(a) 
For any of the activities regulated by this chapter, unless preparation of a stormwater management (SWM) site plan is specifically exempted, the preliminary or final approval of subdivision and/or land development plans, the issuance of any building or occupancy permit, the commencement of any earth disturbance activity shall not proceed until the property owner or applicant or his/her agent has received written approval from the Township of a SWM site plan that demonstrates compliance with the requirements of this chapter, and a written approval of an adequate erosion and sediment (E&S) control plan from the Township or Montgomery County Conservation District, when and as required.
(b) 
SWM site plans approved by the Township, in accordance with Article III, shall be on site throughout the duration of the regulated activity.
(c) 
The Township may, after consultation with the Department of Environmental Protection (DEP), approve measures for meeting the state water quality requirements other than those in this chapter, provided that they meet the minimum requirements of, and do not conflict with, state law including but not limited to the Clean Streams Law.
(d) 
For all regulated earth disturbance activities, erosion and sediment (E&S) control best management practices (BMPs) shall be designed, implemented, operated and maintained during the regulated earth disturbance activities (e.g., during construction) to meet the purposes and requirements of this chapter and to meet all requirements under Title 25 of the Pennsylvania Code and the Clean Streams Law. Various BMPs and their design standards are listed in the Erosion and Sediment Pollution Control Manual, No. 363-2134-008 (April 15, 2000), as amended and updated.
(e) 
Impervious areas:
(1) 
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.
(2) 
For development taking place in stages, the entire development plan must be used in determining conformance with this chapter.
(3) 
For projects that add impervious area to a parcel, the total impervious area within the limits of disturbance is subject to the requirements of this chapter area.
(f) 
Stormwater flows onto adjacent property shall not be created, increased, decreased, relocated, or otherwise altered without written notification of the adjacent property owner(s) by the applicant. Such stormwater flows shall be subject to the requirements of this chapter.
(g) 
All regulated activities shall include such measures as necessary to:
(1) 
Protect health, safety and property;
(2) 
Meet the water quality goals of this chapter by implementing measures to:
A. 
Minimize disturbance to floodplains, wetlands, and wooded areas.
B. 
Create, maintain, repair 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 (i.e., disconnected impervious areas, DIAs) by directing runoff to pervious areas, wherever possible.
(3) 
To the maximum extent practicable, incorporate the techniques for low-impact development practices (e.g., protecting existing trees, reducing areas of impervious surface, cluster development, and protecting open space) described in the Pennsylvania Stormwater Best Management Practices Manual, Pennsylvania Department of Environmental Protection (PADEP) No. 363-0300-002(2006).
(h) 
The design of all facilities over karst features shall include an evaluation of measures to minimize adverse effects.
(i) 
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.
(j) 
Normally dry, open top 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.
(k) 
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/.
(l) 
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.
(m) 
Various BMPs and their design standards are listed in the Pennsylvania Stormwater Best Management Practices Manual (PA BMP Manual).
(n) 
Storm sewers (pipes or other structures) shall be reinforced concrete pipe or high-density polyethylene (when approved by the Township Engineer), have a minimum grade of 0.5% and a minimum inside diameter of 18 inches or a cross-sectional area of 254 square inches. Roof drains or yard drains shall not connect to any sanitary sewer and shall not discharge water directly over any sidewalk. Storm drainage facilities should be located within the paved areas of roadways. Where storm sewers discharge into existing drainage channels at an angle greater than 30° with the downstream channel flow, the far side bank shall be stabilized by the use of riprap or other approved stabilization method. The stabilization shall be designed to prevent erosion and frost heave under and behind the stabilizing media.
(o) 
Maintenance of all drainage facilities and watercourses, both existing and proposed, within the proposed site shall be the responsibility of the developer until such time as one of the following is accomplished:
(1) 
A right-of-way for these facilities is offered for dedication by the developer and is accepted by the Township, at which time it shall then be the responsibility of the Township.
(2) 
An easement acceptable to the Township is established. In the case of a subdivision, the maintenance shall then be the responsibility of the individual lot owners over whose property the easement passes. For land developments or regulated earth disturbance activities, the maintenance shall thereafter be the responsibility of the owner.
(3) 
A homeowners' association, approved by the Township, assumes responsibility for the maintenance of the development, including the maintenance of the watercourses and/or drainage facilities, provided that the Township Engineer approves the assumption of maintenance of such facilities.
