[Ord. 690, 9/10/2009; as amended by Ord. 737, 1/19/2017]
The owner, developer, or subdivider shall provide stormwater management in accordance with Chapter
26, Part
4, Stormwater Management, Chapter
26, Part
5, Neshaminy Creek Watershed Stormwater Management, Pennypack Creek Watershed Stormwater Management Ordinance, and as follows:
A. Applicability.
(1) This section shall apply to all areas of Warminster Township. Regulated activities include:
(c) Construction of new or additional impervious surfaces (driveways, parking lots, etc.).
(d) Construction of new buildings or additions to existing buildings.
(f) Diversion or piping of any natural or man-made stream channel.
(g) Installation of stormwater systems or appurtenances thereto.
(h) Prohibited or polluted discharges.
(i) Alteration of the natural hydrologic regime.
(j) Nonstructural and structural stormwater management best management practices (BMPs) or appurtenances thereto.
(2) Stormwater management design and criteria such as stormwater runoff peak rate requirements and districts, runoff calculation methodology, stormwater management plan requirements, operations and maintenance requirements, etc., shall be as described in this section, the Chapter
26, Part
4, Stormwater Management, Chapter
26, Part
5, Neshaminy Creek Watershed Stormwater Management, and Pennypack Creek Watershed Stormwater Management Ordinance.
(3) The standards contained in this section shall apply as minimum design standards; however federal, state, and other Warminster Township regulations may impose additional standards subject to their jurisdiction. The more stringent requirements of this section, federal, state, and other Warminster Township regulations shall apply to any activity which requires compliance with other ordinances and regulations of Warminster Township.
(4) Earth disturbance activities and associated stormwater management controls are also regulated under existing state law and implementing regulations. This section shall operate in conjunction with those parallel requirements; the requirements of this section shall be no less restrictive in meeting the purposes of this section than state law.
(5) No regulated activities within the municipality shall commence until the requirements of this section and all other applicable stormwater management criteria are met.
(6) All best management practices (BMPs) used to meet the requirements of federal, state, and Warminster Township regulations shall conform to the state water quality requirements and any more stringent requirements as set forth by the Township.
(7) Developers shall construct and/or install stormwater management facilities, on site and off site, as necessary to meet the stormwater management design and criteria provided by these and other Warminster Township requirements and to:
(a) Prevent erosion damage and to satisfactorily carry off, detain or retain, and control the rate of release of stormwater.
(b) Manage the anticipated peak discharge from property being subdivided or developed and existing runoff being contributed from all land at a higher elevation in the same watershed.
(c) Convey stormwater along or through the property to a natural outfall. If a developer concentrates dispersed stormwater flow or redirects stormwater flow to exit at another location on the property, the developer is responsible for constructing an adequate channel on the adjacent property and on all downstream properties until a natural outfall is reached.
1) Natural outfall shall have sufficient capacity to receive stormwater without deterioration of the facility and without adversely affecting property in the watershed. This natural outfall may be a river, creek or other drainage facility so designated by Warminster Township for the proposed system.
(8) Additional studies and higher levels of control than the minimum provided in these and other Warminster Township requirements and criteria may be required by the Board of Supervisors to ensure adequate protection to life and property.
B. Retention of existing watercourses and natural drainage features.
(1) Whenever a watercourse, stream or intermittent stream is located within a development site, it shall remain open in its natural state and location and shall not be piped.
(2) The existing points of natural drainage discharge onto adjacent property(ies) shall not be altered without the written approval of the affected property owners.
(3) No stormwater runoff or natural drainage shall be so diverted as to overload existing drainage systems (including existing stormwater management facilities) or create flooding.
(4) The Board of Supervisors may require a developer to provide a permanent easement along any watercourse located within or along the boundary of any property being subdivided or developed. The purpose of any such easement shall be for the maintenance of the channel of any watercourse; and the terms of the easement shall prohibit excavation, the placing of fill or structures and any alterations which may adversely affect the watercourse. The easement shall be provided for Zone 1 of the riparian buffer in accordance with §
22-602, Subsection
8. The developer will retain the easement until such time as one of the following is accomplished:
(a) The easement is offered for dedication by the developer and accepted by Warminster Township.
