Township of Upper Gwynedd, PA 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 chapter.

For projects that add impervious area to a parcel, §§ 163-21 through 163-26 shall apply to the total impervious area within the limits of earth disturbance.

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

All regulated activities shall include such measures as necessary to:

Protect health, safety and property;

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

Minimize disturbance to floodplains, wetlands and wooded areas.

Maintain or extend riparian buffers.

(c)

Avoid erosive flow conditions in natural flow pathways.

(d)

Minimize thermal impacts to waters of the 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) or the Philadelphia Stormwater Management Guidance 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 chapter.

Storage facilities should completely drain both the volume control and rate control capacities over a period of time not less than 24 hours and not more than 72 hours from the end of the design storm.

Design storm volumes and precipitation intensities to be used in the analysis of discharge or runoff should be obtained from the Precipitation-Frequency Atlas of the United States, Atlas 14, Volume 2, Version 3.0, U.S. Department of Commerce, National Oceanic and Atmospheric Administration (NOAA), National Weather Service, Hydrometeorological Design Studies Center, Silver Springs, 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 chapter and to meet all requirements under Title 25 of the Pennsylvania Code, the Clean Streams Law, and the Storm Water Management Act.

Various BMPs and their design standards are listed in the Pennsylvania Stormwater Best Management Practices Manual (BMP Manual), latest revision. BMPs shall comply with these design standards.

The following standards are in addition to the standards listed in the BMP Manual. All BMPs shall meet the following minimum standards in addition to the standards in the BMP Manual. If there is a conflict between the standards in this chapter and the standards in the BMP Manual, the more restrictive requirements shall apply.

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

The top or toe of any slope associated with a BMP shall be located a minimum of five feet from a property line.

Detention basins shall not be located within floodplains nor within areas of floodplain or alluvial soils.

(4)

Fences may be required for any detention or retention basins where there is a permanent water surface or where conditions warrant.

(5)

The minimum top width of the detention basin berm shall be 10 feet. A cutoff trench (keyway) of relative impervious material shall be provided beneath all embankments requiring fill material. The keyway shall be a minimum of eight feet wide, a minimum of three feet deep, and have 1:1 side slopes.

(6)

All detention and retention basin embankments shall be placed in eight-inch maximum lifts to a minimum ninety-five-percent dry density. 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-1577. Compaction tests shall be run on the leading and trailing edge as well as the top of the berm.

(7)

Emergency overflow facilities shall be provided for retention and detention facilities to accommodate runoff in excess of design flows. Whenever possible, emergency spillway for the retention/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 minimum capacity of all emergency spillways shall be the peak flow rate of the one-hundred-year design storm after development. The construction material of the emergency spillway shall extend along the upstream and downstream berm embankment slopes. The upstream edge of the emergency spillway shall be a minimum of three feet below the spillway crest elevation. The downstream slope of the spillway shall at a minimum extend to the toe of the berm embankment. The emergency spillway shall not discharge over earthen fill or easily erodible material.

(8)

The minimum freeboard for emergency spillways shall be one foot. Freeboard shall be measured from the top of berm elevation to the high-water surface elevation through the spillway for the one-hundred-year storm.

(9)

Anti-seep collars shall be installed around the pipe barrel within the normal saturation zone of detention/retention basin berms. The anti-seep collars and their connections to the pipe barrels shall be watertight. The anti-seep collars shall extend a minimum of two feet beyond the outside of the principal pipe barrel. The maximum spacing between collars shall be 10 times the minimum projection of the collar measured perpendicular to the pipe. A minimum of two anti-seep collars shall be installed on each outlet pipe.

(10)

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

(11)

Energy dissipaters and/or level spreaders shall be installed at points where pipes or drainageways drain to or from the basin. Energy dissipaters shall comply with criteria in the PADEP Erosion and Sediment Pollution Control Program Manual, latest revision. Energy-dissipating device calculations shall be submitted for Township review and approval.

(12)

A perforated riser or similar sediment-control device shall be provided at each outlet of all retention/detention basins during construction for sediment control in accordance with the PADEP Soil Erosion and Sedimentation Control Manual specifications, latest revision.

(13)

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

(14)

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.

(15)

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 Soil Erosion and Sedimentation Control Manual published by the Department of Environmental Protection.

All stormwater conveyance facilities must be designed in accordance with Chapter 162, § 162-12, of the Upper Gwynedd Township Stormwater Management Ordinance.

