Article XIII Stormwater Management

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

For development taking place in stages, the entire development plan must be used in determining conformance with this Part 2.

For projects that add impervious area to a parcel, §§ 172-57 through 172-62 shall apply to the total impervious area within the limits of earth disturbance.

Stormwater discharges onto adjacent property shall not be created, increased, relocated, or otherwise altered without written notification of the adjacent property owner(s). Such stormwater discharges shall be subject to the requirements of this Part 2.

All regulated activities shall include such measures as necessary to:

Protect health, safety and property;

Meet the water quality goals of this Part 2 by implementing measures to:

Minimize disturbance to floodplains, wetlands, and wooded areas.

Maintain or extend riparian buffers.

Avoid erosive flow conditions in natural flow pathways.

(d)

Minimize thermal impacts to waters of this commonwealth.

(e)

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

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

The design of all facilities over karst shall include an evaluation of measures to minimize adverse effects.

Infiltration BMPs should be dispersed on site, made as shallow as practicable, and located to maximize use of natural on-site infiltration features while still meeting the other requirements of this Part 2.

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.

The design storm precipitation amounts to be used in the analysis of peak rates of discharge shall be those from the upper limits of the 90% confidence intervals for the twenty-four-hour precipitation events in the Precipitation-Frequency Atlas of the United States, Atlas 14, Volume 2, Version 3.0, United States Department of Commerce, National Oceanic and Atmospheric Administration (NOAA), National Weather Service (NWS), Hydrometeorological Design Studies Center (HDSC), Silver Spring, Maryland. NOAA's Atlas 14 can be accessed at http://hdsc.nws.noaa.gov/hdsc/pfds/.

For all regulated activities, SWM BMPs shall be designed, implemented, operated, and maintained to meet the purposes and requirements of this Part 2 and to meet all requirements under Title 25 of the Pennsylvania Code, the Clean Streams Law,[4] and the Stormwater Management Act.[5]

[4]

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

[5]

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

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

Any stormwater facility located on state highway rights-of-way shall be subject to approval by the Pennsylvania Department of Transportation (PennDOT), and shall require an agreement to indemnify and hold harmless the Township and PennDOT from any adverse consequences or costs.

Any stormwater management facility (i.e., detention basin BMP) designed to store runoff and requiring a berm or earthen embankment required or regulated by this Part 2 shall be designed to provide an emergency spillway to handle flow up to and including the one-hundred-year proposed conditions. The height of embankment must be set as to provide a minimum 1.0 foot of freeboard above the maximum pool elevation computed when the facility functions for the one-hundred-year proposed conditions' peak inflow. Should any stormwater management facility require a dam safety permit under Pennsylvania Code Title 25, Environmental Protection, Chapter 105, Dam Safety and Waterway Management, the facility shall be designed in accordance with Chapter 105 and meet the regulations of Chapter 105 concerning dam safety which may be required to pass storms larger than the one-hundred-year event.

Any facilities that constitute water obstructions (e.g., culverts, bridges, outfalls, or stream enclosures), and any work involving wetlands as directed in DEP Chapter 105 regulations (as amended or replaced from time to time by DEP)[6] shall be designed in accordance with Chapter 105 and will require a permit from DEP. Any other drainage conveyance facility that does not fall under Chapter 105 regulations must be able to convey, without damage to the drainage structure or roadway, runoff from the twenty-five-year design storm with a minimum 1.0 foot of freeboard measured below the lowest point along the top of the roadway. Roadway crossings located within designated floodplain areas must be able to convey runoff from a one-hundred-year design storm with a minimum 1.0 foot of freeboard measured below the lowest point along the top of the roadway. Any facility that constitutes a dam as defined in DEP Chapter 105 regulations may require a permit under dam safety regulations. Any facility located within a PennDOT right-of-way must meet PennDOT minimum design standards and permit submission requirements.

[6]

Editor's Note: See 25 Pa. Code Ch. 105.

