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 municipality 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 municipality or County Conservation District, when and as required.
B. 
SWM site plan approved by the municipality shall be on-site throughout the duration of the regulated activity.
C. 
The municipality 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.[1]
[1]
Editor's Note: See 35 P.S. § 691.1 et seq.
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, §§ 18-403 through 180-408 shall apply to the total impervious area within the limits of earth disturbance.
F. 
Stormwater discharges 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 discharges 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) 
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.
(3) 
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.
(4) 
Incorporate methods described in the Pennsylvania Stormwater Best Management Practices Manual (BMP Manual).
H. 
The design of all facilities over karst 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.[2]
[2]
Editor's Note: For the Clean Streams Law, see 35 P.S. § 691.1 et seq. For the Storm Water Management Act, see 32 P.S. § 680.1 et seq.
M. 
Various BMPs and their design standards are listed in the BMP Manual1.
Additional erosion and sediment control standards and criteria are recommended to be applied where infiltration BMPs are proposed. They shall include the following:
A. 
These areas shall be protected from sedimentation and compaction during the construction phase.
B. 
BMPs shall not be constructed, nor the areas 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 Nonstructural Project Design Checklist in Appendix C[1] 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.
[1]
Editor's Note: Said appendix is included as an attachment to this chapter.
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 § 180-106:
(1) 
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) 
A minimum soil depth of 24 inches between the bottoms of the infiltration BMPs and bedrock or other limiting zones.
(b) 
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 (Rev) 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 § 180-202) shall be provided prior to infiltration.
(2) 
The size of the infiltration facility shall be 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.
(b) 
The Rev required shall be computed as:
180 180-405A(2) Formula.tif
B. 
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:
(1) 
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 sub-grade stability; infiltration must be implemented if these tests are not completed.
(2) 
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.
(3) 
Design the infiltration structure for the required recharge volume (Rev) 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.
The green stormwater infrastructure and 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 postdevelopment 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. [Note: An asterisk (*) in equations denotes multiplication.]
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 is exempt from the requirements of § 180-408, Stormwater peak rate control and management districts.
A. 
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.
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 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 24-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 replaceable and/or porous media filter cartridge.
A. 
The Wissahickon Creek Watershed has been divided into stormwater management districts as shown on the Management District Map (Ordinance Appendix A[1]). The peak rate requirements specified in Table 408.1 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 100-year storm events are shown in Table 408.1. 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 408.1. For any areas not shown on the release rate maps, the postdevelopment discharge rates shall not exceed the predevelopment discharge rates.
Table 408.1
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
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
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. 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 appendix is included as an attachment to this chapter.
B. 
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 (Ordinance Appendix A).
C. 
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 Ordinance 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.
D. 
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.
E. 
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.
F. 
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.
G. 
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 North Wales Borough;
(1) 
Meet the full requirements specified by Table 408.1 and § 180-408 Subsections A through F; 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.
H. 
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. 
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. Table 409.1 summarizes acceptable computation methods and the method selected by the qualified person shall be based on the individual limitations and suitability of each method for a particular site. The municipality may allow the use of the Rational Method to estimate peak discharges from drainage areas that contain less than 200 acres. The Soil Cover Complex Method shall be used for drainage areas greater than 200 acres.
Table 409.1
Acceptable Computation Methodologies For Stormwater Management Plans
Montgomery County Portion of the Watershed
Method
Method Developed By
Applicability
WINTR-20
USDA NRCS
Applicable where use of full hydrology computer model is desirable or necessary
WINTR-55
USDA NRCS
Applicable for land development plans within limitations described in TR-55
HEC-HMS
US Army Corps of Engineers
Applicable where use of full hydrologic computer model is desirable or necessary
Rational Method or commercial computer package based on Rational Method
Emil Kuichling (1889)
For sites less than 200 acres and with times of concentration less than 60 minutes (tc< 60 min), or as approved by the municipality and/or Municipal Engineer
Other methods
Varies
Other computation methodologies approved by the municipality and/or Municipal Engineer
B. 
If a hydrologic computer model such as HydroCAD or HEC-HMS is used for stormwater runoff calculations, then the duration of rainfall shall be 24 hours. The rainfall distribution should reference NRCS Type II.
C. 
For the purposes of existing conditions flow rate determination, undeveloped land shall be considered as "meadow," unless the natural ground cover generates a lower curve number or Rational 'C' value (i.e., forest).
D. 
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 flow velocities as determined by Manning's equation.
E. 
The Manning equation is preferred for 1-D, gradually-varied, open channel flow. In other cases, appropriate, applicable methods should be applied, however, early coordination with the municipality is necessary.
F. 
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 municipality.
G. 
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. For drainage areas greater than 200 acres in size, the design storm hydrograph shall be computed using a calculation method that produces a full hydrograph. The municipality 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.