Source: https://ecode360.com/28037976
Timestamp: 2019-04-20 23:04:32+00:00

Document:
Applicants proposing regulated activities in the municipality which are not exempt under § 152-106 shall submit a stormwater management site plan (SWM site plan) to the municipality for review and approval in accordance with Articles III and IV. SWM site plans approved by the municipality shall be on site throughout the duration of the regulated activity.
The stormwater management and runoff control criteria and standards in this chapter shall apply to the total proposed regulated activity, even if it is to take place in stages. The measurement of impervious surfaces shall include all of the impervious surfaces in the total proposed regulated activity even if the development is to take place in stages.
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 a SWM site plan under the provisions herein nor compliance with the provisions of this chapter shall relieve any person from responsibility for damage to any person or property otherwise imposed by law.
The applicant shall design the site to minimize disturbances to land, site hydrology, and natural resources, and to maintain the natural hydrologic regime, drainage patterns and flow conditions. The applicant shall apply the procedures set forth in § 152-304 for the overall site design and for selection, location and design of features and BMPs to be used to comply with the requirements of this chapter.
To the maximum extent practicable, postconstruction stormwater shall be discharged within the drainage area of the same stream or water body receiving the runoff prior to construction of the proposed regulated activity.
For regulated activities with one acre or more of proposed earth disturbance, existing drainage peak rate discharges up to and including the one-hundred-year storm onto or through adjacent property(ies) or downgradient property(ies), including diffuse drainage discharge, shall not be altered in any manner without written permission from, and, where applicable as determined by the municipality an easement and agreement with, the affected landowner(s) for conveyance of discharges onto or through their property(ies). Such discharge shall be subject to any applicable discharge criteria specified in this chapter.
Areas located outside of the site (i.e., areas outside of the regulated activity) that drain through a proposed site are not subject to water quality and volume control, infiltration, stream channel protection, or peak flow rate control requirements (as presented in §§ 152-305, 152-306, 152-307 and 152-308). Drainage facilities located on the site shall be designed to safely convey flows from outside of the site through the site.
If site conditions preclude capture of runoff from limited portions of the disturbed area for achieving water quality volume control standards, stream channel protection standards, and the two-year, five-year and ten-year storm event peak runoff rate reduction standards for new development required by this chapter, the applicant shall propose alternate methods to mitigate the bypass of the BMPs, subject to the approval of the municipality. In no case shall resulting peak rate be greater than the predevelopment peak rate for the equivalent design storm.
For all regulated activities, erosion and sediment control BMPs shall be designed, implemented, operated, and maintained during the regulated activities (i.e., during construction) as required to meet the purposes and requirements of this chapter, to meet the erosion and sediment control requirements of the municipality, if applicable, and to meet all requirements under Title 25 of the Pa. Code and the Clean Streams Law.
For all regulated activities, permanent BMPs and conveyances shall be designed, implemented, operated, and maintained to meet the purposes and requirements of this chapter and to meet all requirements under Title 25 of the Pennsylvania Code, the Clean Streams Law, and the Storm Water Management Act.
Editor's Note: See 35 P.S. § 691.1 et seq. and 32 P.S. § 680.1 et seq., respectively.
The design of all BMPs and conveyances shall incorporate sound engineering principles and practices in a manner that does not aggravate existing stormwater problems as identified by the 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).
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.
Other land uses and activities as designated by the municipality.
Contaminated and brownfield sites. Where BMPs may contribute to the migration of contaminants in groundwater, the water quality and runoff volume, stream channel protection, and peak rate control standards shall be met; however, at the municipality's discretion, the minimum infiltration requirement may be reduced or eliminated commensurate with the contaminated area and the required water quality and runoff control measures may be increased to mitigate the reduced infiltration requirement for the contaminated area.
Water bodies listed as "impaired" by PADEP.
Any water body or watershed with an approved total maximum daily load (TMDL).
Areas of known existing flooding problems.
Critical areas with sensitive resources (e.g., state-designated special protection waters, cold-water fisheries, carbonate geology or other groundwater recharge areas that may be highly vulnerable to contamination, drainage areas to water supply reservoirs, etc.).