Any drainage/conveyance facility and/or channel that does not fall under Chapter 105 regulations[7] must be able to convey, without damage to the drainage structure or roadway, runoff from the ten-year design storm. Conveyance facilities to or exiting from stormwater management facilities (i.e., detention basins) shall be designed to convey the design flow to or from that structure. Roadway crossings located within designated floodplain areas must be able to convey runoff from a one-hundred-year design storm. Any facility located within a PennDOT right-of-way must meet PennDOT minimum design standards and permit submission requirements.

[7]

Editor's Note: See 25 Pa. Code Ch. 105.

Storm sewers must be able to convey proposed conditions' runoff from a ten-year design storm without surcharging inlets, where appropriate.

Adequate erosion protection shall be provided along all open channels and at all points of discharge.

Detention facilities will be designed as follows:

The emergency spillway on detention facilities shall be designed to convey the one-hundred-year storm after development with one foot of freeboard to the top of berm elevation. Emergency spillways shall be constructed with a suitable liner, as determined by the Township Engineer, designed in accordance with the manufacturer's specifications, backfilled with topsoil and seeded to protect the berm against erosion.

Basins and all other types of BMPs shall be naturalized with landscaping as recommended in DEP's Best Management Practices Manual as last revised. The plantings shall be in addition to all other landscaping requirements.

Two antiseep collars shall be installed around the basin outlet pipe and shall be centered within the normal saturation zone of the berm. The antiseep collars and their connections to the pipe barrel shall be watertight.

The minimum basin berm width at the design elevation shall be 10 feet. A cutoff trench (keyway) of impervious material shall be provided under all embankments that require fill material. The cutoff trench shall be a minimum of eight feet wide, three feet deep and have side slopes of one horizontal to one vertical.

Any stormwater basin that is intended to maintain a permanent pool shall be designed in accordance with DEP's Pennsylvania Best Management Practices Manual as last revised. All wet basins shall have a ten-foot-wide safety bench, be planted with wet-tolerant vegetation as approved by the Township Engineer and Township Landscape Architect, provide a minimum pond depth of four feet and be lined with clay or other impermeable liner and provide safety bench planting soil above the liner. Additionally, aerators shall be installed in all basins that are designed to maintain a permanent pool.

When an applicant retains ownership of any stormwater management facility, such entity shall be responsible for repair and maintenance of the facility. The site plan shall contain a notation permitting access to such facilities deemed critical to public welfare, for inspection at any reasonable time by the Township or its designee.

(7)

The site plan shall also contain a notation that states the approval of the final plans is conditioned upon the applicant agreeing to be responsible for all repairs and maintenance of the stormwater management facility and permitting access to such facilities deemed critical to public welfare, for inspection at any reasonable time by the Township or its designee. The failure of the applicant to properly maintain any stormwater management facility shall be construed to be a violation of this Part 2 and shall be declared to be a public nuisance, subjecting the violator to any and all penalties provided by law.

Location, maintenance and responsibility for detention facilities.

Detention basins may be located on individual lots if approved by the Township. The maintenance and ownership of these basins will be the responsibility of the lot owner. The responsibility for maintenance of any such basin shall be clearly noted on the subdivision plans and shall appear on any deed conveying said lot from developer to another party.

Detention basins may be located to serve multiple lots. In this case, the ownership, maintenance and responsibility of the basin may be either that of a homeowners' association or lot owners' association. The responsibility for maintenance of any such basin shall be clearly noted on the subdivision plans and shall appear on any deed conveying said lot from the developer to another party.

Lower Moreland Township will not accept ownership of detention facilities.

Lower Moreland Township may require the developer and owner to execute an agreement promising to maintain any stormwater facility and granting access to the Township to inspect any such facility.

Underground detention facilities will be permitted if they meet the following criteria:

Unless otherwise agreed by the Board of Commissioners, underground detention basins shall be located outside of all public rights-of-way.

Underground detention basins shall be constructed of high-density corrugated polyethylene pipe or approved equal.