(b) If an easement acceptable to the Township is established, the maintenance shall then be the responsibility of the individual lot owners over whose property the easement passes. For land developments, the maintenance shall then be the responsibility of the owner.
(c) A homeowners' association or other approved legal entity, approved by Warminster Township, assumes responsibility for the maintenance of the development, including the retention of the watercourse easement.
C. Runoff control measures or best management practices (BMPs).
(1) Stormwater runoff which may result from regulated activities shall be controlled by permanent stormwater runoff BMPs that will provide the required standards within this section, Chapter
26, Part
4, Stormwater Management, Chapter
26, Part
5, Neshaminy Creek Watershed Stormwater Management, and Pennypack Creek Watershed Stormwater Management Ordinance, or other requirements of Warminster Township. The methods of stormwater control or best management practices (BMPs) which may be used to meet the required standards are described in this section and "Pennsylvania Stormwater Best Management Practices Manual," December 2006, as amended. The choice of BMPs is not limited to those referenced in this section and/or the manual. Any selected BMP must meet or exceed the required standards.
(2) Any stormwater facility located on state highway rights-of-way shall be subject to approval by the Pennsylvania Department of Transportation.
(3) Developers are encouraged to use BMPs other than the minimum BMPs specified in this subsection, if appropriate, to provide for additional water quality improvement and groundwater recharge. In evaluating potential stormwater BMPs, the order of preference shall be as follows:
(d) Minimum first flush detention or dual purpose detention (where appropriate).
(4) Infiltration best management practices (BMPs). Infiltration BMPs shall be designed in accordance with the design criteria and specifications in the PA BMP Manual and shall meet the following minimum requirements:
(a) A detailed soils evaluation of the project site shall be performed to determine the suitability of infiltration BMPs per protocol in Chapter
26, Part
4, Stormwater Management, Chapter
26, Part
5, Neshaminy Creek Watershed Stormwater Management, and Pennypack Creek Watershed Stormwater Management Ordinance. 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 site testing shall include adequate sampling of all portions of the site not limited by one-hundered-percent protected natural resources to determine areas of the property which are suitable for infiltration BMPs. The general process for designing infiltration BMPs shall be:
1) Analyze hydrologic soil groups as well as natural and man-made features within the site to determine general areas of suitability for infiltration BMPs.
2) Provide field testing data at the elevation of the proposed infiltration zone (bottom surface of the infiltration facility) to determine appropriate percolation rate and/or hydraulic conductivity. Field testing guidelines are identified in Appendix B of Chapter
26, Part
4, Stormwater Management, Appendix A of Chapter
26, Part
5, Neshaminy Creek Watershed Stormwater Management, and the Pennypack Creek Watershed Stormwater Management Ordinance.
3) Design infiltration BMPs for required stormwater volume based on field-determined capacity per Subsection C(4)(a)2 above.
(b) Soil characteristics. Subject to the specific considerations in Subsection C(4)(f) below:
1) Infiltration BMPs are particularly appropriate in hydrologic soil groups A and B, as described in the SCS TR-55 Manual.
2) Low-erodibility factors ('K' factors) are preferred for the construction of basins.
3) There shall be an infiltration and/or percolation rate sufficient to accept the additional stormwater load and to drain completely as determined by field tests.
4) A minimum of 10 feet of undisturbed fill or compacted impermeable material shall separate the foundation wall of any building and an infiltration BMP.
5) A minimum of 50 feet of undisturbed fill or compacted impermeable material shall separate water supply wells and an infiltration BMP.
6) A minimum of 50 feet shall separate a septic system disposal area and an infiltration BMP unless specific circumstances allow for a reduced separation distance.
7) Infiltration in native soils without prior fill or disturbance is preferred but not always possible. Areas that have experienced historic disturbance or fill are suitable for infiltration provided sufficient time has elapsed and the soil testing indicates that infiltration is feasible. In disturbed areas it may be necessary to infiltrate at a depth beneath soils that have previously been compacted by construction methods or long periods of mowing.
8) The infiltration system shall have positive overflow controls to prevent storage within one foot of the finished surface grade.
9) Surface inflows shall be designed to prevent direct discharge of sediment into the infiltration system.
(c) Any infiltration BMP shall be capable of completely infiltrating the impounded water within 48 hours from the peak of the storm.
