inches in diameter or larger.

F. Manholes, inlets, headwalls, and endwalls shall conform to the requirements of the PennDOT Publication 408, as modified by the adopted Township standards.

G. Proposed channels or swales must be able to convey the increased runoff associated with a proposed one-hundred-year return period event within their banks at velocities consistent with protection of the channels from erosion. Acceptable velocities shall be based upon criteria included in the PA DEP Erosion and Sediment Pollution Control Program Manual.

H. Existing natural or man-made channels or swales must be able to convey proposed one-hundred-year return period runoff without creating any hazard to persons or property.

I. Stormwater runoff on roadways (i.e., gutter spread, lane encroachment, etc.) shall be controlled in accordance with PennDOT Publications 13M, "Design Manual, Part 2" and 584, "Drainage Manual."

3. Design Standards-Detention and Retention Basins.

A. Permanent detention and retention basins shall be designed to meet the following standards:

(1) The maximum permitted depth for detention or retention basins shall be six feet, measured from the bottom of the emergency spillway to the lowest point in the basin.

(2) The minimum top width of all basin embankments shall be eight feet.

(3) The maximum permitted side slopes for detention or retention basins shall be four horizontal to one vertical. In order to obtain a waiver for slopes steeper than 4:1, the plan must include a planting schedule to stabilize the embankments. The proposed vegetation shall be low maintenance varieties. Landscaping shall conform to the requirements of the Subdivision and Land Development Ordinance landscaping requirements (Article 700) and all plans shall be sealed by a landscape architect registered in Pennsylvania.

(4) Any stormwater management facility (i.e., detention basin) designed to store runoff and requiring a berm or earthen embankment required or regulated by this Chapter shall be designed to provide an emergency spillway to handle flow up to and including the one-hundred-year, twenty-four-hour design storm at post-development conditions, assuming the principal outlet structure to be clogged. The height of embankment must be set as to provide a minimum 0.5 feet of freeboard above the maximum elevation computed. Should any stormwater management facility require a dam safety permit under PA DEP 25 Pa.Code, Chapter 105, the facility shall be designed in accordance with PA DEP 25 Pa.Code, Chapter 105, and meet the regulations of PA DEP 25 Pa.Code, Chapter 105, concerning dam safety which may be required to pass storms larger than one-hundred-year event.

(5) A cutoff trench of impervious material shall be provided within all basin embankments.

(6) Where a basin embankment is constructed using fill on an existing 15% or greater slope, the basin must be keyed into the existing grade.

(7) (Reserved)[1]

[1] Editor's Note: Former Subsection 3A(7), regarding fencing, was repealed by Ord. No. 2023-03, 5/3/2023.

(8) All outlet structures and emergency spillways shall include a satisfactory means of energy dissipation at its outlet to assure conveyance and flow without endangering the safety and integrity of the basin and the downstream drainage area.

(9) A concentrated discharge of stormwater to an adjacent property shall be within a natural drainage way or watercourse, or an easement shall be required.

(10) Plans for infiltration must show the locations of existing and proposed septic tank infiltration areas and wells. A minimum twenty-five-foot separation from on-lot disposal systems (OLDS) infiltration areas, including replacement areas, is desired and will be evaluated by the Township on a case by case basis. However, the separation shall not be less than the PA DEP required 10 feet. Infiltration rates shall be based upon perk and probe tests conducted at the site of the proposed facility.

§ 25-309 Carbonate Geology.

[Ord. 2011-09, 9/7/2011, § 309]

1. In areas of carbonate geology, a geologist shall certify to the following:

A. No stormwater management facility will be placed in, over, or immediately adjacent to the following features:

(1) Closer than 100 feet from sinkholes.

(2) Closer than 100 feet from closed depressions.

(3) Closer than 100 feet from caverns, intermittent lakes, or ephemeral streams.

(4) Closer than 50 feet from lineaments in carbonate areas.

(5) Closer than 50 feet from fracture traces.

(6) Closer than 25 feet from bedrock pinnacles (surface or subsurface).