(p) 
All proposed roadways shall be constructed with an underdrain system consisting of six-inch perforated pipe wrapped in a Class 1 geotextile material. The underdrain shall be bedded with a minimum of six inches of 2B stone. The bedding material shall extend to the bottom of the base course. The underdrain shall discharge to an inlet.
(q) 
All pipe and inlets shall meet material capacity and construction specifications as outlined in the latest revisions of the PENNDOT Form 408 and the PENNDOT Design Manual or as outlined in the Township specification as approved by resolution of the Board of Supervisors. Inlets at street intersections shall be placed on the tangent and not on the curved portion.
(r) 
Design flow rate. The storm drain system shall be designed to carry a 100-year peak flow rate. The design 100-year peak flow rate into each inlet shall be indicated on the stormwater drainage plan. The 100-year flow rate shall be determined by the rational formula. [Rational Formula is defined in § 1241.411(b).]
(s) 
Inlet capacity. All inlets must be designed to accommodate the 100-year peak flow rate. The capacity of all C-type, M-type or S-type inlets shall be determined from the Commonwealth of Pennsylvania Department of Transportation Design Manual, Part 2, Highway Design.
(t) 
Allowable headwater depth. At all inlets or manholes, the maximum allowable headwater depth shall be one foot below the top of the inlet grate or the manhole cover.
(u) 
Manholes. Manholes shall not be more than 400 feet apart where pipe sizes of 24 inches or less are used, and not more than 450 feet apart where larger sizes are installed. Manholes shall be installed at all points of abrupt change in alignment.
(v) 
Maximum depth of detention basins. The maximum depth of water in a detention basin shall not exceed four feet.
(w) 
Emergency spillway. Whenever possible, the emergency spillway for detention basins shall be constructed on undisturbed ground. Emergency spillways shall be constructed of reinforced concrete, vegetated earth or other approved material. 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 equal to the peak flow rate from the post-development 100-year design storm. Emergency spillways 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, as a minimum, extend to the top of the berm embankment. The emergency spillway shall not discharge over earthen fill and/or easily erodible material. The discharge path of the emergency spillway shall be evaluated to ensure that downstream properties or structures are not impacted.
(x) 
Antiseep collars. Antiseep collars shall be poured-in-place concrete and installed around the principal pipe barrel within the normal saturation zone of the detention basin berms. The collar shall be poured against the pipe bell. The antiseep collars and their connections to the pipe barrel shall be watertight. The antiseep collars shall extend a minimum of two feet beyond the outside of the principal pipe barrel. The spacing between collars shall be 16 feet. The use of an O-ring pipe shall be required for all basin discharge pipes.
(y) 
Freeboard. "Freeboard" is the difference between the design flow elevation in the emergency spillway and the top of the settled detention basin embankment. The minimum freeboard shall be one foot and the maximum freeboard shall be two feet.
(z) 
Slope of detention basin embankment. The maximum slope of earthen detention basin embankments shall be three to one. The top or toe of any slope shall be located a minimum of 10 feet from any property line. Whenever possible, the side slopes and basin shape shall be amenable to the natural topography. Straight side slopes and rectangular basins shall be avoided whenever possible.
(aa) 
Width of berm. The minimum top width of detention basin berms shall be 10 feet.
(ab) 
Energy dissipaters. Energy dissipating devices (riprap, level spreaders, end sills, etc.) shall be placed at all basin outlets. Forebays or split flow controls are encouraged at the inlet to all basins. When a forebay or split flow control is not used, an energy dissipating device shall be used. Riprap at the inlet to a basin shall extend to the bottom of the basin.
(ac) 
Key trench. All basin berms in fill areas are to have a key trench. The minimum depth shall be one foot and increased to a depth of three feet at the maximum point of fill. The minimum width of the key trench shall be eight feet.
(ad) 
Design flow standard. All culverts and drainage channels shall be designed to carry a flow rate equal to a 100-year, twenty-four-hour storm (Soil Cover Complex Method by the NRCS, Technical Release No. 55).
(a) 
Drainage channels, detention and retention areas. All storm drainage channels, detention and retention areas, whether existing or proposed, shall be graded and planted to effectively naturalize areas so as to become an integral and harmonious part of the landscape by contour and type of plant materials used.
(b) 
All detention basins, side slopes and bottoms shall be seeded with an appropriate seed mixture specifically suited to the levels of periodic inundation for which the system is designed. No-mow basins shall only be allowed as approved by the Township.