Applicants shall utilize the Pennsylvania Stormwater Best Management Practices Manual (PA BMP Manual), as amended, or other sources acceptable to the municipality, for testing and design standards for BMPs, and where there is a conflict with the provisions of this chapter, the most restrictive applies.
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 or other design guidance acceptable to the municipality.
All regulated activities located within a special flood hazard area designated by the Federal Emergency Management Agency (FEMA) shall comply with Zoning Ordinance Article XIII, Flood Hazard District, 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.
Editor's Note: See Ch. 200, Art. XIII.
Disturbance of existing ground cover during construction of the proposed regulated activity is prohibited within 75 feet of top-of-bank of all perennial and intermittent waterways, water bodies (lakes, ponds, etc.) and wetlands, except for activities otherwise approved by state or local agencies (e.g., stream restoration projects, road crossings, subsurface utility projects, etc.).
All regulated activities subject to permit or regulatory requirements by PADEP under regulations at Title 25 Pennsylvania Code Chapter 102, or erosion and sediment control requirements of the municipality.
Work within natural drainageways subject to permit by PADEP under Title 25 Pennsylvania Code Chapter 105.
Any BMP or conveyance that would be located in or adjacent to surface waters of the commonwealth, including wetlands, subject to permit by PADEP under Title 25 Pennsylvania Code Chapter 105.
Any BMP or conveyance that would be located on or discharge to a state highway right-of-way, or require access to or from a state highway and be subject to approval by PennDOT.
Culverts, bridges, storm sewers, or any other facilities which must pass or convey flows from the tributary area and any facility which may constitute a dam subject to permit by PADEP under Title 25 Pennsylvania Code Chapter 105.
An erosion and sediment control plan has been approved by the municipality, if required.
A copy of the erosion and sediment control plan and any required permit(s), as required by PADEP regulations, shall be available on the site at all times.
Additional erosion and sediment control measures shall be applied where infiltration BMPs are proposed, at a minimum including those required in § 152-306M.
First, identify and delineate all existing natural resources and natural and man-made hydrologic features listed in § 152-402B(8) that are located within the site, or receive discharge from, or may be impacted by the proposed regulated activity.
Those that may be considered for alteration, disturbance or removal.
Consider additional guidance for incorporating natural hydrology into the site and BMP designs, methods and techniques that support the objectives of Subsections B and C. Appendix A presents additional discussion of natural hydrology site design and sources of information for conservation design, low-impact design and sustainable design.
The procedures set forth above shall be utilized to the maximum extent practicable for the overall site design and selection, location and design of features and BMPs to be used to comply with the requirements of §§ 152-305, 152-306, 152-307 and 152-308.
The postconstruction total runoff volume shall not exceed the predevelopment total runoff volume for all storms equal to or less than the two-year, twenty-four-hour duration precipitation (design storm). The water quality and runoff volume to be managed shall consist of any runoff volume generated by the proposed regulated activity over and above the predevelopment total runoff volume and shall be captured and permanently retained or infiltrated on the site. Permanent retention options may include, but are not limited to, reuse, evaporation, transpiration, and infiltration.
For modeling purposes, the predevelopment ground cover conditions shall be determined using the corresponding ground cover assumptions presented in § 152-309D of this chapter.
The design of the 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 § 152-306. The infiltration volume required under § 152-306 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 § 152-301P.
When the regulated activity contains or is divided by multiple drainage areas, the water quality and runoff volume shall be separately addressed for each drainage area.
Weighted averaging of runoff coefficients shall not be used for manual computations or input data for water quality and runoff volume calculations.
Areas located outside of the site (i.e., areas outside of the regulated activity) may be excluded from the calculation of the water quality and runoff volume requirements.
Water quality and volume control practices shall be selected and designed to meet the criteria of § 152-304C that apply to water quality and volume control.
Wherever possible, infiltration should be designed to accommodate the entire water quality and runoff volume required in § 152-305.
For regulated activities involving new development, the volume of a minimum of one inch of runoff from all proposed impervious surfaces shall be infiltrated.