Inlets or storm manhole structures with steps shall be located at each corner of the underground basin to provide access for maintenance.

A minimum one foot of freeboard shall be provided between the one-hundred-year water surface elevation and the top of all inlet grates or manhole rims.

The underground basin shall be wrapped in permeable geotextile fabric.

Backfill between the top of the underground basin and the bottom of the pavement section, or the bottom of the topsoil layer in unpaved areas, must be full stone backfill.

(7)

When infiltration rates exceed the acceptable values in the Pennsylvania Best Management Practices Manual, as last revised, a twelve-inch-thick sand filter layer, wrapped in geotextile fabric, should be provided for additional water quality treatment.

(8)

An emergency overflow weir shall be provided for the outlet structure in order to convey basin inflow in excess of design flows or in the event the outlet structure becomes blocked and is unable to convey the design flow. The minimum capacity of the emergency overflow weir shall be equivalent to the one-hundred-year peak inflow rate for the post-development design storm.

(9)

Proposed site improvements and grading downstream of the underground detention basin shall be designed to convey the one-hundred-year peak inflow rate to a natural outfall, storm sewer system or other drainage facility, without adversely affecting downstream properties.

(10)

Underground stormwater facilities that receive runoff from more than one lot within a residential community shall require the creation of a homeowners' association (HOA). The HOA will be responsible for the ownership and continued maintenance of the underground facility.

Detention/retention facility design and construction standards:

Detention basins shall be designed to facilitate regular maintenance, periodic mowing and periodic silt removal and reseeding. Shallow broad basins are preferred to steep-sided basins.

The maximum slope of the earth and detention basin embankment shall be three to one, with the exception that any slope to be maintained by the Township shall be four to one. The top or toe of any slope shall be located a minimum of five feet from a property line. Whenever possible, the side slope and basin shape shall conform to the natural topography.

Unless permitted by the Zoning Hearing Board in accordance with Chapter 108, "Floodplain Conservation and Damage Prevention," detention basins shall not be located within floodplains.

If retention basins are used, the applicant shall demonstrate that such ponds are designed to protect public health, safety and welfare.

The minimum top width of the detention basin berm shall be 10 feet. A cutoff trench (keyway) of relatively impervious material shall be provided beneath all embankments requiring fill material. The keyway shall be a minimum eight feet wide, minimum three feet deep and have one-to-one side slopes.

In order to ensure proper drainage on the basin bottom, a minimum grade of 2% shall be maintained for sheet flow. Where a slope of 2% cannot be maintained, infiltration channels or trenches will be required and shall be constructed between all basin inlets and the basin outlet.

(7)

All detention and retention basin embankments shall be placed in eight-inch maximum lifts to a minimum dry density of 95%. Prior to proceeding to the next lift, compaction shall be checked by the Township Engineer or an approved soils engineer, who shall provide the Township Engineer with a written report. Compaction tests shall be performed using the Modified Proctor Method in accordance with ASTM D-1557. Compaction tests shall be run on the leading and trailing edge as well as the top of the berm.

(8)

Emergency overflow facilities shall be provided for detention facilities to accommodate runoff in excess of design flows. Whenever possible, emergency spillways for the detention basins shall be constructed on undisturbed ground. Emergency spillways shall be constructed of concrete pavers, gabions or other similar materials approved by the Township Engineer. All emergency spillways shall be constructed so that the detention basin berm is protected against erosion. The construction material of the emergency spillway shall extend along the upstream and downstream berm embankment slopes. The downstream slope of the spillway shall, as a minimum, extend to the toe of the berm embankment. The emergency spillway shall not discharge over earthen fill or easily erodible material.

(9)

Antiseep collars shall be installed around the pipe barrel within the normal saturation zone of the detention basin berms. The antiseep collars and their connections to the pipe barrels 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. A minimum of two antiseep collars shall be installed on each outlet pipe.