(c) 
When a detention or retention basin is proposed in any district, the basin(s) should conform to the requirements of the Lower Gwynedd Township Subdivision and Land Development Chapter[1] and the following landscape requirements:
(1) 
All stormwater management facilities shall complement the adjacent land use. Creative, natural, nonsymmetrical appearance and design shall be utilized whenever deemed feasible by the Township Engineer and may include:
A. 
The use of free flowing shapes such as peninsulas and islands. Gradation of side slopes shall be no steeper than 4:1 maximum. Other design approaches may be approved by the Township Engineer and the Lower Gwynedd Township Planning Commission. Basins designed as geometric shapes, especially rectangles, are discouraged, unless, in the opinion of the Township Engineer or the Lower Gwynedd Township Planning Commission, no other approach is feasible.
[1]
Editor's Note: See Ch. 1230.
(d) 
Basin perimeter landscaping shall consist of approved plant materials which provide wildlife habitat, shelter and/or food sources, and which are seasonally wet tolerant as classified in the Wetlands Manual.
(1) 
There shall be a minimum of one approved tree and five approved shrubs per 2,500 cubic feet of basin storage at the 100-year storm design elevation.
(2) 
The trees shall be a combination of shade, flowering and evergreen tree types, and shrubs shall be a combination of deciduous, needled and broadleaf evergreen types.
(3) 
A wildflower mixture with meadow grasses shall be utilized for ground cover for a minimum of 20% of the basin perimeter where the probability of erosion is minimal. Wildflower sod, if used, shall follow the same criteria as set forth below:
A. 
An approved Northeastern mix of a minimum of 12 varieties of wildflowers with a blend of native clump meadow grasses that will attract butterflies and hummingbirds, flowering from late spring through the fall season.
B. 
Minimum germination rate: 60%.
C. 
Seeding rate: four pounds per acre or as approved.
D. 
The remaining basin perimeter shall be seeded or sodded with permanent low-maintenance fine grasses.
(e) 
Fences may be required at the discretion of the Board of Supervisors.
(f) 
Trees and shrubs shall be placed so as to provide a suitable visual barrier at the time of planting. If evergreen trees are used, they shall be of six feet to eight feet in height, placed in a double, staggered row, 10 feet on center. Trees shall not be placed on the top of the detention basin.
(g) 
Native deciduous trees shall be a minimum of three to 3 1/2 inches in caliper, native shrubs shall be a minimum of 24 inches to 36 inches in height, and native evergreen trees shall be a minimum of six feet to eight feet in height at the time of installation. Trees and shrubs shall be spaced in accordance with the characteristic spread of each specific species at maturity.
(h) 
The screening requirement shall be waived only with the expressed consent of the Board of Supervisors.
(a) 
Cuts. No excavation shall be made with a cut face steeper than 3:1, horizontal to vertical, except under the condition in which the material in which the excavation is made is sufficiently stable to sustain a slope of steeper than three horizontal to one vertical. A written statement to that effect is required from a licensed civil engineer having experience in soils engineering and shall be submitted to the Township Engineer and approved by him. The statement shall affirm that the site has been inspected and that the deviation from the slope will not result in injury to persons or damage to property. Retaining walls will be required if a stable slope cannot be maintained. Any retaining wall design must be approved by the Township Engineer. The toe of the slope or headwall of any cut must be located a minimum of five feet from property lines. No excavation shall endanger adjoining properties.
(b) 
Fills. No fill shall be made which creates any exposed surface steeper in slope than three horizontal to one vertical, except where the fill is located so that settlement, sliding or erosion will not result in property damage or be a hazard to adjoining property, streets or buildings. A written statement from a licensed civil engineer licensed by the Commonwealth of Pennsylvania and having experience in soils engineering, certifying that he has inspected the site and that any proposed deviation from the slope specified above will not endanger any property or result in property damage, must be submitted to and approved by the Township Engineer.
(1) 
A concrete or stone masonry wall constructed in accordance with Township specifications may be required to support the face of the fill where above slopes are exceeded.
(2) 
The top of any fill or toe of the slope of any fill shall be located five feet from any property line.