The total water quality and runoff volume required in § 152-305 of this chapter.
If the requirements of Subsection B or C cannot be physically accomplished, then the applicant shall be responsible for demonstrating with data or calculations to the satisfaction of the municipality why this infiltration volume cannot be physically accomplished on the site (e.g., shallow depth to bedrock or limiting zone, open voids, steep slopes, etc.) and what alternative volume can be infiltrated; however in all cases at least the first 0.5 inch of runoff volume shall be infiltrated.
Only if a minimum of at least 0.5 inch infiltration requirement cannot be physically accomplished on the site shall a waiver from § 152-306 be considered by the municipality.
If site conditions preclude capture of runoff from portions of the impervious surfaces, the infiltration volume for the remaining area shall be increased an equivalent amount to offset the loss.
When a project contains or is divided by multiple watersheds, the infiltration volume shall be separately addressed for each watershed.
Existing impervious surfaces located in areas outside of the site (i.e., outside of the regulated activity) may be excluded from the calculation of the required infiltration volume.
Provide field tests such as double-ring infiltrometer or other hydraulic conductivity tests (at the elevation of the proposed infiltration surface) to determine the appropriate hydraulic conductivity rate. Standard septic/sewage percolation tests are not acceptable for design purposes.
Design the infiltration facility for the required retention (infiltration) volume based on field-determined infiltration capacity (and apply safety factor as per applicable design guidelines) at the elevation of the proposed infiltration surface.
On-lot infiltration features are encouraged; however, it shall be demonstrated to the municipality that the soils are conducive to infiltration on the identified lots.
A minimum depth of 24 inches between the bottom of the BMP and the top of the limiting zone. Additional depth may be required in areas underlain by karst or carbonate geology (see Subsection N).
An infiltration rate sufficient to accept the additional stormwater volume and drain completely as determined by field tests conducted by the applicant.
The infiltration facility shall completely drain the retention (infiltration) volume within three days (72 hours) from the end of the design storm.
For any infiltration practice that collects runoff from shared or multiple features and that is located within 25 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.
When a hotspot is located in the area draining to a proposed infiltration facility, an evaluation of the potential of groundwater contamination from the proposed infiltration facility shall be performed, including a hydrogeologic investigation (if necessary) by a qualified licensed professional to determine what, if any, pretreatment or additional design considerations are needed to protect groundwater quality.
When located within a "wellhead protection area" of a public water supply well, infiltration practices shall be in conformance with the applicable approved source water protection assessment or source water protection plan.
The applicant shall provide appropriate safeguards against groundwater contamination for land uses that may cause groundwater contamination should there be a mishap or spill.
During site construction, all infiltration practice components shall be protected from compaction due to heavy equipment operation or storage of fill or construction material. Infiltration areas shall also be protected from sedimentation. Areas that are accidentally compacted or graded shall be remediated to restore soil composition and porosity. Adequate documentation to this effect shall be submitted to the municipality for review. All areas designated for infiltration shall not receive runoff until the contributory drainage area has achieved final stabilization.
Consideration of infiltration BMPs for areas underlain by karst or carbonate geology is encouraged, but only where the design, supporting calculations, results of soils or other site investigations or other documentation are provided to the municipality demonstrating that the potential or likelihood of subsidence or sinkholes is minimal. Evaluation of site conditions and infiltration design shall rely on guidance in the PA BMP Manual (as amended) or other guidance acceptable to the municipality.
Groundwater quality of the carbonate aquifer shall be protected from infiltration of pollutants. At a minimum, stormwater runoff from hotspots (i.e., sources of significant pollutant runoff) shall first be discharged through a water quality BMP(s) to remove pollutants prior to infiltration. Where soil characteristics are insufficient to provide removal of pollutants from sources other than hotspots, stormwater runoff shall first be discharged through a water quality BMP(s) to remove pollutants prior to infiltration.
Where sediment transport in the stormwater runoff is anticipated to reach the infiltration system, appropriate permanent measures to prevent or collect sediment shall be installed prior to discharge to the infiltration system.