(d) Special attention shall be paid to proper installation of infiltration-oriented stormwater management systems during construction and to careful avoidance of soil compaction during site development.
(e) Caution shall be exercised where salt or chloride would be a pollutant since soils do little to filter this pollutant, and it may contaminate the groundwater. Caution shall be exercised where infiltration is proposed in source water protection areas. The qualified design professional shall evaluate the possibility of groundwater contamination from the proposed infiltration/recharge facility and perform a hydrogeologic justification study if required by the Township Engineer.
(f) Infiltration BMPs shall be constructed on soils with an infiltration rate of a minimum of 0.25 inch per hour, including a safety factor of 50% applied for design purposes (e.g., for soil which measured 0.5 in/hr, the BMP design should use 0.25 in/hr to insure safe infiltration rates after construction) and shall have the following separation between the bottom of the facility and seasonal high water table and/or bedrock and/or groundwater (limiting zones);
1) For runoff from impervious surfaces associated with residential land use and/or pervious surface runoff associated with both residential uses and nonresidential uses: a minimum depth of 36 inches between the intended bottom of facility and limiting zones.
2) For runoff from impervious surface associated with nonresidential uses: a minimum depth of 36 inches between the bottom of facility and limiting zones. The minimum required separation between the limiting zone may be increased at the Township's discretion if project specific conditions, such as anticipated contaminants, dictate greater prevention of groundwater contamination.
3) For rooftop runoff: a minimum depth of 24 inches between the intended bottom of the facility and limiting zones.
(g) Soils with infiltration rates in excess of 6.0 inches per hour may require an additional soil buffer, such as an organic layer over the bed bottom, if the cation exchange capacity (CEC) is less than five and pollutant loading is expected to be significant.
(h) All infiltration BMPs shall be designed to, as a minimum:
1) Provide adequate storage to accommodate the volume of runoff calculated as the difference between the predevelopment runoff volume and the post-development runoff volume based on the largest required design storm.
2) Control the post-development peak rate of runoff to the predevelopment peak rate of runoff for design storms greater than one-year, twenty-four-hour storm.
3) Provide an overflow or spillway which safely permits the passing of runoff greater than that occurring during the largest design storm.
4) Release rate requirements of the Chapter
26, Part
4, Stormwater Management, Chapter
26, Part
5, Neshaminy Creek Watershed Stormwater Management, and Pennypack Creek Watershed Stormwater Management Ordinance may be met by use of infiltration BMPs alone or in combination with other facilities.
(5) Wet pond and artificial wetland best management practices (BMPs).
(a) Wet pond and artificial wetland BMPs shall be designed in accordance with the design criteria and specifications in the PA BMP Manual.
(b) Artificial wetland facilities shall meet the following minimum requirements:
1) At least 80% of the wetland shall be developed as a shallow water emergent wetland, with a water depth between eight inches and 12 inches. It is recommended that the shallow water emergent wetland area be separated into high marshes (zero to six inches below normal water surface) and low marshes (six to 18 inches below normal water surface). The remainder shall be constructed as open water with depths between two feet and four feet.
2) The length of the constructed treatment wetland between the inflow and outflow points shall be maximized. The minimum length to width ratio for all wetland systems shall be 3:1. A medial berm may be used to reduce flow width and improve flow distribution and length.
3) Constructed treatment wetlands shall be constructed on hydric or wet soils and/or soils which have an infiltration rate of less than 0.1 inch per hour. A base flow of 0.10 cfs/50 acres is recommended for all constructed treatment wetland facilities, unless a source of recharge is available such as a natural spring or well.
4) A shallow forebay shall be provided adjacent to all inflow areas, unless the inflow area provides less than 10% of the total inflow to the facility. The forebay serves to enhance sediment trapping and pollutant removal, as well as concentrating accumulated sediment in an area where it can be readily removed. To create a forebay, a baffle may be introduced to restrict hydraulic communication between the inlet and the remainder of the retention basin. The minimum standards for forebays are as follows:
a) Minimum length shall be 10 feet.
b) Sufficient storage volume shall be provided to trap sediment between clean-out intervals (typically five to 15 years). At a minimum, the forebay shall be sized to contain 0.25 inch per impervious acre of contributing drainage.
c) The forebay shall be three to five feet deep.
d) The height of the berm or baffle separating the forebay from the pond shall be one foot below the one-year water surface elevation or at the normal water surface elevation, whichever is higher.
e) The forebay shall be accessible and stabilized to accommodate equipment used in removing accumulated sediment.