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

C. If the developer can prove through analysis that the project site is an area underlain by carbonate geology, and such geologic conditions may result in sinkhole formations, then the project site is exempt from recharge requirements as described in § 25-303, "Volume Control." However, the project site shall still be required to meet all other standards found in this Chapter.

D. It shall be the developer's responsibility to verify if the project site is underlain by carbonate geology. The following note shall be attached to all stormwater management plans and signed and sealed by the developer's geologist:

"I, ________________________, certify that the proposed stormwater management facility (circle one) is/is not underlain by carbonate geology."

E. Whenever a stormwater management facility will be located in an area underlain by carbonate geology, a geological evaluation of the proposed location by a geologist shall be conducted to determine susceptibility to sinkhole formation. The evaluation may include the use of impermeable liners to reduce or eliminate the separation distances listed in the BMP Manual. Additionally, the evaluation shall, at a minimum, address soil permeability, depth to bedrock, seasonally high groundwater table, susceptibility for sinkhole formation, suitability of stormwater management facilities, subgrade stability and maximum infiltration capacity in depth of water per unit area.

F. A detailed soils evaluation of the project site shall be performed to determine the suitability of recharge facilities. The evaluation shall be performed by a qualified professional, and at a minimum, address soil permeability, depth to bedrock, susceptibility to sinkhole formation, and subgrade stability. The general process for designing the infiltration BMP shall be:

(1) Site evaluation to determine general areas of suitability for infiltration practices.

(2) Provide field test throughout the area proposed for development to determine appropriate percolation rate and/or hydraulic conductivity. At least one infiltration test must be included in each soil group and at least one infiltration test must be conducted for each five lots proposed for development. Infiltration tests must be taken at the location and depth of all proposed infiltration structures.

(3) Design infiltration structure for required storm volume based on all available data.

G. Extreme caution shall be exercised where infiltration is proposed in geologically susceptible areas such as strip mine or limestone areas It is also extremely important that the design professional evaluate the possibility of groundwater contamination from the proposed infiltration/recharge facility and recommend a hydrogeologic justification study be performed if necessary. Whenever a basin will be located in an area underlain by limestone, a geological evaluation of the proposed location shall be conducted to determine susceptibility to sinkhole formations. The design of all facilities over carbonate formations shall include measures to prevent ground water contamination and, where necessary, sinkhole formation. The infiltration requirement in the high quality/exceptional waters shall be subject to the Department's 25 Pa.Code, Chapter 93, and anti-degradation regulations. A detailed hydrogeologic investigation may be required by the Township and where appropriate, the Township may require the installation of an impermeable liner in detention basins.

H. All stormwater BMPs proposed within wellhead protection areas, or in close proximity to public wells, shall be in accordance with the Wellhead Protection Ordinance [Chapter 26, Part 1B] as well as Chapter 7, "Special Management Areas" of the BMP Manual.

§ 25-310 Erosion and Sedimentation Control Requirements.

[Ord. 2011-09, 9/7/2011, § 310]

1. As required in § 25-301.4, whenever the vegetation and topography are to be disturbed, such activity must be in conformance with PA DEP 25 Pa.Code, Chapter 105, Rules and Regulations, Part I, Subpart C, Protection of Natural Resources, Article II, Water Resources, Chapter 102, "Erosion Control," and in accordance with the York County Conservation District.

2. It is extremely important that strict erosion and sedimentation control measures be applied surrounding infiltration structures during installation to prevent the infiltrative surfaces from becoming clogged. Additional erosion and sedimentation control design standards and criteria must be applied where infiltration BMPs are proposed shall include the following:

A. Areas proposed for infiltration BMPs shall be protected from sedimentation and compaction during the construction phase, so as to maintain their maximum infiltration capacity.

3. (Reserved)[1]

[1] Editor's Note: Former Subsection 3, regarding fencing, was repealed by Ord. No. 2023-03, 5/3/2023.

4. The developer shall demonstrate that the post-development hydrograph flows during erosion and sedimentation control phase are less than or equal to the pre- development hydrograph flows to assure the rate and volume of runoff leaving the site is controlled for the two-, five-, and ten-year frequency storms. All calculation methodology shall be in accordance with §§ 25-303 through 25-309.