(3) 
Fill placement and compaction. All fill shall be placed mechanically and shall be free of any debris or organic material, roots and stumps. Fill shall be placed in consecutive, maximum eight-inch layers and compressed or compacted mechanically with equipment weighing not less than 10 tons or with similar sheepsfoot roller or compactors having equivalent compression capability. The Township shall require, at the owner's expense, a test certifying the adequate compaction of fill materials as required above or in accordance with the requirements of the Township Engineer. All detention basin fill shall be compacted to 95% modified proctor density (ASTM 1557).
(a) 
No regulated earth disturbance activities within the Township shall commence until approval by the Township of an erosion and sediment control plan for construction activities. Where a regulated earth disturbance activity occurs in connection with a subdivision or land development, compliance with this chapter shall be a component of the subdivision or land development application.
(b) 
The applicant must comply with Title 25 Chapter 102 of the Pennsylvania Code and any other applicable state, county and Township codes. PADEP requires an NPDES construction activities permit and an engineered postconstruction SWM plan for projects proposing earth disturbance greater than one acre.
(c) 
Evidence that the necessary permit(s) for regulated earth disturbance activities have been received from the appropriate PADEP regional office or the MCCD must be provided to the Township.
(d) 
A copy of the approved erosion and sediment control plan and any applicable permits, as required by DEP regulations, shall be available at the project site at all times.
(e) 
Additional erosion and sediment control standards and criteria are recommended to be applied where infiltration BMPs are proposed. They shall include the following:
(1) 
Areas proposed for infiltration BMPs shall be protected from sedimentation and compaction during the construction phase to maintain maximum infiltration capacity.
(2) 
Infiltration BMPs shall not be constructed nor receive runoff until the entire drainage areas tributary to the infiltration BMPs have achieved final stabilization.
The design of all regulated activities should include the following to minimize stormwater impacts:
(a) 
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.
(b) 
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.
(a) 
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 § 1241.106:
(1) 
Infiltration BMPs intended to receive runoff from developed areas shall be selected based on suitability of soils and site conditions. Infiltration BMPs shall be constructed on soils that have the following characteristics:
A. 
A minimum soil depth of 24 inches between the bottom of the infiltration BMPs and bedrock or seasonally high water table.
B. 
An infiltration rate sufficient to accept the additional stormwater load and dewater completely as determined by field tests. A minimum of 0.2 inch/hour (in/hr) shall be utilized and for acceptable rates a safety factor of 50% should be applied for design purposes (e.g., for soil which measured 0.4 in/hr, the BMP design should be 0.2 in/hr to ensure safe infiltration rates after construction).
C. 
All open-air infiltration facilities shall be designed to completely infiltrate the runoff volume within three days (72 hours) from the start 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.
E. 
Pretreatment shall be provided prior to infiltration.
(2) 
Soils. A detailed soils evaluation of the project site shall be prepared for all regulated activities and to determine the suitability for infiltration facilities. The evaluation shall be performed by a qualified professional, and, at a minimum, address soil permeability, depth to bedrock, and subgrade stability. The general process for designing an infiltration BMP shall be:
A. 
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 suitability; infiltration may not be ruled out without conducting these tests.
B. 
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.
C. 
Design the infiltration structure based on field-determined capacity at the level of the proposed infiltration surface and based on a safety factor of 50%.
D. 
If on-lot infiltration structures are proposed, it must be demonstrated to the Township that the soils are conducive to infiltrate on the lots identified.
E. 
An impermeable liner shall be required in detention basins where the possibility of groundwater contamination exists. A detailed hydrogeologic investigation may be required by the Township.
(3) 
The infiltration facility shall be sized based upon the following volume criteria:
A. 
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 Rev required shall be computed as:
Rev = (1 inch/12 inches) * (I)
Where:
Rev
=
Recharge volume (cubic feet)
I
=
Impervious area (square feet) [per § 1241.401(e)]
An asterisk (*) in equations denotes multiplication.
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.
(a) 
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.
(1) 
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.
(2) 
For modeling purposes:
A. 
Existing (predevelopment) nonforested pervious areas must be considered meadow.
B. 
Twenty percent of existing impervious area, when present, shall be considered meadow in the model for existing conditions.
(b) 
The Simplified Method (CG-2 in the BMP 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:
(1) 
Stormwater facilities shall capture at least the first two inches of runoff from all new impervious surfaces.
Volume (cubic feet) = (2/12) * impervious surfaces (square feet)
(2) 
At least the first one inch of runoff from new impervious surfaces shall be permanently removed from the runoff flow — i.e., it shall not be released into the surface waters of the commonwealth. Removal options include reuse, evaporation, transpiration, and infiltration.