(10)

All outlet pipes through the basin berm shall be reinforced concrete pipe, designed to withstand the loading caused by a fully saturated berm, and shall have watertight joints using zero-ring joint pipe. Outlet pipe shall be backfilled with material similar to the core material (semi-impervious).

(11)

The invert of the inlet pipe(s) into a basin shall be six inches above the basin floor or lining so that it can adequately drain after rainstorms. Inlet pipe(s) shall discharge to areas of the basin that slope toward the outlet structure.

(12)

Energy dissipaters and/or level spreaders shall be installed at points where pipe, or drainageways drain to or from the basin. Energy dissipaters shall comply with the Montgomery County Soil Conservation District/Department of Environmental Protection criteria. NCRS energy-dissipating device calculations shall be submitted for Township Engineer review and approval.

(13)

Inlet and outlet structures shall be located at a maximum distance from one another in order to promote water quality benefits. The Township Engineer may require sediment trap devices for entrapping sediments carried in stormwater if sufficient separation of inlet and outlet structures cannot be achieved.

(14)

A perforated riser or similar sediment control device shall be provided at each outlet of all detention basins during construction for sediment control. The riser shall be constructed of metal or concrete. The riser shall extend to a maximum elevation of one foot below the crest elevation of the emergency spillway. The perforated riser shall be designed so that the rate of outflow is controlled by the pipe barrel through the basin berm when the depth of water within the basin exceeds the height of the riser. Circular perforations with a maximum diameter of one inch shall be spaced 12 inches vertically. The horizontal spacing shall be in accordance to DEP Soil Erosion and Sedimentation Control Manual specifications. The perforations shall be clearly cut and shall not be susceptible to enlargement. All metal risers shall be suitably coated to prevent corrosion. A trash rack or similar appurtenance shall be provided to prevent debris from entering the pipe. All risers shall have a concrete base attached with a watertight connection. The base shall be of sufficient weight to prevent flotation of the riser. An antivortex device consisting of a thin vertical plate normal to the base and berm shall be provided at the top of the riser. Unless this structure is part of the permanent outlet control, it shall be removed from the site when the site has been adequately stabilized, as determined by the Township Engineer.

(15)

All drainage channels shall be designed to prevent erosion of the bed and batiks. The maximum permissible flow velocity shall not exceed the design requirements outlined in the current Design Manual published by the Pennsylvania Department of Environmental Protection. Suitable stabilization shall be provided where required to prevent erosion of the drainage channels.

(16)

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.

(17)

Because of the critical nature of vegetated drainage channels, the design of all vegetated channels shall, as a minimum, conform to the design requirements outlined in the current Design Manual published by the Pennsylvania Department of Environmental Protection.

Stormwater conveyance system design and construction standards:

Storm sewers, culverts, bridges and related installation shall be provided to:

Permit unimpeded flow of natural watercourses and in such a manner as to protect the natural character of the watercourses and to provide regulated discharge.

Ensure adequate drainage of all low points along the line of streets,

Intercept stormwater runoff along streets at intervals reasonably related to the extent and grade of the area drained and to prevent substantial flow of water across intersections.

All storm sewer system components shall conform to current PennDOT standards.

Drainage structures which drain watershed areas in the excess of 1/2 square mile (320 acres) or which have a span of eight feet or more shall be designed using the Natural Resources Conservation Service Technical Release 55, Urban Hydrology for Small Watersheds.

If a water obstruction permit is required from the Pennsylvania Department of Environmental Protection, the design storm shall be a one-hundred-year storm.

The culvert or bridge shall be as wide as the ultimate width of the roadway. Additional width may be required to provide sidewalks on one or both sides of the cartway.

Storm sewer design and construction requirements:

Minimum pipe size is 18 inches.

Minimum pipe slope shall be 0.5% per foot.

§ 172-56 Permit requirements by other governmental entities.

Minimum drop across junctions shall be two inches. At changes in pipe diameter, pipe crowns shall be matched at junctions (manhole, inlet or junction box).

(d)

Maximum distance between junctions shall be 300 feet.