Where roof drains are designed to discharge to infiltration practices, they shall have appropriate measures to prevent clogging by unwanted debris (for example, silt, leaves and vegetation). Such measures shall include but are not limited to leaf traps, gutter guards and cleanouts.
All infiltration practices shall have appropriate positive overflow controls.
No sand, salt or other particulate matter may be applied to a porous surface material for winter ice conditions.
Excavation for the infiltration facility shall be performed with equipment that will not compact the bottom of the seepage bed/trench or like facility.
The bottom of the bed and/or trench shall be scarified prior to the placement of aggregate.
Only clean aggregate with documented porosity, free of fines, shall be allowed.
The tops, bottoms and sides of all seepage beds, trenches, or like facilities shall be covered with drainage fabric. Fabric shall be nonwoven fabric acceptable to the municipality.
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.
The peak flow rate of the postconstruction 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 municipality, the postconstruction 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).
The minimum orifice size in the outlet structure to the BMP shall be three inches in diameter unless otherwise approved by the municipality, 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 municipality's satisfaction. When filters are utilized, maintenance provisions shall be provided to ensure filters meet the design function.
All proposed stormwater facilities shall make use of measures to extend the flow path and increase the travel time of flows in the facility.
When a regulated activity contains or is divided by multiple drainage areas, the peak flow rate control shall be separately addressed for each drainage area.
The applicant shall comply with the following peak flow rate control requirements for all regulated activities including those that involve new development and redevelopment.
Postconstruction 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 308.1.
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 308.1, using the redevelopment ground cover assumptions presented in § 152-309D.
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.
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 309.1 summarizes acceptable computation methods. The method selected for use shall be based on the individual limitations and suitability of each method for a particular site. The use of the Rational Method to estimate peak discharges for drainage areas greater than five acres shall be permitted only upon approval by the municipality.
Applicable where use of full hydrology computer model is desirable or necessary.
Applicable for land development plans where limitations described in TR-55 are met.
Applicable where use of a full hydrologic computer model is desirable or necessary.
For sites up to five acres, or as approved by the municipality.
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.
For areas that are woods (as defined in Article II of this chapter), predevelopment calculations shall assume ground cover of "woods in good condition."
For all other areas (including all impervious surfaces), predevelopment calculations shall assume ground cover of "meadow."
For areas that are not woods or not impervious surfaces, predevelopment calculations shall assume ground cover of "meadow."
For areas that are impervious surfaces, predevelopment calculations shall assume at least 20% of the existing impervious surface area to be disturbed as "meadow" ground cover.
Stormwater standards for new development shall apply to all proposed regulated activities that involve only new development activities as defined in this chapter.
Stormwater standards for redevelopment shall apply to all proposed regulated activities that involve only redevelopment activities as defined in this chapter.
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 (postconstruction) conditions to be used in the Soil Cover Complex Method shall be obtained from Table B-1 in Appendix B of this chapter.
Editor's Note: Appendix B is included at the end of this chapter.
Runoff coefficients (C) for both predevelopment and proposed (postconstruction) conditions for use in the Rational Method shall be obtained from Table B-2 in Appendix B of this chapter.
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 B-3 in Appendix B of the chapter.
Average antecedent moisture conditions (for the Soil Cover Complex Method only, for example, TR-55, TR-20).
A Type II distribution storm (for the Soil Cover Complex Method only, for example, TR-55, TR-20).
Any BMP intended to hold standing water for four days or longer shall be designed to incorporate biologic controls consistent with the West Nile Guidance found in Appendix C, PADEP Document 363-0300-001 "Design Criteria – Wetlands Replacement/Monitoring," as amended (or contact the Pennsylvania State Cooperative Wetland Center or the Penn State Cooperative Extension Office for design information).
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 one-hundred-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 one-hundred-year proposed conditions inflow. Should any BMP require a dam safety permit under Pa. Chapter 105 regulations, the facility shall be designed in accordance with and meet the regulations of Pa. Chapter 105 concerning dam safety. Pennsylvania Chapter 105 may require the safe conveyance of storms larger than one-hundred-year event.