Volume (cubic feet) = (1/12) * impervious surfaces (square feet)
(3) 
Wherever possible, infiltration facilities should be designed to accommodate infiltration of the entire permanently removed runoff; however, in all cases at least the first 0.5 inch of the permanently removed runoff should be infiltrated.
(4) 
This method may only be used for projects that are exempt from § 1241.410, Peak rate controls.
(a) 
If a perennial or intermittent stream passes through the site, the applicant shall create a riparian buffer extending a minimum of 10 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 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.
(b) 
Applicants shall adhere to the following stream bank erosion/channel protection requirements:
(1) 
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).
(2) 
The minimum orifice diameter in the outlet control structure shall be three inches, in addition a trash rack shall be installed to prevent clogging. On sites with drainage areas that do not provide enough runoff volume to allow a twenty-four-hour attenuation with the three-inch orifice, 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.
(a) 
Standard for managing runoff from all sites in the Township for the two-, five-, ten-, twenty-five-, fifty-, and 100-year design storms are shown in Table 410.A.1 below.
Table 410.A.1
Peak Rate Control Standards for All Sites in Lower Gwynedd Township
Proposed Condition Design Storm
Existing Condition Design Storm
2-year
reduce to
1-year
5-year
reduce to
2-year
10-year
reduce to
5-year
25-year
reduce to
10-year
50-year
reduce to
25-year
100-year
reduce to
50-year
(b) 
Off-site areas. When calculating the allowable peak runoff rates, developers do not have to account for runoff draining into the subject development site from an off-site area. On-site drainage facilities shall be designed to safely convey off-site flows through the development site.
(c) 
Site areas. The stormwater management site area is the only area subject to the management district criteria. Nonimpacted areas or nonregulated activities bypassing the stormwater management facilities would not be subject to the management district criteria.
(d) 
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:
(1) 
Meet the full requirements specified by Table 410.1 and § 1241.410(a) through (d); or
(2) 
Reduce the total impervious surface on the site by at least 20% based upon a comparison of existing impervious surface to proposed impervious surface.
(e) 
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, so long as peak outflow is not increased.
(a) 
The following criteria shall be used for runoff calculations:
(1) 
For development sites not considered redevelopment, the ground cover used to determine the existing conditions runoff volume and flow rate shall be as follows:
A. 
Wooded sites shall use a ground cover of "woods in good condition." A site is classified as wooded if a continuous canopy of trees exists over a 1/4 acre.
B. 
The undeveloped portion of the site including agriculture, bare earth, and fallow ground shall be considered as "meadow in good condition," unless the natural ground cover generates a lower curve number (CN) or Rational "c" value (i.e., woods) as listed in Tables A-4 or A-7 in Appendix A.[1]
[1]
Editor's Note: The appendices are included as attachments to this chapter.
(2) 
For development and redevelopment sites, the ground cover used to determine the existing conditions runoff volume and flow rate for the developed portion of the site shall be based upon actual land cover conditions. If the developed site contains impervious surfaces, 20% of the impervious surface area shall be considered meadow in the model for existing conditions.
(b) 
Stormwater runoff peak discharges from all development sites with a drainage area of greater than two acres shall be calculated using a generally accepted calculation technique that is based on the NRCS Soil Cover Complex Method. Table 411.1 summarizes acceptable computation methods. The method selected by the qualified professional shall be based on the individual limitations and suitability of each method for a particular site. The Township Engineer may allow the use of the Rational Method (Q=CIA) to estimate peak discharges from drainage areas that contain less than two acres.
Q
=
Peak flow rate, cubic feet per second (CFS)
C
=
Runoff coefficient, dependent on land use/cover
I
=
Design rainfall intensity, inches per hour
A
=
Drainage area, acres.
(c) 
All calculations consistent with this chapter using the Soil Cover Complex Method shall use the appropriate design rainfall depths for the various return period storms according to Table A-1 in Appendix A. If a hydraulic computer model such as PSRM or HEC-1/HEC-HMS is used for stormwater runoff calculations, then the duration of rainfall shall be 24 hours.