(e)

Runoff to proposed storm sewers and inlets shall be calculated using the Rational Method.

(f)

The time of concentration shall be assumed to be five minutes for pipes under 30 inches. For pipes 30 inches or greater, the calculated time of concentration can be utilized.

(g)

The time of concentration to inlets for grate capacity calculations shall be assumed to be five minutes.

(h)

All storm sewer pipes shall be designed at a minimum to accommodate a minimum of a ten-year storm.

(i)

The one-hundred-year storm shall be transported to the basin. The plans shall reflect this requirement as required by the Township Engineer.

(j)

All storm sewer pipes at inlets in sump condition shall be designed to accommodate the fifty-year storm.

(k)

All storm sewer pipes and inlets intended to drain to detention facilities shall be designed to accommodate the one-hundred-year storm if the bypass or overflow runoff will not reach the basin by overland flow.

(l)

All inlets in sump condition shall be six-foot inlets or dual four-foot inlets, separated by eight feet, as needed.

(m)

All storm sewer systems shall be analyzed for both inlet and outlet control (including tailwater effects) by using the equations and nomographs as shown in the FHA's Hydraulic Design Services No. 5. In lieu of this, computer programs that calculate the actual hydraulic grade line for the storm sewer system can be used, provided that all losses (friction, bend, junction, etc.) are taken into account. Documentation for the program must be submitted for approval.

(n)

Minimum cover over pipes shall be two feet from the top of the outside of pipe bell to the final grade.

(o)

Inlet capacities shall be calculated using PennDOT or manufacturer's nomographs. Documentation for manufacturer's nomograph must be provided to the Township Engineer.

(p)

All stormwater pipes must be reinforced concrete pipe (RCP), high-density polyethylene pipe (HDPE) or other equivalent material approved by the Township Engineer.

Approvals issued and actions taken under this Part 2 do not relieve the applicant of the responsibility to secure required permits or approvals for activities regulated by any other code, law, regulation or ordinance.

§ 172-57 Erosion and sediment control during regulated earth disturbance activities.

Evidence of any necessary permit(s) for regulated earth disturbance activities from the appropriate DEP regional office or County Conservation District must be provided to the municipality.

Additional erosion and sediment control design standards and criteria are recommended to be applied where infiltration BMPs are proposed. They shall include the following:

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

§ 172-58 Nonstructural project design to minimize stormwater impacts.

Infiltration BMPs shall not be constructed nor receive runoff until the entire drainage area contributory to the infiltration BMP has achieved final stabilization.

The design of all regulated activities should include the following to minimize stormwater impacts:

The applicant should find practicable alternatives to the surface discharge of stormwater, the creation of impervious surfaces, and the degradation of waters of the commonwealth and must maintain as much as possible the natural hydrologic regime of the site.

An alternative is practicable if it is available and capable of implementation after taking into consideration existing technology and logistics in light of overall project purposes and other municipal requirements.

All practicable alternatives to the discharge of stormwater are presumed to have less adverse impact on quantity and quality of waters of the commonwealth unless otherwise demonstrated.

§ 172-59 Groundwater recharge requirements.

Infiltration best management practices (BMPs) shall meet the following minimum requirements unless the site qualifies for an exemption from the infiltration requirements of this Part 2 as listed in § 172-45:

Infiltration BMPs intended to receive runoff from developed areas shall be selected based on suitability of soils and site conditions and shall be constructed on soils that have the following characteristics.

A minimum soil depth of 24 inches between the bottoms of the infiltration BMPs and bedrock or other limiting zones.

An infiltration rate sufficient to accept the additional stormwater load and dewater completely as determined by field tests conducted by the applicant's qualified person.

All open-air infiltration facilities shall be designed to completely infiltrate the recharge (infiltration) volume (Re_v) within three days (72 hours) from the end of the design storm.

(d)

All subsurface and contained facilities such as capture-and-reuse systems must have storage available equivalent to the water volume control amount within three days (72 hours) from the end of the design storm.