5) A planting plan prepared by a landscape architect shall be developed for the artificial wetland showing all proposed aquatic, emergent, and upland plantings. The planting plan shall be developed to provide a diversity of species resulting in a dense stand of wetland vegetation.
(c) Retention basins/wet ponds shall meet the following minimum requirements:
1) Constructed treatment wetlands shall be constructed on hydric or wet soils and/or soils which have an infiltration rate of less than 0.1 inch per hour. A base flow of 0.10 cfs/50 acres is recommended for all constructed treatment wetland facilities, unless a source of recharge is available such as a natural spring or well.
2) The length of the pond between the inflow and outflow points shall be maximized, and an irregular shoreline shall be provided. The minimum length to width ratio for all wetland systems shall be 2:1.
3) A shallow forebay shall be provided adjacent to all inflow areas, unless the inflow area provides less than 10% of the total inflow to the facility. The forebay serves to enhance sediment trapping and pollutant removal, as well as concentrating accumulated sediment in an area where it can be readily removed. To create a forebay, a baffle may be introduced to restrict hydraulic communication between the inlet and the remainder of the retention basin. The minimum standards for forebays are as follows:
a) Minimum length shall be 10 feet.
b) Sufficient storage volume shall be provided to trap sediment between clean-out intervals (typically five to 15 years). At a minimum, the forebay shall be sized to contain 0.25 inch per impervious acre of contributing drainage.
c) The forebay shall be three to five feet deep.
d) The height of the berm or baffle separating the forebay from the pond shall be one foot below the one-year water surface elevation or at the normal water surface elevation, whichever is higher.
e) The forebay shall be accessible and stabilized to accommodate equipment used in removing accumulated sediment.
f) Nonerodible material shall be used to construct the forebay berm. Armoring, such as riprap, is generally preferred to stabilize the upgradient face of the berm.
4) Wet ponds shall be designed with public safety as a primary concern. An aquatic safety bench shall be provided around the perimeter of the permanent pool. The aquatic safety bench shall be a minimum of 10 feet wide with a maximum slope of 10:1. A 3:1 slope shall lead from the edge of the safety bench toward the deep water portion of the pond for at least 15 feet. Slopes on the remainder of the pond below the permanent pool elevation shall be a maximum of 2:1. The portion of the pond devoted to storage and peak attenuation of storm runoff, the terrestrial bench, shall be nearly level and shall be planted with shrubs and trees that shade the pool and mitigate thermal impacts. Terrestrial bench shall be at least 10 feet wide.
5) A planting plan prepared by a landscape architect shall be developed for the wet pond showing all proposed aquatic, emergent, and upland plantings.
6) The perimeter slope above the terrestrial bench shall have a maximum slope of 4:1.
7) Wet ponds shall have a deep water zone to encourage gravity settling of suspended fines and to prevent stagnation and possible eutrophication. The maximum depth shall not exceed eight feet.
8) Wet ponds shall be capable of being substantially drained by gravity flow. Where possible, wet ponds shall be equipped with a manually operated drain that can be secured against unauthorized operation.
9) The preferred method of low flow discharge from a pond is a submerged, reverse-slope pipe that extends downward from the outlet structure to an inflow point one foot below the normal water surface elevation. Alternative methods may be employed and shall include provisions to prevent debris clogging of the structure.
(6) Bioretention facilities and rain gardens.
(a) All concentrated discharges directed to a bioretention facility shall be conveyed through a pretreatment filter strip. The filter strip shall be designed to reduce the incoming velocities and to filter out coarser sediment particles. Examples of pretreatment filter strips include sand or gravel diaphragms, grass swales, sand filters, stone check dams, etc.
(b) Bioretention facilities shall meet the following minimum standards:
1) Minimum width of 10 feet, excluding side slopes.
2) Minimum length of 20 feet.
3) Minimum length to width ratio of 2:1.
4) Maximum shallow ponding depth of six inches.
5) Minimum planting soil bed depth of two feet for herbaceous plants and three feet for trees and shrubs.