Table 411.1
Acceptable Computation Methodologies for Stormwater Management Plans
Method
Method Developed By
Applicability
TR-20 (or commercial computer package based on TR-20)
USDA NRCS
Applicable where use of full hydrology computer model is desirable or necessary
TR-55 (or commercial computer package based on TR-55)
USDA NRCS
Applicable for land development plans within limitations described in TR-55
HEC-1/HEC-HMS
U.S. Army Corps of Engineers
Applicable where use of full hydrologic computer model is desirable or necessary
PSRM
Penn State University
Applicable where use of full hydrologic computer model is desirable or necessary; simpler than TR-20 or HEC-1
Rational Method (or commercial computer package based on Rational Method)
Emil Kuichling (1889)
Peak discharge only for sites less than two acres or as approved by the Township and/or Township Engineer
Other methods
Varies
Other computation methodologies approved by the Township and/or Township Engineer
(d) 
All calculations using the Rational Method shall use rainfall intensities consistent with appropriate times of concentration for overland flow and return periods from Table A-1 in Appendix A. Times of concentration for overland flow shall be calculated using the methodology presented in Chapter 3 of Urban Hydrology for Small Watersheds, NRCS, TR-55 (as amended or replaced from time to time by NRCS). Times of concentration for channel and pipe flow shall be computed using Manning's equation.
(e) 
Runoff curve numbers (CN) for both existing and proposed conditions to be used in the Soil Cover Complex Method shall be based on Table A-4 in Appendix A.
(f) 
Runoff coefficients (C) for both existing and proposed conditions for use in the Rational Method shall be consistent with Table A-7 in Appendix A.
(g) 
The Manning equation is preferred for one-dimensional, gradually varied, open channel flow. In other cases, appropriate, applicable methods should be employed; however, early coordination with the Township is necessary.
(h) 
Outlet structures for stormwater management facilities shall be designed to meet the performance standards of this chapter using the generally accepted hydraulic analysis technique or method of the Township.
(i) 
The design of any stormwater detention facilities intended to meet the performance standards of this chapter shall be verified by routing the design storm hydrograph through these facilities using the Storage-Indication Method. The design storm hydrograph shall be computed using an acceptable calculation method (Table 411.1) that produces a full hydrograph. The Township may approve the use of any generally accepted full hydrograph approximation technique that shall use a total runoff volume that is consistent with the volume from a method that produces a full hydrograph.
(a) 
Hot spots.
(1) 
The use of infiltration BMPs is prohibited on hot spot land use areas. Examples of hot spots are listed in Appendix B.[1]
[1]
Editor's Note: The appendices are included as attachments to this chapter.
(2) 
Stormwater runoff from hot spot land uses shall be pretreated. In no case may the same BMP be employed consecutively to meet this requirement. Guidance regarding acceptable methods of pretreatment is located in Appendix B.
(b) 
West Nile Guidance requirements. All wet basin designs shall incorporate controls consistent with the West Nile Guidance found in Appendix C.
(a) 
In order to protect and improve water quality, a riparian buffer easement shall be created and recorded as part of any subdivision or land development that encompasses a riparian buffer.
(b) 
Except as required by Chapter 102, the riparian buffer easement shall be measured to be the greater of the limit of the 100 year floodplain or a minimum of 35 feet from the top of the streambank (on each side).
(c) 
Minimum management requirements for riparian buffers.
(1) 
Existing native vegetation shall be protected and maintained within the riparian buffer easement.
(2) 
Whenever practicable invasive vegetation shall be actively removed and the riparian buffer easement shall be planted with native trees, shrubs and other vegetation to create a diverse native plant community appropriate to the intended ecological context of the site.
(d) 
The riparian buffer easement shall be enforceable by the municipality and shall be recorded in the Montgomery County Recorder of Deeds Office, so that it shall run with the land and shall limit the use of the property located therein. The easement shall allow for the continued private ownership and shall count toward the minimum lot area required by the Zoning Ordinance, unless otherwise specified in the Zoning Ordinance.
(e) 
Any permitted use within the riparian buffer easement shall be conducted in a manner that will maintain the extent of the existing 100-year floodplain, improve or maintain the stream stability, and preserve and protect the ecological function of the floodplain.
(f) 
The following conditions shall apply when public and/or private recreation trails are permitted within riparian buffers:
(1) 
Trails shall be for nonmotorized use only.
(2) 
Trails shall be designed to have the least impact on native plant species and other sensitive environmental features.
(g) 
Septic drainfields and sewage disposal systems shall not be permitted withing the riparian buffer easement and shall comply with setback requirements established under 25 Pa. Code Chapter 73.