(e)

Pretreatment (See § 172-49.) shall be provided prior to infiltration.

The size of the infiltration facility shall be based upon the following volume criteria:

Where practicable and appropriate, the recharge volume shall be infiltrated on site. The recharge volume shall be equal to one inch of runoff (I) over all proposed impervious surfaces.

The Re_v required shall be computed as:

Re_v = (1/12) * (I)

Where:
	Re_v	=	Recharge volume (cubic feet)
	I	=	Impervious area within the limits of earth disturbance (square feet)
An asterisk (*) in equations denotes multiplication.

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

Analyze hydrologic soil groups as well as natural and man-made features within the site to determine general areas of suitability for infiltration practices. In areas where development on fill material is under consideration, conduct geotechnical investigations of subgrade stability; infiltration may not be ruled out without conducting these tests.

Provide field tests such as double ring infiltrometer or hydraulic conductivity tests (at the level of the proposed infiltration surface) to determine the appropriate hydraulic conductivity rate. Percolation tests are not recommended for design purposes.

Design the infiltration structure for the required recharge volume (Re_v) based on field determined capacity at the level of the proposed infiltration surface.

If on-lot infiltration structures are proposed by the applicant's qualified person, it must be demonstrated to the municipality that the soils are conducive to infiltrate on the lots identified.

§ 172-60 Water volume control requirements.

An impermeable liner will be required in detention basins where the possibility of groundwater contamination exists. A detailed hydrogeologic investigation may be required by the municipality.

The low-impact development practices provided in the BMP Manual shall be utilized for all regulated activities to the maximum extent practicable. Water volume controls shall be implemented using the Design Storm Method in Subsection A or the Simplified Method in Subsection B below. For regulated activity areas equal to or less than one acre that do not require hydrologic routing to design the stormwater facilities, this Part 2 establishes no preference for either methodology; therefore, the applicant may select either methodology on the basis of economic considerations, the intrinsic limitations on applicability of the analytical procedures associated with each methodology, and other factors. All regulated activities greater than one acre must use the Design Storm Method.

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

The post-development total runoff volume for all storms equal to or less than the two-year, twenty-four-hour storm event shall not be increased.

For modeling purposes:

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

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

The Simplified Method (CG-2 in the BMP Manual) 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, or for projects that require design of stormwater storage facilities. For new impervious surfaces.

Stormwater facilities shall capture at least the first two inches of runoff from all new impervious surfaces. [Note: An asterisk (*) in equations denotes multiplication.]

Volume (cubic feet) = (2/12) * impervious surfaces (square feet)

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 the commonwealth. Removal options include reuse, evaporation, transpiration, and infiltration.

Volume (cubic feet) = (1/12) * impervious surfaces (square feet)

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

§ 172-61 Stream bank erosion; channel protection.

This method is exempt from the requirements of § 172-62, Peak rate controls and management districts.

If a perennial or intermittent stream passes through the site, the applicant shall create a stream buffer extending a minimum of 50 feet to either side of the top-of-bank of the channel. The buffer area shall be established and maintained with appropriate native vegetation. (Refer to Appendix B of the BMP Manual for plant lists.) If the applicable rear or side yard setback is less than 50 feet, the buffer width may be reduced to 25% of the setback to a minimum of 10 feet. If an existing buffer is legally prescribed (e.g., deed, covenant, easement, etc.) and it exceeds the requirements of this Part 2, the existing buffer shall be maintained. Applicants shall adhere to the following stream bank erosion/channel protection requirements:

In addition to the control of water quality volume (in order to minimize the impact of stormwater runoff on downstream stream bank erosion), the primary requirement is to design a BMP to detain the proposed conditions' two-year, twenty-four-hour storm event to the existing conditions' one-year flow using the SCS Type II distribution. Additionally, provisions shall be made (such as adding a small office at the bottom of the outlet structure or a sand filter) so that the proposed conditions' one-year twenty-four-hour storm event takes a minimum of 24 hours to drain from the facility from a point when the maximum volume of water from the one-year twenty-four-hour storm event is captured (i.e., 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 water volume control orifice is at the invert of the facility).