(c) All bioretention facilities shall incorporate an organic mulch layer. The organic mulch layer shall be standard landscape style, single or double, shredded hardwood mulch or chips. The mulch layer shall be well aged, uniform in color, and free of other materials such as weed seed, soil roots, etc. The mulch layer shall be applied to maximum depth of three inches. Grass clippings shall not be used as mulch material.
(d) Planting soil shall be a loam soil capable of supporting healthy vegetative cover. Soils shall be amended with a composted organic material consisting of 20% to 30% compost material and 70% to 80% soil base, preferably topsoil. A soil analysis of the planting bed soil shall verify that the soils meet the following:
1) pH range between 5.5 and 6.5.
2) Organic matter content 5% to 10%.
3) Magnesium provided at 35 pounds per acre minimum.
4) Phosphorus provided at 75 pounds per acre minimum.
5) Potassium provided at 85 pounds per acre minimum.
6) Soluble salts shall be less than 500 ppm.
7) Clay content between 0% and 10%.
8) Silt content between 30% and 55%.
9) Sand content between 35% and 60%.
(e) All bioretention facilities shall incorporate landscaping and shall be provided with a landscaping plan prepared by a landscape architect that meets the following standards:
1) Plant species shall be selected based on the ability to tolerate urban stresses, such as pollutants, variable soil moisture and ponding fluctuations. A list of acceptable native plants is provided in the PA BMP Manual.
2) A minimum of three species each of trees, shrubs, and/or herbaceous plants shall be selected to insure diversity.
(f) In cases where the coefficient of permeability of the existing soils is less than 0.1 inch per hour, an underdrain facility shall be provided. The underdrain facility shall be protected from sediment accumulation with filter fabric.
(g) Where underdrain is proposed, an eight-inch deep sand bed shall be provided between the planting soil bed and the underdrain system.
(h) An overflow spillway or storm drain shall be provided to adequately convey the fifty-year storm event. The spillway or storm drain facility shall be set above the maximum proposed ponding depth.
(7) Permeable pavement.
(a) The type of permeable pavement material (perforated brick pavers, concrete grid pavers, porous bituminous concrete, porous concrete, etc.) shall be determined based on required surface infiltration rates, total surface area of pavement, and proposed use of the pavement/parking area.
(b) Permeable pavements shall be constructed with a perimeter overflow edge. The edge is intended to intercept runoff from the pavement if for any reason the permeable surface were to become clogged. The perimeter overflow edges shall connect directly into the base layer of the pavement.
(c) Soil percolation/infiltration rates shall equal or exceed 0.2 inch per hour. Soils with infiltration rates in excess of 6.0 inches per hour may require an additional soil buffer, such as an organic layer over the bed bottom, if the cation exchange capacity (CEC) is less than five and pollutant loading is expected to be significant.
(d) Permeable pavement recharge beds shall dewater within 72 hours.
(e) A minimum depth of 24 inches between the intended bottom of the facility and the seasonal high water table and/or bedrock and/or groundwater (limiting zones). The minimum required separation between the limiting zone may be increased at the Township's discretion if project specific conditions, such as anticipated contaminants, dictate greater prevention of groundwater contamination.
(f) All recharge bed bottoms shall be flat (0% slope). Porous pavement surface slopes shall not exceed 2% and are encouraged to be designed at 1%. In certain cases, the Township may allow surface slopes up to 3% if natural grades are left largely undisturbed.
(8) Extended detention/dual-purpose detention.
(a) Detention basin BMPs shall be designed in accordance with the design criteria and specifications in the PA BMP Manual and shall meet the following the following minimum requirements.
(b) Grading of detention basins shall be designed to utilize the natural contours of land whenever possible. When such design is impractical, the construction of the basin shall utilize slopes as shallow as possible to blend the structure into the terrain.
1) Maximum depth of stored water in detention basins shall be five feet, unless a greater depth is approved by the Board of Supervisors.
2) Maximum side slopes shall be a four-to-one ratio, horizontal to vertical. A ten-foot wide access shall be provided for maintenance of the facility with a maximum slope of 10:1 ratio.
3) A minimum grade of 2% shall be maintained for all areas in the basin where sheet flow occurs. A one-percent minimum grade shall be maintained for channel flow in the basin.