Editor's Note: See 25 Pa. Code Chapter 105.
Any drainage conveyance facility and/or channel not governed by Pa. Chapter 105 regulations shall be designed to convey, without damage to the drainage facility or roadway, runoff from the twenty-five-year storm event. Larger storm events (fifty-year and one-hundred-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 one-hundred-year design storm consistent with Federal Emergency Management Agency National Flood Insurance Program – Floodplain Management Requirements.
Any 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 municipality.
Storm drainage requirements and facility design standards.
Stormwater management system required. A stormwater management system designed to achieve the objectives of this section, including a storm sewer system and all appurtenances where necessary, shall be required to be constructed by the owner or subdivider consistent with the provisions of this section and shall, in particular, be designed to avoid any situation where surface or subsurface drainage could impair public safety or cause physical damage to adjacent lands or public property. The system, after proper management in accordance with this chapter, shall discharge to the nearest practical subsurface location in order to recharge groundwater, to a natural drainage channel, and/or to an existing stormwater management system. The applicant shall show evidence of consideration of all discharge options, including documentation as to why the chosen discharge location(s) was/were selected.
Off-site improvements. Any increase in runoff off site, measured both in total volume and peak rate between pre- and postdevelopment conditions, shall require necessary corrective measures deemed appropriate by the Board of Supervisors to alleviate any off-site drainage problem affected by the subdivision or land development. The costs for such measures will be borne entirely by the developer or subdivider, including deed of easement, wherever applicable.
Total watershed evaluation. The future use of undeveloped lands upstream of a subject property, as are shown on the Future Land Use Plan of the adopted Township Comprehensive Plan, shall be taken into account in the design of all stormwater management systems. The capacity and maximum anticipated present flow of the system receiving the proposed discharge shall be evaluated to determine whether it is capable of receiving additional flow generated by proposed and potential future development. This evaluation shall also include the anticipated runoff from other existing or proposed developments within the same watershed. Upon the recommendation of the Township Engineer, stormwater management facilities, designed in accordance with this chapter, may serve more than one property or development in the same watershed. Consultation with the Township is required prior to the design of such facilities.
Underdrains. All building foundations, grade slabs, and cellar floors located in soils that have a community development limitation degree of moderate to severe seasonal high water table (as defined in the Chester County Soil Survey) shall be provided with an underdrain system. This system shall provide for drainage of the enclosed volume above the slab, and relief of subsurface water to a depth of not less than 18 inches below the slab or foundation bottom. The system shall consist of a pipe field of the herringbone or gridiron configuration in course, gravel-filled trenches that are in direct contact with the slab or foundation subbase. The excavation shall provide a minimum of 0.05 foot/foot slope to the gravel-filled trenches. The underdrain system(s) shall discharge to location(s) sufficiently downgradient of the building being drained and other nearby buildings in order so as not to exacerbate high water table problems.
Contribution to comprehensive drainage plan. In addition to any other requirement of this chapter, the developer may be required to participate in off-site storm drainage improvements in the drainage basin within which the proposed development is located. The specific off-site drainage improvements required shall be those specified by the Township Board of Supervisors.
Storm drain pipes and inlets. Except where alternative design approaches are recommended by the Township Engineer, a storm drain system shall be designed to collect water at any point where three cubic feet per second to five cubic feet per second accumulates, at the bottom of all vertical grades, and immediately upgrade from all street intersections.
Design flow rate. Storm drain system(s), where provided, shall be designed to carry a twenty-five-year peak flow rate, and a fifty-year peak flow rate at the sump area. The design twenty-five-year peak flow rate into each inlet shall be indicated on the stormwater drainage plan.
Overflow system. An overflow system shall be provided to carry storm flow when the capacity of the storm drainpipe system is exceeded. The overflow system shall be of sufficient capacity to carry the difference between the one-hundred-year and the ten-year peak flow rates.
The capacity of each inlet shall be indicated on the stormwater drainage plan.
All inlet boxes shall be prefabricated concrete. The pipe locations shall be precut.
All inlet grates shall be bicycle-safe grates.