The minimum orifice size in the outlet structure to the BMP shall be three inches in diameter where possible, and a trash rack shall be installed to prevent clogging. On sites with small drainage areas contributing to this 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. Orifice sizes less than three inches can be utilized, provided that the design will prevent clogging of the intake. It is recommended that the design, to accommodate maintenance, include a sand or porous media filter.

§ 172-62 Peak rate controls and management districts.

The Pennypack Creek Watershed has been divided into stormwater management districts as shown on the Management District Map (Figure 172-62)[1].

In addition to the requirements specified in Table 172-62 below, the erosion and sedimentation control (§ 172-57), the nonstructural project design (§ 172-58), the groundwater recharge (§ 172-59), the water volume control (§ 172-60), and the stream bank erosion (§ 172-61) requirements shall be implemented.

Standards for managing runoff from each subarea in the Pennypack Creek Watershed for the two-, five-, ten-, twenty-five-, fifty-, and one-hundred-year storm events are shown in 172-62. Development sites located in each of the management districts must control proposed condition runoff rates to existing condition runoff rates for the design storms in accordance with Table 172-62.

Table 172-62 Peak Rate Control Standards by Stormwater Management District in the Pennypack Creek Watershed
District	Proposed Condition Design Storm		Existing Condition Design Storm
A	2-year 5-year 10-year 25-year 50-year 100-year	Reduce to	1-year 5-year 10-year 25-year 50-year 100-year
B	2-year 5-year 10-year 25-year 50-year 100-year	Reduce to	1-year 2-year 5-year 10-year 25-year 50-year

	NOTES:
	In District C, development sites that can discharge directly to the Pennypack Creek Main Channel (east of I-95) and to the Delaware River main channel without use of city infrastructure may do so without control of proposed conditions' peak rate of runoff.
	Projects that are required to obtain a NPDES permit for stormwater discharges associated with construction activities are required to show no increase in peaks from existing conditions.
	When adequate capacity in the downstream system does not exist and will not be provided through improvements, the proposed conditions' peak rate of runoff must be controlled to the predevelopment conditions' peak rate as required in District A provisions for the specified design storms. The predevelopment condition for new development is the existing condition. For redevelopment purposes in Philadelphia County, the predevelopment condition shall be determined according to the procedures found in the Philadelphia Stormwater Guidance Manual.

[1]

Editor's Note: Said figure is located in Subsection B, below.

General. Proposed condition rates of runoff from any regulated activity shall not exceed the peak release rates of runoff from existing conditions for the design storms specified on the Stormwater Management District Watershed Map (Figure 172-62).

Figure 172-62

Management District Watershed Map

District boundaries. The boundaries of the stormwater management districts are shown on an Official Map that is available for inspection at the municipal and county planning offices. A copy of the Official Map at a reduced scale is included as Figure 172-62. The exact location of the stormwater management district boundaries as they apply to a given development site shall be determined by mapping the boundaries using the two-foot topographic contours (or most accurate data required) provided as part of the drainage plan.

Sites located in more than one district. For a proposed development site located within two or more stormwater management districts, the peak discharge rate from any subarea shall meet the management district criteria in which the discharge is located.

Off-site areas. Off-site areas that 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.

Site areas. Where the site area to be impacted by a proposed development activity differs significantly from the total site area, only the proposed impact area utilizing stormwater management measures shall be subject to the management district criteria. In other words, unimpacted areas bypassing the stormwater management facilities would not be subject to the management district criteria.

§ 172-63 Calculation methodology.

Stormwater runoff from all development sites with a drainage area of greater than 20 acres shall be calculated using a generally accepted calculation technique that is based on the NRCS Soil Cover Complex Method. The qualified person must consult with the municipality to gain approval of design methods prior to design.