(c) The top width of the embankment shall be a minimum of 10 feet for storage volumes greater than 15,000 cubic feet. The top width of the embankment may be reduced to five feet for maximum storage volumes under 15,000 cubic feet.
(d) A cutoff trench shall be provided along the center line of any dam or earth fill embankments. The trench shall have a bottom width of not less than four feet, but adequate to allow use of equipment necessary to obtain proper compaction. Side slopes of cutoff trench shall be no steeper than 1:1 ratio. The trench shall be filled with successive thin layers of relatively impervious material, each layer being thoroughly compacted.
(e) All basin embankments shall be placed in lifts not to exceed eight inches in thickness and each lift shall be compacted to a minimum of 95% of modified proctor density as established by ASTM D-1557. Prior to proceeding to the next lift, the compaction shall be checked by a soils engineer employed by the applicant/developer. Compaction tests shall be run on the leading and trailing edge of the berm along with the top of the berm. Verification of required compaction shall be submitted to the Township prior to utilization of any basin for stormwater management.
(f) Spillways shall be provided to convey storm runoff around or under the embankment in a controlled manner to prevent overtopping. The spillway also must convey the water from the basin to a stable outlet below without damage to the downstream slopes.
1) Primary spillway. The riser shall be solidly attached to the barrel and all connections for riser and pipe barrel shall be watertight. The barrel and riser shall be placed on a firm foundation. The fill material around the primary spillway shall be placed in four-inch lifts and compacted to at least the same density as the adjacent embankment. If the basin is being used as a temporary sediment control facility during construction, the riser shall be constructed in accordance with specifications set forth by PADEP and NRCS in the "Erosion and Sediment Pollution Control Manual," latest edition.
2) Emergency spillway. Whenever possible, the emergency spillway for detention basins shall be constructed on undisturbed ground. Emergency spillways constructed on undisturbed ground may be constructed of reinforced vegetated earth. All other spillways shall be constructed of riprap, concrete checkerblocks, or similar materials approved by the Township Engineer.
a) The minimum capacity of all emergency spillways shall be equal to the peak flow rate from the one-hundred-year design storm.
b) Emergency spillways shall extend along the upstream and downstream berm embankment slopes. The upstream edge of the spillway material shall be a minimum of three feet below the spillway crest elevation. The downstream slope of the spillway shall extend to the toe of the berm embankment. The emergency spillway shall not discharge over earthen fill or easily erodible material.
3) The minimum freeboard through the emergency spillway shall be one foot. Freeboard is defined as the difference between the design flow elevation through the spillway and the elevation of the top of the settled basin berm. Six inches, minimum, is required between the one-hundred-year water surface elevation and the emergency spillway crest.
4) Antiseep collars shall be installed around the principal spillway pipe barrel within the normal saturation zone. The collars and their connections to the pipe shall be watertight. The maximum spacing shall be approximately 14 times the minimum projection of the collar measured perpendicular to the pipe. The minimum projection shall be two feet.
5) All basin outlets which discharge to surface waters shall have energy dissipating devices designed in accordance with the PADEP "Erosion and Sediment Pollution Control Manual," latest edition.
6) Basins shall be protected against erosion by vegetative means as soon as practical after construction and before runoff is directed to the facility via storm conveyances. Areas that have eroded during construction shall be regraded and stabilized prior to final seeding procedures. Basins shall receive a minimum of eight inches of topsoil, conditioned as needed, prior to seeding.
7) Vegetative cover. Basins shall be landscaped in accordance with §
22-523.
(9) All stormwater control facility designs shall conform to the applicable standards and specifications of the following governmental and institutional agencies:
(a) American Society of Testing and Materials (ASTM).
(c) Bucks County Conservation District.
(d) Federal Highway Administration (FHWA).
(e) National Crushed Stone Association (NCSA).
(f) National Sand and Gravel Association (NSGA).
(g) Pennsylvania Department of Environmental Protection (DEP).
(h) Pennsylvania Department of Transportation (PennDOT).
(i) U.S. Department of Agriculture, Natural Resources Conservation Service, Pennsylvania (NRCS).
(10) If special geological hazards or soil conditions, such as carbonate derived soils, are identified on the site, the developer's engineer shall consider the effect of proposed stormwater management measures on these conditions. In such cases, the Township may require an in-depth report by a competent soils engineer.