All inlet grates shall be flush with the top of the wearing course.
All inlet risers shall be prefabricated concrete.
All pipe within the paved cartway shall be reinforced concrete or high-density polyethylene pipe.
All pipe outside of the paved cartway can consist of interior polyethylene pipe.
Straight pipe sections. Wherever possible, all storm drain pipes shall be designed to follow straight courses. No angular deflections of storm sewer pipe sections in excess of 5° shall be permitted. No vertical curves shall be permitted in the storm drain pipe system.
Minimum grade and size. All storm drain pipes shall be designed to maintain a minimum grade of 1/2%. All storm pipes shall have a minimum inside diameter of 15 inches, except that pipes under 25 feet or more of fill, where permitted, shall not be less than 24 inches or a cross-sectional area of 453 square inches.
Pipe arches. Where headroom is restricted, equivalent pipe arches may be used in lieu of circular pipes.
Allowable headwater depth. At all inlets or manholes, the maximum allowable headwater depth shall be one foot below the top of the inlet grate of the manhole cover.
Horizontal pipe deflections. A manhole or inlet shall be provided at all horizontal deflections in the storm pipe system exceeding 5°.
Minimum and maximum cover. A minimum of 18 inches of cover shall be maintained over all storm drain pipes. The top of storm drain pipes shall be at least 1/2 foot below subgrade elevation. The maximum cover over storm drainpipes shall be 10 feet except where, due to specific site conditions, approval of greater cover (fill) is recommended by the Township Engineer.
Diversion or runoff. All storm drain pipes shall be designed to carry stormwater runoff into detention, retention or infiltration facilities utilized to control the rate and volume of stormwater runoff.
The top of all inlet grating shall be designed to be sumped two inches from the road wearing surface.
All storm sewers in dedicated rights-of-way shall be reinforced concrete pipes.
Maximum depth of detention or retention basins. The maximum depth of water in a detention or retention basin shall be two feet as a result of a two-year, twenty-four-hour storm event and five feet as a result of a one-hundred-year, twenty-four-hour storm event.
An acceptable outfall shall be provided at the outlet of all detention basins and shall not be permitted to discharge onto adjacent properties.
Emergency spillway. All stormwater flow into any permanent detention, retention or infiltration facility exceeding design capacity shall be handled by an emergency spillway. Whenever possible, any emergency spillway shall be constructed on undisturbed ground. Emergency spillways shall be paved with concrete monoslab pavers which shall run off the sides and down each side of the berm. All emergency spillways shall be constructed so that any earthen berms are protected against erosion. The minimum capacity of all emergency spillways shall be such that the combined capacity of the emergency spillway and the principal outfall provide for the passage of the peak flow rate from the one-hundred-year, twenty-four-hour storm event with a flow depth across the spillway not greater than one foot. The dimensions of the emergency spillways can be determined from the Environmental Protection Handbook – Chester County Conservation District. Emergency spillways shall extend along the upstream and downstream berm embankment slopes. The upstream edge of the emergency spillway shall be a minimum of two feet below the crest evaluation of the subject stormwater management facility. The downstream slope of the spillway shall, as a minimum, extend to the toe of any affected embankment. The emergency spillway shall not discharge directly over earthen fill and/or other easily erodible material.
Antiseep collars. Antiseep collars shall be installed around the principal pipe barrel within the normal saturation zone of the detention basin berms. The antiseep collars and their connections to the pipe barrel shall be watertight. The antiseep collars shall extend a minimum of two feet beyond the outside of the principal pipe barrel. The maximum spacing between collars shall be 14 times the minimum projection of the collar measured perpendicular to the pipe.
Slope of detention basin embankment. The maximum slope of earthen detention basin embankments shall be three to one. The top or toe of any slope shall be located a minimum of five feet from any property line. Whenever possible, the side slopes and basin shape shall be amendable to the natural topography. Straight side slopes and rectangular basins shall be avoided whenever possible.
Width and construction of berm. The minimum top width of detention basin berms shall be 10 feet. Berms shall be constructed of clay and shall have a key cut into virgin soil.