If in the course of preparing or reviewing the stormwater management plan the Township Engineer determines that off-site improvements are necessary to satisfactorily control the stormwater from the site, the applicant shall be responsible for such off-site improvements.

§ 163-20 Permit requirements by other governmental entities.

Approvals issued and actions taken under this chapter 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.

§ 163-21 Erosion and sediment control during regulated earth disturbance activities.

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

These areas shall be protected from sedimentation and compaction during the construction phase.

BMPs shall not be constructed, nor the areas receive runoff, until the entire drainage areas contributory to the infiltration BMPs have achieved final stabilization.

§ 163-22 Nonstructural project design to minimize stormwater impacts.

The design of all regulated activities should include the following to minimize stormwater impacts (See Appendix C for a Nonstructural Project Design Checklist[1].):

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.

[1]

Editor's Note: Appendix C is included as an attachment to this chapter.

§ 163-23 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 chapter as listed in § 163-6.

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.

(c)

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 § 163-13) 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 1.0 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 developed by the applicant 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 an 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 must be implemented if these tests are not completed.

Perform 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 accepted for design purposes.

Design the infiltration structure for the required recharge volume (Re_v), based on field tests at the elevation of the proposed infiltration surface.

(4)

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

(5)

The applicant must install an impermeable liner in detention basins where the possibility of groundwater contamination exists. A detailed hydrogeologic investigation may be required by the municipality.

§ 163-24 Water volume control requirements.

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 chapter 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.

(4)

This method is exempt from the requirements of § 163-26, Stormwater peak rate control and management districts.

§ 163-25 Stream bank erosion requirements (channel protection).

If a perennial or intermittent stream passes through the site, the applicant shall create a riparian 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 in an undisturbed state. This buffer area may be maintained as a meadow with minimal mowing of the grassed area or as a forested buffer, being planted 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 (i.e., deed, covenant, easement, etc.) and it exceeds the requirements of this chapter, the existing buffer shall be maintained. This does not include lakes or wetlands.

The following uses are permitted within a riparian buffer when authorized as a special exception and when authorization is obtained from PADEP and when all other applicable rules and requirements are followed, including all federal, state and county requirements and floodplain ordinances:

Construction or placement of roads, bridges, driveways, trails, storm drainage, utilities or other similar structures.

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 orifice 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, 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 replaceable and/or porous media filter cartridge.

§ 163-26 Stormwater peak rate control and management districts.

The Wissahickon Creek Watershed has been divided into stormwater management districts as shown on the Management District Map (Appendix A) and the Upper Gwynedd Township Stormwater Management District Map (Appendix F).[1]

The peak rate requirements specified in Table 163-26 below shall be implemented in addition to all other applicable requirements. Standards for managing peak rates of runoff from each subarea in the Wissahickon Creek Watershed for the two-, five-, ten-, twenty-five-, fifty- and one-hundred-year storm events are shown in Table 163-26. 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 163-26.

Table 163-26
Peak Rate Control Standards by Stormwater Management District in the Wissahickon Creek Watershed
District	Proposed Condition Design Storm		Existing Condition Design Storm
A	2-year	Reduce to	1-year
	5-year		5-year
	10-year		10-year
	25-year		25-year
	50-year		50-year
	100-year		100-year
B	2-year	Reduce to	1-year
	5-year		2-year
	10-year		5-year
	25-year		10-year
	50-year		25-year
	100-year		50-year
C*	Conditional Direct Discharge District

	NOTES:
	*	In District C, development sites that can discharge directly to the Wissahickon Creek main channel and to the Schuylkill 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 an 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.

[1]

Editor's Note: Appendixes A and F are included as attachments to this chapter.

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 (Appendix A).[2]

[2]

Editor's Note: Appendix A is included as an attachment to this chapter.

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 Appendix A. 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.

Alternate criteria for redevelopment sites. For redevelopment sites, one of the following minimum design parameters shall be accomplished, whichever is most appropriate for the given site conditions as determined by the Township:

Meet the full requirements specified by Table 163-26 and § 163-26A through F; or

Reduce the total impervious surface on the site by at least 20% based upon a comparison of existing impervious surface to proposed impervious surface.

Stormwater control measures which increase storage or infiltration volume, and which are not associated with new land development or redevelopment activity that increases runoff volume above existing levels, are exempt from the peak rate requirements of this chapter.

§ 163-27 Calculation methodology.

Stormwater runoff from all development sites with a drainage area of greater than 200 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.