Slope of basin bottom. In order to ensure proper drainage of the detention basin, except where infiltration is proposed within the floor of the basin, a minimum grade of 2% shall be maintained for all sheet flow, and a minimum grade of 1% shall be maintained for all channel flow.
Energy dissipators. Energy dissipating devices (rip-rap, end sills, etc.) shall be placed at all basin outlets.
Grading standards for stormwater management facilities.
Cuts. No excavation shall be made with a cut face steeper than three horizontal to one vertical, except under the condition that the materials in which the excavation is made are sufficiently stable to sustain a slope of steeper than three horizontal to one vertical. A written statement to that effect is required from a civil engineer, licensed by the Commonwealth of Pennsylvania, having experience in soils engineering and must be submitted to the Township Engineer and approved by him. The statement shall affirm that the site has been inspected and that the deviation from the slope should not result in injury to persons or damage to property. Retaining walls will be required if a stable slope cannot be maintained. Any retaining wall design must be approved by the Township Engineer. The toe of the slope or headwall of any cut must be located a minimum of five feet from property lines.
Fills. No fill shall be made which creates any exposed surfaces steeper in slope than three horizontal to one vertical, except where the fill is located so that settlement, sliding, or erosion will not result in property damage or be a hazard to adjoining property, streets, or buildings. A written statement is required from a civil engineer licensed by the Commonwealth of Pennsylvania having experience in soils engineering certifying that he has inspected the site and that any proposed deviation from the slope specified above should not endanger any property or result in property damage, and must be submitted to and approved by the Township Engineer.
A concrete or stone masonry wall designed and constructed in accordance with these specifications and standards may be required to support the face of the fill where the above-specified slopes are exceeded.
The top of any fill or toe of the slope of any fill shall be located five feet from any property line.
Landscaping of storm drainage, retention and detention areas.
All storm drainage channels, retention areas, and detention basins, whether existing or proposed, shall be graded and planted with permanent vegetative cover acceptable to the Township and in a manner which effectively naturalizes such areas as an integral and harmonious part of the surrounding landscape.
Fence or screening. A fence or suitable vegetative screen shall be provided around all detention basins upon the recommendation of the Township Engineer. All fencing shall be at least 3 1/2 feet in height. Where provided in lieu of fencing, vegetative screening shall be composed of shrubs intended to provide a barrier to access into the detention basin area, including, for example, Barberry (Barberis species), Eleagnus (Eleagnus species), Firethorn (Pyracantha species), or Rose (Rose species). This fencing or vegetative screening requirement shall be waived only with the expressed consent of the Board of Supervisors.
A minimum of six inches of topsoil material shall be placed on all areas affected by construction of detention basins (bottom of basin, side, slopes, top of berm, etc.). The material must meet the requirements of the Pennsylvania Department of Transportation, Form 408 Specification, as amended.
Culverts and drainage channels. Culverts shall generally be required at points of vehicular or pedestrian crossing over drainage channels receiving drainage from an area in excess of 2 1/2 acres.
Design flow standard. All culverts and drainage channels shall be designed to carry the flow rate resulting from a fifty-year, twenty-four-hour storm event (Soil Conservation Service, Technical Release No. 55).
Erosion prevention. All drainage channels shall be designed to prevent the erosion of the bed and bank areas. The flow velocity in all vegetated drainage channels shall not exceed three feet per second to prevent erosion. Suitable bank stabilization shall be provided where required to prevent erosion of the drainage channels. Where storm sewers discharge into existing drainage channels at an angle greater than 30° from parallel with the downstream channel flow, the far side bank shall be stabilized by the use of rip-rap or masonry, and/or concrete walls. The stabilization shall be designed to prevent erosion and frost heave under and behind the stabilizing media.
Maximum side slope. Any vegetated drainage channel requiring mowing of the vegetation shall have a maximum grade of three horizontal to one vertical on those areas to be mowed.

References: § 152
 § 152
 § 691
 § 680
 § 152
 § 152
 § 152
 § 152
 § 152
 § 152
 § 152
 § 152
 § 152
 § 152
 § 152